Defining and Declaring a C++ Function

One of the rules of the C/C++ language is the following, and we know it very well for variables: Any identifier, in order to be used, must first be declared. This rule also applies to functions, so we identify the following notions, seemingly similar. Understanding them well will save us from many errors!!

• Defining a function means writing it in its entirety; it consists of the function header and the function body;
• Declaring a function means announcing its existence. Declaring the function is done in a declarative statement and is also called the function prototype. Declaring the function can be replaced by defining it.

Let's consider the following example, without practical significance:

#include < iostream >
using namespace std;

void F(){
    cout << "Hello";
}

int main()
{
    F();
    return 0;
}

The program is syntactically correct. The part:

void F()
{
   cout << "Hello";
}

represents the definition of the function F(), but here it also gets declared. If we change the order of functions F() and main(), we get:

#include < iostream >
using namespace std;

int main()
{
    F();
    return 0;
}

void F()
{
    cout << "Hello";
}

This time the program is not correct. We notice that the identifier F is not declared. It can be declared by specifying the function prototype before the main() function (practically, before calling it), as below:

#include < iostream >
using namespace std;

void F();

int main()
{
    F();
    return 0;
}

void F()
{
    cout << "Hello";
}

We observe that the prototype (declaration) is a regular C++ statement, which ends with a ; !!