Parameter (computer programming)
A parameter is an intrinsic property of the procedure, included in its definition. For example, in many languages, a procedure to add two supplied integers together and calculate the sum would need two parameters, one for each integer. In general, a procedure may be defined with any number of parameters, or no parameters at all. If a procedure has parameters, the part of its definition that specifies the parameters is called its parameter list.// Naming the first parameter 'pThreadArgument' is correct, rather than
To better understand the difference, consider the following function written in C:
Because of the difference between parameters and arguments, it is possible to supply inappropriate arguments to a procedure. The call may supply too many or too few arguments; one or more of the arguments may be a wrong type; or arguments may be supplied in the wrong order. Any of these situations causes a mismatch between the parameter and argument lists, and the procedure will often return an unintended answer or generate a runtime error.
Default arguments can be seen as a special case of the variable-length argument list.
Some languages allow subroutines to be defined to accept a variable number of arguments. For such languages, the subroutines must iterate through the list of arguments.
In some cases only input and input/output are distinguished, with output being considered a specific use of input/output, and in other cases only input and output (but not input/output) are supported. The default mode varies between languages: in Fortran 90 input/output is default, while in C# and SQL extensions input is default, and in TScript each parameter is explicitly specified as input or output.
Syntactically, parameter mode is generally indicated with a keyword in the function declaration, such as
void f(out int x) in C#. Conventionally output parameters are often put at the end of the parameter list to clearly distinguish them, though this is not always followed. TScript uses a different approach, where in the function declaration input parameters are listed, then output parameters, separated by a colon (:) and there is no return type to the function itself, as in this function, which computes the size of a text fragment:
This is semantically different in that when called, the function is simply evaluated – it is not passed a variable from the calling scope to store the output in.
For example, to return two variables from a function in C, one may write:
Similar considerations apply to returning a value of one of several possible types, where the return value can specify the type and then value is stored in one of several output variables.
Output and input/output parameters prevent function composition, since the output is stored in variables, rather than in the value of an expression. Thus one must initially declare a variable, and then each step of a chain of functions must be a separate statement. For example, in C++ the following function composition:
In the special case of a function with a single output or input/output parameter and no return value, function composition is possible if the output or input/output parameter (or in C/C++, its address) is also returned by the function, in which case the above becomes:
The micro-optimization of not requiring a local variable and copying the return when using output variables can also be applied to conventional functions and return values by sufficiently sophisticated compilers.