Question

When building I come across these errors:

Error 5 error LNK2005: “int __cdecl numGen(void)” (?numGen@@YAHXZ)
already defined in main.obj
Error 6 error LNK1169: one or more multiply defined symbols found

numGen.cc:

#include <stdlib.h>
#include <stdio.h>
#include <time.h>

int numGen()
{
    int rNum;
    srand(time(NULL)); //--Seeds a random number.
    rNum = 1 + (rand() % 100);

    return rNum;
}

main.cc:

#include <iostream>
#include "NumGen.cc"

int main()
{
    std::cout << numGen();
    return 0;
}

Answer 1

By #includeing NumGen.cc in your main.cc file, you are causing the preprocessor to create two files like the following:

NumGen.cc:

#include <stdlib.h>
#include <stdio.h>
#include <time.h>

int numGen()
{
    int rNum;
    srand(time(NULL)); //--Seeds a random number.
    rNum = 1 + (rand() % 100);

    return rNum;
}

and main.cc:

#include <iostream>
#include <stdlib.h>
#include <stdio.h>
#include <time.h>

int numGen()
{
    int rNum;
    srand(time(NULL)); //--Seeds a random number.
    rNum = 1 + (rand() % 100);

    return rNum;
}

int main()
{
    std::cout << numGen();
    return 0;
}

because the #include preprocessor directive just inserts the contents of the file you are including where you are including it (unless of course it’s been included before and is wrapped in an include guard. I’m also assuming that you are compiling both NumGen.cc and main.cc on the same command line, so naturally you’ll get a multiply defined symbol error given the function numGen is now defined and implemented in both files.

What you have to do is forward declare numGen in a header file, let’s call it NumGen.h:

int numGen();

and then put the line #include "NumGen.h" in both NumGen.cc and main.cc.