1 |
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2 | GCC Bugs
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3 |
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4 | The latest version of this document is always available at
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5 | [1]http://gcc.gnu.org/bugs.html.
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6 | _________________________________________________________________
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7 |
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8 | Table of Contents
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9 |
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10 | * [2]Reporting Bugs
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11 | + [3]What we need
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12 | + [4]What we DON'T want
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13 | + [5]Where to post it
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14 | + [6]Detailed bug reporting instructions
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15 | + [7]Detailed bug reporting instructions for GNAT
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16 | + [8]Detailed bug reporting instructions when using a
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17 | precompiled header
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18 | * [9]Frequently Reported Bugs in GCC
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19 | + [10]C++
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20 | o [11]Missing features
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21 | o [12]Bugs fixed in the 3.4 series
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22 | + [13]Fortran
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23 | * [14]Non-bugs
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24 | + [15]General
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25 | + [16]C
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26 | + [17]C++
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27 | o [18]Common problems when upgrading the compiler
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28 | _________________________________________________________________
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29 |
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30 | Reporting Bugs
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31 |
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32 | The main purpose of a bug report is to enable us to fix the bug. The
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33 | most important prerequisite for this is that the report must be
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34 | complete and self-contained, which we explain in detail below.
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35 |
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36 | Before you report a bug, please check the [19]list of well-known bugs
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37 | and, if possible in any way, try a current development snapshot. If
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38 | you want to report a bug with versions of GCC before 3.1 we strongly
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39 | recommend upgrading to the current release first.
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40 |
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41 | Before reporting that GCC compiles your code incorrectly, please
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42 | compile it with gcc -Wall and see whether this shows anything wrong
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43 | with your code that could be the cause instead of a bug in GCC.
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44 |
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45 | Summarized bug reporting instructions
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46 |
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47 | After this summary, you'll find detailed bug reporting instructions,
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48 | that explain how to obtain some of the information requested in this
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49 | summary.
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50 |
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51 | What we need
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52 |
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53 | Please include in your bug report all of the following items, the
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54 | first three of which can be obtained from the output of gcc -v:
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55 | * the exact version of GCC;
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56 | * the system type;
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57 | * the options given when GCC was configured/built;
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58 | * the complete command line that triggers the bug;
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59 | * the compiler output (error messages, warnings, etc.); and
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60 | * the preprocessed file (*.i*) that triggers the bug, generated by
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61 | adding -save-temps to the complete compilation command, or, in the
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62 | case of a bug report for the GNAT front end, a complete set of
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63 | source files (see below).
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64 |
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65 | What we do not want
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66 |
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67 | * A source file that #includes header files that are left out of the
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68 | bug report (see above)
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69 | * That source file and a collection of header files.
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70 | * An attached archive (tar, zip, shar, whatever) containing all (or
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71 | some :-) of the above.
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72 | * A code snippet that won't cause the compiler to produce the exact
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73 | output mentioned in the bug report (e.g., a snippet with just a
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74 | few lines around the one that apparently triggers the bug, with
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75 | some pieces replaced with ellipses or comments for extra
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76 | obfuscation :-)
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77 | * The location (URL) of the package that failed to build (we won't
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78 | download it, anyway, since you've already given us what we need to
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79 | duplicate the bug, haven't you? :-)
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80 | * An error that occurs only some of the times a certain file is
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81 | compiled, such that retrying a sufficient number of times results
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82 | in a successful compilation; this is a symptom of a hardware
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83 | problem, not of a compiler bug (sorry)
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84 | * E-mail messages that complement previous, incomplete bug reports.
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85 | Post a new, self-contained, full bug report instead, if possible
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86 | as a follow-up to the original bug report
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87 | * Assembly files (*.s) produced by the compiler, or any binary
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88 | files, such as object files, executables, core files, or
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89 | precompiled header files
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90 | * Duplicate bug reports, or reports of bugs already fixed in the
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91 | development tree, especially those that have already been reported
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92 | as fixed last week :-)
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93 | * Bugs in the assembler, the linker or the C library. These are
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94 | separate projects, with separate mailing lists and different bug
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95 | reporting procedures
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96 | * Bugs in releases or snapshots of GCC not issued by the GNU
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97 | Project. Report them to whoever provided you with the release
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98 | * Questions about the correctness or the expected behavior of
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99 | certain constructs that are not GCC extensions. Ask them in forums
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100 | dedicated to the discussion of the programming language
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101 |
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102 | Where to post it
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103 |
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104 | Please submit your bug report directly to the [20]GCC bug database.
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105 | Alternatively, you can use the gccbug script that mails your bug
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106 | report to the bug database.
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107 | Only if all this is absolutely impossible, mail all information to
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108 | [21]gcc-bugs@gcc.gnu.org.
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109 |
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110 | Detailed bug reporting instructions
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111 |
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112 | Please refer to the [22]next section when reporting bugs in GNAT, the
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113 | Ada compiler, or to the [23]one after that when reporting bugs that
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114 | appear when using a precompiled header.
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115 |
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116 | In general, all the information we need can be obtained by collecting
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117 | the command line below, as well as its output and the preprocessed
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118 | file it generates.
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119 |
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120 | gcc -v -save-temps all-your-options source-file
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121 |
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122 | Typically the preprocessed file (extension .i for C or .ii for C++,
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123 | and .f if the preprocessor is used on Fortran files) will be large, so
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124 | please compress the resulting file with one of the popular compression
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125 | programs such as bzip2, gzip, zip or compress (in decreasing order of
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126 | preference). Use maximum compression (-9) if available. Please include
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127 | the compressed preprocessor output in your bug report, even if the
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128 | source code is freely available elsewhere; it makes the job of our
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129 | volunteer testers much easier.
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130 |
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131 | The only excuses to not send us the preprocessed sources are (i) if
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132 | you've found a bug in the preprocessor, (ii) if you've reduced the
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133 | testcase to a small file that doesn't include any other file or (iii)
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134 | if the bug appears only when using precompiled headers. If you can't
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135 | post the preprocessed sources because they're proprietary code, then
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136 | try to create a small file that triggers the same problem.
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137 |
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138 | Since we're supposed to be able to re-create the assembly output
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139 | (extension .s), you usually should not include it in the bug report,
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140 | although you may want to post parts of it to point out assembly code
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141 | you consider to be wrong.
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142 |
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143 | Whether to use MIME attachments or uuencode is up to you. In any case,
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144 | make sure the compiler command line, version and error output are in
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145 | plain text, so that we don't have to decode the bug report in order to
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146 | tell who should take care of it. A meaningful subject indicating
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147 | language and platform also helps.
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148 |
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149 | Please avoid posting an archive (.tar, .shar or .zip); we generally
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150 | need just a single file to reproduce the bug (the .i/.ii/.f
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151 | preprocessed file), and, by storing it in an archive, you're just
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152 | making our volunteers' jobs harder. Only when your bug report requires
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153 | multiple source files to be reproduced should you use an archive. This
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154 | is, for example, the case if you are using INCLUDE directives in
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155 | Fortran code, which are not processed by the preprocessor, but the
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156 | compiler. In that case, we need the main file and all INCLUDEd files.
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157 | In any case, make sure the compiler version, error message, etc, are
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158 | included in the body of your bug report as plain text, even if
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159 | needlessly duplicated as part of an archive.
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160 |
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161 | If you fail to supply enough information for a bug report to be
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162 | reproduced, someone will probably ask you to post additional
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163 | information (or just ignore your bug report, if they're in a bad day,
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164 | so try to get it right on the first posting :-). In this case, please
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165 | post the additional information to the bug reporting mailing list, not
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166 | just to the person who requested it, unless explicitly told so. If
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167 | possible, please include in this follow-up all the information you had
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168 | supplied in the incomplete bug report (including the preprocessor
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169 | output), so that the new bug report is self-contained.
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170 |
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171 | Detailed bug reporting instructions for GNAT
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172 |
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173 | See the [24]previous section for bug reporting instructions for GCC
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174 | language implementations other than Ada.
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175 |
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176 | Bug reports have to contain at least the following information in
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177 | order to be useful:
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178 | * the exact version of GCC, as shown by "gcc -v";
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179 | * the system type;
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180 | * the options when GCC was configured/built;
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181 | * the exact command line passed to the gcc program triggering the
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182 | bug (not just the flags passed to gnatmake, but gnatmake prints
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183 | the parameters it passed to gcc)
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184 | * a collection of source files for reproducing the bug, preferably a
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185 | minimal set (see below);
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186 | * a description of the expected behavior;
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187 | * a description of actual behavior.
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188 |
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189 | If your code depends on additional source files (usually package
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190 | specifications), submit the source code for these compilation units in
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191 | a single file that is acceptable input to gnatchop, i.e. contains no
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192 | non-Ada text. If the compilation terminated normally, you can usually
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193 | obtain a list of dependencies using the "gnatls -d main_unit" command,
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194 | where main_unit is the file name of the main compilation unit (which
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195 | is also passed to gcc).
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196 |
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197 | If you report a bug which causes the compiler to print a bug box,
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198 | include that bug box in your report, and do not forget to send all the
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199 | source files listed after the bug box along with your report.
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200 |
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201 | If you use gnatprep, be sure to send in preprocessed sources (unless
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202 | you have to report a bug in gnatprep).
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203 |
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204 | When you have checked that your report meets these criteria, please
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205 | submit it according to our [25]generic instructions. (If you use a
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206 | mailing list for reporting, please include an "[Ada]" tag in the
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207 | subject.)
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208 |
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209 | Detailed bug reporting instructions when using a precompiled header
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210 |
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211 | If you're encountering a bug when using a precompiled header, the
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212 | first thing to do is to delete the precompiled header, and try running
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213 | the same GCC command again. If the bug happens again, the bug doesn't
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214 | really involve precompiled headers, please report it without using
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215 | them by following the instructions [26]above.
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216 |
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217 | If you've found a bug while building a precompiled header (for
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218 | instance, the compiler crashes), follow the usual instructions
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219 | [27]above.
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220 |
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221 | If you've found a real precompiled header bug, what we'll need to
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222 | reproduce it is the sources to build the precompiled header (as a
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223 | single .i file), the source file that uses the precompiled header, any
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224 | other headers that source file includes, and the command lines that
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225 | you used to build the precompiled header and to use it.
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226 |
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227 | Please don't send us the actual precompiled header. It is likely to be
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228 | very large and we can't use it to reproduce the problem.
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229 | _________________________________________________________________
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230 |
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231 | Frequently Reported Bugs in GCC
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232 |
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233 | This is a list of bugs in GCC that are reported very often, but not
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234 | yet fixed. While it is certainly better to fix bugs instead of
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235 | documenting them, this document might save people the effort of
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236 | writing a bug report when the bug is already well-known.
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237 |
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238 | There are many reasons why a reported bug doesn't get fixed. It might
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239 | be difficult to fix, or fixing it might break compatibility. Often,
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240 | reports get a low priority when there is a simple work-around. In
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241 | particular, bugs caused by invalid code have a simple work-around: fix
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242 | the code.
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243 | _________________________________________________________________
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244 |
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245 | C++
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246 |
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247 | Missing features
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248 |
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249 | The export keyword is not implemented.
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250 | Most C++ compilers (G++ included) do not yet implement export,
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251 | which is necessary for separate compilation of template
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252 | declarations and definitions. Without export, a template
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253 | definition must be in scope to be used. The obvious workaround
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254 | is simply to place all definitions in the header itself.
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255 | Alternatively, the compilation unit containing template
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256 | definitions may be included from the header.
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257 |
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258 | Bugs fixed in the 3.4 series
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259 |
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260 | The following bugs are present up to (and including) GCC 3.3.x. They
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261 | have been fixed in 3.4.0.
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262 |
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263 | Two-stage name-lookup.
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264 | GCC did not implement two-stage name-lookup (also see
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265 | [28]below).
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266 |
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267 | Covariant return types.
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268 | GCC did not implement non-trivial covariant returns.
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269 |
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270 | Parse errors for "simple" code.
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271 | GCC gave parse errors for seemingly simple code, such as
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272 |
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273 | struct A
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274 | {
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275 | A();
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276 | A(int);
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277 | };
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278 |
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279 | struct B
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280 | {
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281 | B(A);
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282 | B(A,A);
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283 | void foo();
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284 | };
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285 |
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286 | A bar()
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287 | {
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288 | B b(A(),A(1)); // Variable b, initialized with two temporaries
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289 | B(A(2)).foo(); // B temporary, initialized with A temporary
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290 | return (A()); // return A temporary
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291 | }
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292 |
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293 | Although being valid code, each of the three lines with a
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294 | comment was rejected by GCC. The work-arounds for older
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295 | compiler versions proposed below do not change the semantics of
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296 | the programs at all.
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297 |
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298 | The problem in the first case was that GCC started to parse the
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299 | declaration of b as a function called b returning B, taking a
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300 | function returning A as an argument. When it encountered the 1,
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301 | it was too late. To show the compiler that this should be
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302 | really an expression, a comma operator with a dummy argument
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303 | could be used:
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304 |
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305 | B b((0,A()),A(1));
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306 |
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307 | The work-around for simpler cases like the second one was to
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308 | add additional parentheses around the expressions that were
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309 | mistaken as declarations:
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310 |
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311 | (B(A(2))).foo();
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312 |
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313 | In the third case, however, additional parentheses were causing
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314 | the problems: The compiler interpreted A() as a function
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315 | (taking no arguments, returning A), and (A()) as a cast lacking
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316 | an expression to be casted, hence the parse error. The
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317 | work-around was to omit the parentheses:
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318 |
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319 | return A();
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320 |
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321 | This problem occurred in a number of variants; in throw
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322 | statements, people also frequently put the object in
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323 | parentheses.
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324 | _________________________________________________________________
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325 |
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326 | Fortran
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327 |
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328 | Fortran bugs are documented in the G77 manual rather than explicitly
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329 | listed here. Please see [29]Known Causes of Trouble with GNU Fortran
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330 | in the G77 manual.
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331 | _________________________________________________________________
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332 |
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333 | Non-bugs
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334 |
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335 | The following are not actually bugs, but are reported often enough to
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336 | warrant a mention here.
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337 |
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338 | It is not always a bug in the compiler, if code which "worked" in a
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339 | previous version, is now rejected. Earlier versions of GCC sometimes
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340 | were less picky about standard conformance and accepted invalid source
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341 | code. In addition, programming languages themselves change, rendering
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342 | code invalid that used to be conforming (this holds especially for
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343 | C++). In either case, you should update your code to match recent
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344 | language standards.
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345 | _________________________________________________________________
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346 |
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347 | General
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348 |
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349 | Problems with floating point numbers - the [30]most often reported
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350 | non-bug.
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351 | In a number of cases, GCC appears to perform floating point
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352 | computations incorrectly. For example, the C++ program
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353 |
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354 | #include <iostream>
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355 |
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356 | int main()
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357 | {
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358 | double a = 0.5;
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359 | double b = 0.01;
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360 | std::cout << (int)(a / b) << std::endl;
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361 | return 0;
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362 | }
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363 |
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364 | might print 50 on some systems and optimization levels, and 49
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365 | on others.
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366 |
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367 | This is the result of rounding: The computer cannot represent
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368 | all real numbers exactly, so it has to use approximations. When
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369 | computing with approximation, the computer needs to round to
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370 | the nearest representable number.
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371 |
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372 | This is not a bug in the compiler, but an inherent limitation
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373 | of the floating point types. Please study [31]this paper for
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374 | more information.
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375 | _________________________________________________________________
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376 |
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377 | C
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378 |
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379 | Increment/decrement operator (++/--) not working as expected - a
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380 | [32]problem with many variations.
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381 | The following expressions have unpredictable results:
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382 |
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383 | x[i]=++i
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384 | foo(i,++i)
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385 | i*(++i) /* special case with foo=="operator*" */
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386 | std::cout << i << ++i /* foo(foo(std::cout,i),++i) */
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387 |
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388 | since the i without increment can be evaluated before or after
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389 | ++i.
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390 |
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391 | The C and C++ standards have the notion of "sequence points".
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392 | Everything that happens between two sequence points happens in
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393 | an unspecified order, but it has to happen after the first and
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394 | before the second sequence point. The end of a statement and a
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395 | function call are examples for sequence points, whereas
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396 | assignments and the comma between function arguments are not.
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397 |
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398 | Modifying a value twice between two sequence points as shown in
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399 | the following examples is even worse:
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400 |
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401 | i=++i
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402 | foo(++i,++i)
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403 | (++i)*(++i) /* special case with foo=="operator*" */
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404 | std::cout << ++i << ++i /* foo(foo(std::cout,++i),++i) */
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405 |
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406 | This leads to undefined behavior (i.e. the compiler can do
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407 | anything).
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408 |
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409 | Casting does not work as expected when optimization is turned on.
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410 | This is often caused by a violation of aliasing rules, which
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411 | are part of the ISO C standard. These rules say that a program
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412 | is invalid if you try to access a variable through a pointer of
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413 | an incompatible type. This is happening in the following
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414 | example where a short is accessed through a pointer to integer
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415 | (the code assumes 16-bit shorts and 32-bit ints):
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416 |
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417 | #include <stdio.h>
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418 |
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419 | int main()
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420 | {
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421 | short a[2];
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422 |
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423 | a[0]=0x1111;
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424 | a[1]=0x1111;
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425 |
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426 | *(int *)a = 0x22222222; /* violation of aliasing rules */
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427 |
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428 | printf("%x %x\n", a[0], a[1]);
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429 | return 0;
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430 | }
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431 |
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432 | The aliasing rules were designed to allow compilers more
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433 | aggressive optimization. Basically, a compiler can assume that
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434 | all changes to variables happen through pointers or references
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435 | to variables of a type compatible to the accessed variable.
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436 | Dereferencing a pointer that violates the aliasing rules
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437 | results in undefined behavior.
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438 |
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439 | In the case above, the compiler may assume that no access
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440 | through an integer pointer can change the array a, consisting
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441 | of shorts. Thus, printf may be called with the original values
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442 | of a[0] and a[1]. What really happens is up to the compiler and
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443 | may change with architecture and optimization level.
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444 |
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445 | Recent versions of GCC turn on the option -fstrict-aliasing
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446 | (which allows alias-based optimizations) by default with -O2.
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447 | And some architectures then really print "1111 1111" as result.
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448 | Without optimization the executable will generate the
|
---|
449 | "expected" output "2222 2222".
|
---|
450 |
|
---|
451 | To disable optimizations based on alias-analysis for faulty
|
---|
452 | legacy code, the option -fno-strict-aliasing can be used as a
|
---|
453 | work-around.
|
---|
454 |
|
---|
455 | The option -Wstrict-aliasing (which is included in -Wall) warns
|
---|
456 | about some - but not all - cases of violation of aliasing rules
|
---|
457 | when -fstrict-aliasing is active.
|
---|
458 |
|
---|
459 | To fix the code above, you can use a union instead of a cast
|
---|
460 | (note that this is a GCC extension which might not work with
|
---|
461 | other compilers):
|
---|
462 |
|
---|
463 | #include <stdio.h>
|
---|
464 |
|
---|
465 | int main()
|
---|
466 | {
|
---|
467 | union
|
---|
468 | {
|
---|
469 | short a[2];
|
---|
470 | int i;
|
---|
471 | } u;
|
---|
472 |
|
---|
473 | u.a[0]=0x1111;
|
---|
474 | u.a[1]=0x1111;
|
---|
475 |
|
---|
476 | u.i = 0x22222222;
|
---|
477 |
|
---|
478 | printf("%x %x\n", u.a[0], u.a[1]);
|
---|
479 | return 0;
|
---|
480 | }
|
---|
481 |
|
---|
482 | Now the result will always be "2222 2222".
|
---|
483 |
|
---|
484 | For some more insight into the subject, please have a look at
|
---|
485 | [33]this article.
|
---|
486 |
|
---|
487 | Cannot use preprocessor directive in macro arguments.
|
---|
488 | Let me guess... you used an older version of GCC to compile
|
---|
489 | code that looks something like this:
|
---|
490 |
|
---|
491 | memcpy(dest, src,
|
---|
492 | #ifdef PLATFORM1
|
---|
493 | 12
|
---|
494 | #else
|
---|
495 | 24
|
---|
496 | #endif
|
---|
497 | );
|
---|
498 |
|
---|
499 | and you got a whole pile of error messages:
|
---|
500 |
|
---|
501 | test.c:11: warning: preprocessing directive not recognized within macro arg
|
---|
502 | test.c:11: warning: preprocessing directive not recognized within macro arg
|
---|
503 | test.c:11: warning: preprocessing directive not recognized within macro arg
|
---|
504 | test.c: In function `foo':
|
---|
505 | test.c:6: undefined or invalid # directive
|
---|
506 | test.c:8: undefined or invalid # directive
|
---|
507 | test.c:9: parse error before `24'
|
---|
508 | test.c:10: undefined or invalid # directive
|
---|
509 |
|
---|
510 | This is because your C library's <string.h> happens to define
|
---|
511 | memcpy as a macro - which is perfectly legitimate. In recent
|
---|
512 | versions of glibc, for example, printf is among those functions
|
---|
513 | which are implemented as macros.
|
---|
514 |
|
---|
515 | Versions of GCC prior to 3.3 did not allow you to put #ifdef
|
---|
516 | (or any other preprocessor directive) inside the arguments of a
|
---|
517 | macro. The code therefore would not compile.
|
---|
518 |
|
---|
519 | As of GCC 3.3 this kind of construct is always accepted and the
|
---|
520 | preprocessor will probably do what you expect, but see the
|
---|
521 | manual for detailed semantics.
|
---|
522 |
|
---|
523 | However, this kind of code is not portable. It is "undefined
|
---|
524 | behavior" according to the C standard; that means different
|
---|
525 | compilers may do different things with it. It is always
|
---|
526 | possible to rewrite code which uses conditionals inside macros
|
---|
527 | so that it doesn't. You could write the above example
|
---|
528 |
|
---|
529 | #ifdef PLATFORM1
|
---|
530 | memcpy(dest, src, 12);
|
---|
531 | #else
|
---|
532 | memcpy(dest, src, 24);
|
---|
533 | #endif
|
---|
534 |
|
---|
535 | This is a bit more typing, but I personally think it's better
|
---|
536 | style in addition to being more portable.
|
---|
537 |
|
---|
538 | Cannot initialize a static variable with stdin.
|
---|
539 | This has nothing to do with GCC, but people ask us about it a
|
---|
540 | lot. Code like this:
|
---|
541 |
|
---|
542 | #include <stdio.h>
|
---|
543 |
|
---|
544 | FILE *yyin = stdin;
|
---|
545 |
|
---|
546 | will not compile with GNU libc, because stdin is not a
|
---|
547 | constant. This was done deliberately, to make it easier to
|
---|
548 | maintain binary compatibility when the type FILE needs to be
|
---|
549 | changed. It is surprising for people used to traditional Unix C
|
---|
550 | libraries, but it is permitted by the C standard.
|
---|
551 |
|
---|
552 | This construct commonly occurs in code generated by old
|
---|
553 | versions of lex or yacc. We suggest you try regenerating the
|
---|
554 | parser with a current version of flex or bison, respectively.
|
---|
555 | In your own code, the appropriate fix is to move the
|
---|
556 | initialization to the beginning of main.
|
---|
557 |
|
---|
558 | There is a common misconception that the GCC developers are
|
---|
559 | responsible for GNU libc. These are in fact two entirely
|
---|
560 | separate projects; please check the [34]GNU libc web pages for
|
---|
561 | details.
|
---|
562 | _________________________________________________________________
|
---|
563 |
|
---|
564 | C++
|
---|
565 |
|
---|
566 | Nested classes can access private members and types of the containing
|
---|
567 | class.
|
---|
568 | Defect report 45 clarifies that nested classes are members of
|
---|
569 | the class they are nested in, and so are granted access to
|
---|
570 | private members of that class.
|
---|
571 |
|
---|
572 | G++ emits two copies of constructors and destructors.
|
---|
573 | In general there are three types of constructors (and
|
---|
574 | destructors).
|
---|
575 |
|
---|
576 | 1. The complete object constructor/destructor.
|
---|
577 | 2. The base object constructor/destructor.
|
---|
578 | 3. The allocating constructor/deallocating destructor.
|
---|
579 |
|
---|
580 | The first two are different, when virtual base classes are
|
---|
581 | involved.
|
---|
582 |
|
---|
583 | Global destructors are not run in the correct order.
|
---|
584 | Global destructors should be run in the reverse order of their
|
---|
585 | constructors completing. In most cases this is the same as the
|
---|
586 | reverse order of constructors starting, but sometimes it is
|
---|
587 | different, and that is important. You need to compile and link
|
---|
588 | your programs with --use-cxa-atexit. We have not turned this
|
---|
589 | switch on by default, as it requires a cxa aware runtime
|
---|
590 | library (libc, glibc, or equivalent).
|
---|
591 |
|
---|
592 | Classes in exception specifiers must be complete types.
|
---|
593 | [15.4]/1 tells you that you cannot have an incomplete type, or
|
---|
594 | pointer to incomplete (other than cv void *) in an exception
|
---|
595 | specification.
|
---|
596 |
|
---|
597 | Exceptions don't work in multithreaded applications.
|
---|
598 | You need to rebuild g++ and libstdc++ with --enable-threads.
|
---|
599 | Remember, C++ exceptions are not like hardware interrupts. You
|
---|
600 | cannot throw an exception in one thread and catch it in
|
---|
601 | another. You cannot throw an exception from a signal handler
|
---|
602 | and catch it in the main thread.
|
---|
603 |
|
---|
604 | Templates, scoping, and digraphs.
|
---|
605 | If you have a class in the global namespace, say named X, and
|
---|
606 | want to give it as a template argument to some other class, say
|
---|
607 | std::vector, then std::vector<::X> fails with a parser error.
|
---|
608 |
|
---|
609 | The reason is that the standard mandates that the sequence <:
|
---|
610 | is treated as if it were the token [. (There are several such
|
---|
611 | combinations of characters - they are called digraphs.)
|
---|
612 | Depending on the version, the compiler then reports a parse
|
---|
613 | error before the character : (the colon before X) or a missing
|
---|
614 | closing bracket ].
|
---|
615 |
|
---|
616 | The simplest way to avoid this is to write std::vector< ::X>,
|
---|
617 | i.e. place a space between the opening angle bracket and the
|
---|
618 | scope operator.
|
---|
619 |
|
---|
620 | Copy constructor access check while initializing a reference.
|
---|
621 | Consider this code:
|
---|
622 |
|
---|
623 | class A
|
---|
624 | {
|
---|
625 | public:
|
---|
626 | A();
|
---|
627 |
|
---|
628 | private:
|
---|
629 | A(const A&); // private copy ctor
|
---|
630 | };
|
---|
631 |
|
---|
632 | A makeA(void);
|
---|
633 | void foo(const A&);
|
---|
634 |
|
---|
635 | void bar(void)
|
---|
636 | {
|
---|
637 | foo(A()); // error, copy ctor is not accessible
|
---|
638 | foo(makeA()); // error, copy ctor is not accessible
|
---|
639 |
|
---|
640 | A a1;
|
---|
641 | foo(a1); // OK, a1 is a lvalue
|
---|
642 | }
|
---|
643 |
|
---|
644 | Starting with GCC 3.4.0, binding an rvalue to a const reference
|
---|
645 | requires an accessible copy constructor. This might be
|
---|
646 | surprising at first sight, especially since most popular
|
---|
647 | compilers do not correctly implement this rule.
|
---|
648 |
|
---|
649 | The C++ Standard says that a temporary object should be created
|
---|
650 | in this context and its contents filled with a copy of the
|
---|
651 | object we are trying to bind to the reference; it also says
|
---|
652 | that the temporary copy can be elided, but the semantic
|
---|
653 | constraints (eg. accessibility) of the copy constructor still
|
---|
654 | have to be checked.
|
---|
655 |
|
---|
656 | For further information, you can consult the following
|
---|
657 | paragraphs of the C++ standard: [dcl.init.ref]/5, bullet 2,
|
---|
658 | sub-bullet 1, and [class.temporary]/2.
|
---|
659 |
|
---|
660 | Common problems when upgrading the compiler
|
---|
661 |
|
---|
662 | ABI changes
|
---|
663 |
|
---|
664 | The C++ application binary interface (ABI) consists of two components:
|
---|
665 | the first defines how the elements of classes are laid out, how
|
---|
666 | functions are called, how function names are mangled, etc; the second
|
---|
667 | part deals with the internals of the objects in libstdc++. Although we
|
---|
668 | strive for a non-changing ABI, so far we have had to modify it with
|
---|
669 | each major release. If you change your compiler to a different major
|
---|
670 | release you must recompile all libraries that contain C++ code. If you
|
---|
671 | fail to do so you risk getting linker errors or malfunctioning
|
---|
672 | programs. Some of our Java support libraries also contain C++ code, so
|
---|
673 | you might want to recompile all libraries to be safe. It should not be
|
---|
674 | necessary to recompile if you have changed to a bug-fix release of the
|
---|
675 | same version of the compiler; bug-fix releases are careful to avoid
|
---|
676 | ABI changes. See also the [35]compatibility section of the GCC manual.
|
---|
677 |
|
---|
678 | Remark: A major release is designated by a change to the first or
|
---|
679 | second component of the two- or three-part version number. A minor
|
---|
680 | (bug-fix) release is designated by a change to the third component
|
---|
681 | only. Thus GCC 3.2 and 3.3 are major releases, while 3.3.1 and 3.3.2
|
---|
682 | are bug-fix releases for GCC 3.3. With the 3.4 series we are
|
---|
683 | introducing a new naming scheme; the first release of this series is
|
---|
684 | 3.4.0 instead of just 3.4.
|
---|
685 |
|
---|
686 | Standard conformance
|
---|
687 |
|
---|
688 | With each release, we try to make G++ conform closer to the ISO C++
|
---|
689 | standard (available at [36]http://www.ncits.org/cplusplus.htm). We
|
---|
690 | have also implemented some of the core and library defect reports
|
---|
691 | (available at
|
---|
692 | [37]http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html &
|
---|
693 | [38]http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html
|
---|
694 | respectively).
|
---|
695 |
|
---|
696 | Non-conforming legacy code that worked with older versions of GCC may
|
---|
697 | be rejected by more recent compilers. There is no command-line switch
|
---|
698 | to ensure compatibility in general, because trying to parse
|
---|
699 | standard-conforming and old-style code at the same time would render
|
---|
700 | the C++ frontend unmaintainable. However, some non-conforming
|
---|
701 | constructs are allowed when the command-line option -fpermissive is
|
---|
702 | used.
|
---|
703 |
|
---|
704 | Two milestones in standard conformance are GCC 3.0 (including a major
|
---|
705 | overhaul of the standard library) and the 3.4.0 version (with its new
|
---|
706 | C++ parser).
|
---|
707 |
|
---|
708 | New in GCC 3.0
|
---|
709 |
|
---|
710 | * The standard library is much more conformant, and uses the std::
|
---|
711 | namespace (which is now a real namespace, not an alias for ::).
|
---|
712 | * The standard header files for the c library don't end with .h, but
|
---|
713 | begin with c (i.e. <cstdlib> rather than <stdlib.h>). The .h names
|
---|
714 | are still available, but are deprecated.
|
---|
715 | * <strstream> is deprecated, use <sstream> instead.
|
---|
716 | * streambuf::seekoff & streambuf::seekpos are private, instead use
|
---|
717 | streambuf::pubseekoff & streambuf::pubseekpos respectively.
|
---|
718 | * If std::operator << (std::ostream &, long long) doesn't exist, you
|
---|
719 | need to recompile libstdc++ with --enable-long-long.
|
---|
720 |
|
---|
721 | If you get lots of errors about things like cout not being found,
|
---|
722 | you've most likely forgotten to tell the compiler to look in the std::
|
---|
723 | namespace. There are several ways to do this:
|
---|
724 | * Say std::cout at the call. This is the most explicit way of saying
|
---|
725 | what you mean.
|
---|
726 | * Say using std::cout; somewhere before the call. You will need to
|
---|
727 | do this for each function or type you wish to use from the
|
---|
728 | standard library.
|
---|
729 | * Say using namespace std; somewhere before the call. This is the
|
---|
730 | quick-but-dirty fix. This brings the whole of the std:: namespace
|
---|
731 | into scope. Never do this in a header file, as every user of your
|
---|
732 | header file will be affected by this decision.
|
---|
733 |
|
---|
734 | New in GCC 3.4.0
|
---|
735 |
|
---|
736 | The new parser brings a lot of improvements, especially concerning
|
---|
737 | name-lookup.
|
---|
738 | * The "implicit typename" extension got removed (it was already
|
---|
739 | deprecated since GCC 3.1), so that the following code is now
|
---|
740 | rejected, see [14.6]:
|
---|
741 |
|
---|
742 | template <typename> struct A
|
---|
743 | {
|
---|
744 | typedef int X;
|
---|
745 | };
|
---|
746 |
|
---|
747 | template <typename T> struct B
|
---|
748 | {
|
---|
749 | A<T>::X x; // error
|
---|
750 | typename A<T>::X y; // OK
|
---|
751 | };
|
---|
752 |
|
---|
753 | B<void> b;
|
---|
754 |
|
---|
755 | * For similar reasons, the following code now requires the template
|
---|
756 | keyword, see [14.2]:
|
---|
757 |
|
---|
758 | template <typename> struct A
|
---|
759 | {
|
---|
760 | template <int> struct X {};
|
---|
761 | };
|
---|
762 |
|
---|
763 | template <typename T> struct B
|
---|
764 | {
|
---|
765 | typename A<T>::X<0> x; // error
|
---|
766 | typename A<T>::template X<0> y; // OK
|
---|
767 | };
|
---|
768 |
|
---|
769 | B<void> b;
|
---|
770 |
|
---|
771 | * We now have two-stage name-lookup, so that the following code is
|
---|
772 | rejected, see [14.6]/9:
|
---|
773 |
|
---|
774 | template <typename T> int foo()
|
---|
775 | {
|
---|
776 | return i; // error
|
---|
777 | }
|
---|
778 |
|
---|
779 | * This also affects members of base classes, see [14.6.2]:
|
---|
780 |
|
---|
781 | template <typename> struct A
|
---|
782 | {
|
---|
783 | int i, j;
|
---|
784 | };
|
---|
785 |
|
---|
786 | template <typename T> struct B : A<T>
|
---|
787 | {
|
---|
788 | int foo1() { return i; } // error
|
---|
789 | int foo2() { return this->i; } // OK
|
---|
790 | int foo3() { return B<T>::i; } // OK
|
---|
791 | int foo4() { return A<T>::i; } // OK
|
---|
792 |
|
---|
793 | using A<T>::j;
|
---|
794 | int foo5() { return j; } // OK
|
---|
795 | };
|
---|
796 |
|
---|
797 | In addition to the problems listed above, the manual contains a
|
---|
798 | section on [39]Common Misunderstandings with GNU C++.
|
---|
799 |
|
---|
800 | References
|
---|
801 |
|
---|
802 | 1. http://gcc.gnu.org/bugs.html
|
---|
803 | 2. http://gcc.gnu.org/bugs.html#report
|
---|
804 | 3. http://gcc.gnu.org/bugs.html#need
|
---|
805 | 4. http://gcc.gnu.org/bugs.html#dontwant
|
---|
806 | 5. http://gcc.gnu.org/bugs.html#where
|
---|
807 | 6. http://gcc.gnu.org/bugs.html#detailed
|
---|
808 | 7. http://gcc.gnu.org/bugs.html#gnat
|
---|
809 | 8. http://gcc.gnu.org/bugs.html#pch
|
---|
810 | 9. http://gcc.gnu.org/bugs.html#known
|
---|
811 | 10. http://gcc.gnu.org/bugs.html#cxx
|
---|
812 | 11. http://gcc.gnu.org/bugs.html#missing
|
---|
813 | 12. http://gcc.gnu.org/bugs.html#fixed34
|
---|
814 | 13. http://gcc.gnu.org/bugs.html#fortran
|
---|
815 | 14. http://gcc.gnu.org/bugs.html#nonbugs
|
---|
816 | 15. http://gcc.gnu.org/bugs.html#nonbugs_general
|
---|
817 | 16. http://gcc.gnu.org/bugs.html#nonbugs_c
|
---|
818 | 17. http://gcc.gnu.org/bugs.html#nonbugs_cxx
|
---|
819 | 18. http://gcc.gnu.org/bugs.html#upgrading
|
---|
820 | 19. http://gcc.gnu.org/bugs.html#known
|
---|
821 | 20. http://gcc.gnu.org/bugzilla/
|
---|
822 | 21. mailto:gcc-bugs@gcc.gnu.org
|
---|
823 | 22. http://gcc.gnu.org/bugs.html#gnat
|
---|
824 | 23. http://gcc.gnu.org/bugs.html#pch
|
---|
825 | 24. http://gcc.gnu.org/bugs.html#detailed
|
---|
826 | 25. http://gcc.gnu.org/bugs.html#where
|
---|
827 | 26. http://gcc.gnu.org/bugs.html#detailed
|
---|
828 | 27. http://gcc.gnu.org/bugs.html#detailed
|
---|
829 | 28. http://gcc.gnu.org/bugs.html#new34
|
---|
830 | 29. http://gcc.gnu.org/onlinedocs/g77/Trouble.html
|
---|
831 | 30. http://gcc.gnu.org/PR323
|
---|
832 | 31. http://www.validlab.com/goldberg/paper.ps
|
---|
833 | 32. http://gcc.gnu.org/PR11751
|
---|
834 | 33. http://mail-index.NetBSD.org/tech-kern/2003/08/11/0001.html
|
---|
835 | 34. http://www.gnu.org/software/libc/
|
---|
836 | 35. http://gcc.gnu.org/onlinedocs/gcc/Compatibility.html
|
---|
837 | 36. http://www.ncits.org/cplusplus.htm
|
---|
838 | 37. http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html
|
---|
839 | 38. http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html
|
---|
840 | 39. http://gcc.gnu.org/onlinedocs/gcc/C---Misunderstandings.html
|
---|