In C++, a function prototype is a declaration of a function that provides information about the function's parameters and return type. Function prototypes are typically included in a header file or at the beginning of a source file.

Here's an example of a function prototype:

int add(int x, int y);

This prototype declares a function named add that takes two int parameters and returns an int.

Function prototypes are useful when you need to use a function in a source file before it's been defined. By including a prototype, you can tell the compiler about the function's signature, which allows it to check that the function is being called correctly.

Here's an example of how to use a function prototype:

// Function prototype
int add(int x, int y);

int main() {
    int result = add(3, 4);
    return 0;
}

// Function definition
int add(int x, int y) {
    return x + y;
}

In this example, the function add is declared using a prototype before the main function. This allows the add function to be used in the main function, even though it hasn't been defined yet.

Note that you don't need to include a function prototype for every function you define. If a function is only used in the same source file where it's defined, you can define the function before any other code that uses it, and the compiler will be able to infer the function's signature based on its definition. However, if a function is used in multiple source files, it's a good practice to provide a function prototype in a header file.