Why "int i" has multiple definitions?

Question

I have two files as below:

Test1.h

#ifndef TEST_H
#define TEST_H

int i = 10;

#endif

Test2.cpp

#include <iostream>

#include "Test1.h"

int main()
{
    std::cout << i << std::endl;
}

I know I can solve this by using extern or const in Test1.h.

But my question is that "I don't understand the error".

error LNK2005: "int i" (?i@@3HA) already defined in Test1.obj
error LNK1169: one or more multiply defined symbols found

How can int i have multiple definitions?

The header file has include guards.
When I include the header file it should mean that everything gets copied in Test2.cppand it should become:

Test2.cpp

#include <iostream>

int i = 10

int main()
{
    std::cout << i << std::endl;
}

And header file should become irrelevant at this point after everything being included.

My other question is if I declare int i with extern in header file and include it in .cpp, then would it be an example of external linkage? Because generally I have seen external linkage between two .c or .cpp as in here but if you explicitly include the file is it still regarded as i having external linkage?

You're absolutely on the right track looking at the effect of the #include directive; just don't forget that it's done both source files. Test1.cpp also has #include "Test1.h", so it has the same definition of i as Test2.cpp has. That's why the linker is complaining. — Pete Becker
The error message indicates that there is also a Test1.obj involved, which you failed to mention in your question. Please show the commandline you used to compile, as well as all source files involved — M.M
I think you included the header in the compile command. You probably did something like g++ Test1.h Test2.cpp -o main. You don't need to specify the header in the compile command. — hgiesel
Re: "I really don't have it." -- then there's something messed up in your project. Start a new one. — Pete Becker
It was likely a bug in compiler. I renamed the Test1.h to Test2.h and the error is completely gone. It happened because I renamed Test1.cpp to Test1.h. — user103214

CashCow CashCow · Accepted Answer · 2016-07-15T11:08:04

Each compilation unit (a .cpp file) produces its own set of symbols individually which are then linked together by the linker.

A header file "becomes" part of the compilation unit it is included in, which compile to an object file (.obj in Windows, .o in Unix systems)

Therefore it is like you have defined a global 'i' in each compilation unit. The correct solution (as you know, if you have to have a global) is to declare it as "extern" in the header then have one compilation unit actually define it.

Include guards only prevent the same header being included twice in the same compilation unit, which can happen if I include and and one of those includes the other.

Why "int i" has multiple definitions?

5 Answers