C function prototype

 Is that for Class/Object function prototype, I must define the
function in header file or .cpp file.

MyClass::functionA();
MyClass::functionB();

but for C function prototype, I don't have to define if it's put
before the main() function the following is not needed -

void stradd (char *s1, char *s2);
void stradd (char *s1, int i);

=========
#include <iostream> // cannot be iostream.h??
#include <stdio.h>
#include <string.h>

#include <comdef.h>
#include <conio.h>
#include <windows.h> // must need for SYSTEMTIME

//must need C/C++ > General > Debug Information Format to debug
working

using namespace std; // for cout must have??

// concatenate two strings
void stradd (char *s1, char *s2)
{
    strcat (s1, s2); // CRT <string.h> function
}

// concatenate a string with a "stringized" integer
void stradd (char *s1, int i)
{
    char temp[80];

    sprintf (temp, "%d", i);
    strcat (s1, temp);
}

int main()
{
    //SYSTEMTIME st = {0,0,0,0,0,0,0,0}; // cannot divide into 2
lines - must init all in one line
    SYSTEMTIME st = {0}; // OK too

    char str[80];
    //char* str; // not OK will crash program

    strcpy (str, "Hello ");
    stradd (str, "there");
    cout << str << "\n";

    stradd (str, 100);
    cout << str << "\n";

    stradd (str, "hihi");
    cout << str << "\n";

    return 0;
}