Question

I have the following 3 files:

error.h

#ifndef error_h
#define error_h
#include <string>
#include <iostream>
#include <cstdio>
void Error(std::string msg);
#endif

error.cpp

#ifdef error_h
#include "error.h"
void Error(std::string msg)
{
    std::cerr
     << "\n=========================================================\n"
     << msg
     << "\n=========================================================\n";
    exit(EXIT_FAILURE);
}
#endif

foobar.cpp

#include "error.h"
int main()
{
    for(int i=0; i<99; i++)
        if(i==55)
            Error("this works");
    return 0;
}

Now I do:

$ g++ -c error.cpp foobar.cpp
$ g++ error.o foobar.o -o exampleprogram

And I get:

foobar.o: In function `main':
foobar.cpp:(.text+0x4b): undefined reference to `Error(std::basic_string<char,
std::char_traits<char>, std::allocator<char> >)'
collect2: ld returned 1 exit status

What am I doing wrong? What do I need to understand to resolve this, and similar issues in the future without asking questions? Thanks!

Answer 1

Why do you have these lines in error.cpp?

#ifdef error_h
  ...
#endif

Since the preprocessor symbol error_h is not defined the entire contents of error.cpp are being omitted by the preprocessor. Remove those lines and your program will link successfully.

You seem to have a misunderstanding of how (and maybe why) #include guards are to be used. Refer to this answer for an explanation.

Also, there’s no need to include iostream and cstdio in error.h, since that file is not using anything declared in either of those headers. Those files should be included in error.cpp.