Function prototype

{{Short description|Declaration of a function's name and type signature but not body}} {{About|function declarations|the prototype property of JavaScript functions and objects|JavaScript#Object-orientation (prototype-based)|other uses|Software prototyping}} {{more citations needed|date=September 2016}}

In computer programming, a '''function prototype''' is a declaration of a function that specifies the function's name and type signature (arity, data types of parameters, and return type), but omits the function body. While a function definition specifies ''how'' the function does what it does (the "implementation"), a function prototype merely specifies its interface, i.e. ''what'' data types go in and come out of it. The term "function prototype" is particularly used in the context of the programming languages C and C++ where placing forward declarations of functions in header files allows for splitting a program into translation units, i.e. into parts that a compiler can separately translate into object files, to be combined by a linker into an executable or a library. The function declaration precedes the function definition, giving details of name, return type, and storage class along with other relevant attributes.<ref>{{Cite web |last=TylerMSFT |date=2023-01-25 |title=Function Prototypes |url=https://learn.microsoft.com/en-us/cpp/c-language/function-prototypes?view=msvc-170 |access-date=2023-08-09 |website=learn.microsoft.com |language=en-us}}</ref>

Function prototypes can be used when either:<ref>{{Cite web |date=2018-10-25 |title=Function prototypes |url=https://www.ibm.com/docs/es/rbd/9.1.1.2?topic=functions-function-prototypes |access-date=2023-08-09 |website=www.ibm.com |language=en-us}}</ref>

* Defining an ExternalType * Creating an Interface part

In a prototype, parameter names are optional (and in C/C++ have function prototype scope, meaning their scope ends at the end of the prototype), however, the type is necessary along with all modifiers (e.g. if it is a pointer or a reference to {{mono|const}} parameter) except {{mono|const}} alone.

In object-oriented programming, interfaces and abstract methods serve much the same purpose.

== Example == Consider the following function prototype:

or, without named parameters:

or, with trailing return types (C++ only):

Function prototypes include the function signature, the name of the function, return type and access specifier. In this case the name of the function is "sum". The function signature defines the number of parameters and their types. The return type is "void". This means that the function is not going to return any value. Note that the parameter names in the first example are optional.

== Uses == In early versions of C, if a function was not previously declared and its name occurred in an expression followed by a left parenthesis, it was implicitly declared as a function that returns an <code>int</code> and nothing was assumed about its arguments. In this case the compiler would not be able to perform compile-time validity checking of the number and type(s) of arguments. The C99 standard requires the use of prototypes.

<syntaxhighlight lang="c"> #include <limits.h> #include <stdio.h>

// Function prototype char myFunction(int a);

int main(void) { putchar(myFunction(-1)); // Correctly formatted call putchar(myFunction(1.5)); // Compiler warning: type mismatch putchar(myFunction("IncorrectArgType")); // Compiler warning: type mismatch putchar(myFunction()); // Compiler error: too few arguments

// Although adding 1 to INT_MAX is an integer overflow error, // it cannot be detected at compile time putchar(myFunction(INT_MAX + 1));

return 0; }

// Function definition char myFunction(int n) { if (n > 0) { return '>'; } else if (n < 0) { return '<'; } else { return '='; } } </syntaxhighlight>

The function {{mono|MyFunction}} expects to be called with an integer argument. By including the function prototype, you inform the compiler that the function takes one integer argument and you enable the compiler to catch incorrectly specified calls.

=== Creating library interfaces === By placing function prototypes in a header file, one can specify an interface for a library.

=== Class declaration === In C++, function prototypes are also used in class definitions.

== References == {{Reflist}} {{refbegin|}} * {{cite book|title=The C Programming Language|last1=Kernighan|first1=Brian W.|author-link1=Brian Kernighan|last2=Ritchie|first2=Dennis M.|author-link2=Dennis Ritchie|year=1988|publisher=Prentice Hall PTR|place=Upper Saddle River, NJ|edition=2nd|isbn=0-13-110362-8|url-access=registration|url=https://archive.org/details/cprogramminglang00bria}} {{refend}}

Category:Subroutines Category:C programming language family Category:Computer programming