Question

I understand that pre-processor commands are an important part of header files to prevent vars and classes from being defined more than once.

I have been running into issues with my vars being defined multiple times – even with pre-processor wrappers. Here is a sample project that is experiencing compiler errors:

Header:

// TestInclude.h
#ifndef TESTINCLUDE_H_
#define TESTINCLUDE_H_

int myInt;

#endif /*TESTINCLUDE_H_*/

C++:

// TestInclude.cpp
#include <iostream>
#include "IncludeMe.h"
#include "TestInclude.h"

int main( int argc, char* args[] )
{
    std::cin >> myInt;

    IncludeMe thisClass;

    std::cin >> myInt;
}

Header:

// IncludeMe.h
#ifndef INCLUDEME_H_
#define INCLUDEME_H_

class IncludeMe
{
private:
    int privateInt;
public:
    IncludeMe();
};

#endif /*INCLUDEME_H_*/

C++:

// IncludeMe.cpp
#include <iostream>
#include "IncludeMe.h"
#include "TestInclude.h"

IncludeMe::IncludeMe()
{
    std::cout << "myInt: " << myInt;
}

Then I compile like this:

Makefile:

all:
g++ -g -o TestInclude TestInclude.cpp IncludeMe.cpp

And I get the following error:

/tmp/ccrcNqqO.o: In function `IncludeMe’:
/home/quakkels/Projects/c++/TestInclude/IncludeMe.cpp:6: multiple definition of `myInt’
/tmp/ccgo6dVT.o:/home/quakkels/Projects/c++/TestInclude/TestInclude.cpp:7: first defined here
collect2: ld returned 1 exit status
make: *** [all] Error 1

Why am I getting this error when I’m using pre-processor conditionals in my header files?

Answer 1

Include guards do not protect against multiple definitions. They only protect against infinite recursive inclusion. (You can of course include the same header in multiple translation units!)

You should never have object definitions* in the header; only declarations:

header.hpp:

extern int a;

file.cpp:

#include "header.hpp"

int a = 12;

_{*) You can have class definitions in a header file, as well as inline functions and class member functions.}