Failed to work with pointers

Your program has very serious problems, it is rare that it even works in a harmless way.

In the function trocear you create the variable salida of pointer type to character char * and you return that pointer, which, since it belongs to the function trocear when you leave this function, it ceases to exist and misses. Using the values of a variable that has ceased to exist is a undefined behavior .

The variable salida is a pointer that does not point to any existing data. The pointers exist to point to data (read this thread ) and only when they point to something do they make sense. In your case, you create the pointer but do not assign it value:

char *salida;

This implies that it points to an unknown place in the memory (uninitialized variables receive an indeterminate value when created), from that unknown place you do arithmetic pointers to point to different points starting from an unknown origin:

 *(salida+i)=num%10+'0';

This line is translated as: " From the point where you are, it advances i positions and at that point it writes the indicated value ", the problem is that without knowing the point from the that you start, you will hardly know the point at which you are going ... this is another indefinite behavior.

Proposal.

If you want to work with pointers (I would not do it, but if it is a requirement we will need to stick to it), you will need to use dynamic memory instead of automatic memory (automatic memory is released when leaving the area in which it is created, the dynamic memory is released by hand) so your trocear function should look like this:

char * trocear(int num) {
    // Un entero de 32 bits puede contener números de hasta 10 dígitos
    char *salida = new char[11];

    // hacer cosas...

    return salida;
}

With this proposal, the memory you created in trocear should be released after use:

char *n = trocear(123456);
cout << n;
delete[] n; // <---- MUY IMPORTANTE

But doing this is a major headache, I advise you to use smart pointers:

std::unique_ptr<char[]> trocear(int num) {
    // Un entero de 32 bits puede contener números de hasta 10 dígitos
    auto salida = std::make_unique<char[]>(11);

    for (int digito = 0; num; ++digito)
    {
        salida[digito] = '0' +  num % 10;
        num /= 10;
    }

    return salida;
}

The smart pointer will take care of releasing the memory at the end of its use, freeing you from the responsibility of doing it by hand:

auto n = trocear(123456);
cout << n.get();

You can see the code working in Wandbox .

I am not entirely in agreement with the answers you have been given, that is why I give you mine. There are many ways to make your code with pointers keeping the same purpose, I think that this topic is fundamental to understand the programming in C ++ and understand the programming itself, although there are languages that abstract the programmer from the pointers, in reality that is what that is always done in the machine. that's why I decided to make an example as similar to your guideline although there are many ways to do it.

char* trocear(int num){ 
    char *salida = new char[20];
    char c;
    int mod;
    int i=0;
    while (num>0){

        mod = num%10;           
        salida[i] = mod+48; 
        num = (num-mod)/10;     
        i++;
    }
    salida[i] = '
int main(){

    int num = 123456;

    char *cadenaTroceada = trocear(num);    

    //cout<<cadenaTroceada[2]<<endl;    

    cadenaTroceada += 2;
    cout<<*cadenaTroceada<<endl;

    return 0;
}
';

    return salida;

}

int main(){

    int num = 123456;

    char *cadenaTroceada = trocear(num);

    cout<<cadenaTroceada<<endl;

    return 0;
}

Then if in the main you want a specific character remember that you can treat your pointer as a vector. I leave an annotated form and a form without comment.

char* trocear(int num){ 
    char *salida = new char[20];
    char c;
    int mod;
    int i=0;
    while (num>0){

        mod = num%10;           
        salida[i] = mod+48; 
        num = (num-mod)/10;     
        i++;
    }
    salida[i] = '
int main(){

    int num = 123456;

    char *cadenaTroceada = trocear(num);    

    //cout<<cadenaTroceada[2]<<endl;    

    cadenaTroceada += 2;
    cout<<*cadenaTroceada<<endl;

    return 0;
}
';

    return salida;

}

int main(){

    int num = 123456;

    char *cadenaTroceada = trocear(num);

    cout<<cadenaTroceada<<endl;

    return 0;
}

I leave a link of some tutorials where they explain the subject link