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Concept
Function prototype
Summary
In computer programming, a function prototype or function interface 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 s allows for splitting a program into translation units, i.e. into parts that a compiler can separately translate into s, 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.
Function prototypes can be used when either:
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 parameter) except alone.
In object-oriented programming, interfaces and abstract methods serve much the same purpose.
Consider the following function prototype:
void Sum( int a, int b );
OR
void Sum( int, int );
OR
auto Sum( int, int ) -> void; // 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.
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 int and nothing was assumed about its arguments.
Official source
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