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parameters

summary

    This subchapter looks at parameters.

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parameters

    This subchapter looks at parameters.

    Here are five basic methods for transmitting parameters:

    (1) Value (Ada, ALGOL 60, Pascal) The formal parameter identifies a local variable in the procedure or function. The local variable is initialized to the argument value at the time of the procedure or function call. Assignments to the local variable do not affect the caller. Callby value may be a strict value transmission, in which case the formal parameter identifies a local constant rather than a lcoal variable.

    (2) Result (Ada) The formal parameter identifies a local variable in the procedure or function. The local variable has an undefined initial value. Upon return the contents of the local variable are assigned to the argument in the caller (if the argument is a variable).

    (3) Value/ResultAda, FORTRAN)> The formal parameter identifies a local variable in the procedure or function. The local variable is initialized to the argument value at the time of the procedure or function call. Upon return the contents of the local variable are assigned to argument in the caller (if the argument is a variable). The argument may be fixed at or before the time of the call or it may be redetermined upon return.

    (4) Reference (Ada, FORTRAN, Pascal) A reference (usually a pointer) to the argument is transmitted to the procedure or function. Operations on the formal parameter that occur within the procedure or function are performed via the reference. If the argument is an expression, the compiler constructs a temporary variable.

    (5) Name (ALGOL 60) A parameterless procedure p is transmitted to the procedure or function. The procedure p creates a reference to the argument. If the argument is an expression, then p computes the value of the expression, stores that value in a temporary variable, and provides a reference to the temporary variable. All operations on the formal parameter invoke p and use the reference generated by p.

    Very few programming languages explicitly specify parameter transmission mechanisms. The mechanism chosen can vary from compiler to compiler for the same language. Depending on a particular compiler’s choices for transmission methods can make a program difficult to understand and can cause lack of portability, not just to different machines, but even to later versions of the same compiler.

    Of course, if the language specifies the transmission methods and gives specific constructs to indicate the method chosen, then the programmer is free to make use of the choices available.

Ada

other

   “11. If you have a procedure with 10 parameters, you probably missed some.” —Alan Perlis, Epigrams on Programming, ACM’s SIGPLAN Notices Volume 17, No. 9, September 1982, pages 7-13

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