What is the difference between using prototypes and not using them in c++?

Question:

I have a question about functions in c++, I have learned how to make functions using prototypes as in the following code:

#include<iostream>

using namespace std;

int suma(int, int);

int main(){

    int a = 5, b = 4;

    cout<<"\nLa suma de los numeros "<<a<<" y "<<b<<" es: "<<suma(a,b)<<endl;

    return 0;
}

int suma(int x, int y){
    int resultado;
    resultado = x + y;
    return resultado;
}

But I don't know what is the difference with the following where I don't use the prototype of the "sum" function:

#include<iostream>

using namespace std;

int suma(int x, int y){
    int resultado;
    resultado = x + y;
    return resultado;
}

int main(){

    int a = 5, b = 4;

    cout<<"\nLa suma de los numeros "<<a<<" y "<<b<<" es: "<<suma(a,b)<<endl;

    return 0;
}

I have seen that several people use the functions in c++ without defining the prototype, I am not very clear if there is any difference between these two forms, since both work for me. I have also read that prototypes are required in c++, but the prototype is not used in the second code. So what is the difference between using or not using prototypes in c++ and if they work the same in c?

Answer:

The only difference is in non-member-functions, that is, in functions that do not belong to a class.

In these non-member-functions, the compiler has to know the prototype before the first call :

int suma( int x, int y ){ return x + y; }

int main( ) {
    std::cout << "Suma: " << suma( 1, 2 ) << std::endl;

    return 0;
}

Ok, it works correctly. The compiler knows what suma( ) is and what arguments it expects before you use it in main( ) . But nevertheless:

int main( ) {
    std::cout << "Suma: " << suma( 1, 2 ) << std::endl;

    return 0;
}

int suma( int x, int y ){ return x + y; }

error: 'sum' was not declared in this scope

In this case, the compiler doesn't know what suma and protest is. Here, it is mandatory to use the prototype:

int suma( int x, int y );

int main( ) {
    std::cout << "Suma: " << suma( 1, 2 ) << std::endl;

    return 0;
}

int suma( int x, int y ){ return x + y; }