Why am I getting a 'multiple definition' error? how do i fix it?

Question

command given by terminal:

g++ main.cpp test.cpp

Error message:

/tmp/ccvgRjlI.o: In function `test2()':
test.cpp:(.text+0x0): multiple definition of `test2()'
/tmp/ccGvwiUE.o:main.cpp:(.text+0x0): first defined here
collect2: error: ld returned 1 exit status main.cpp

Source code:

#include "test.hpp"
int main(int argc, char *argv[])
{
    test2();
    return 0;
}

test.hpp

#ifndef _TEST_HPP_
#define _TEST_HPP_

#include <iostream>

void test();
void test2() { std::cerr << "test2" << std::endl; }

#endif

test.cpp

#include "test.hpp"

using std::cerr;
using std::endl;

void test() { cerr << "test" << endl; }

btw the following compiles fine:

g++ main.cpp

Add the inline keyword before test2 to get around the one definition rule. — 0x5453
Nothing to do with your problem, but names like _TEST_HPP_ are reserved in C++. Lose the leading underscore. — user2100815
Possible duplicate of Multiple definition of a function error, even when using #if guard clauses — Julien Lopez

Vlad from Moscow Vlad from Moscow · Accepted Answer · 2017-01-11T18:22:23

The header test.hpp is included in two compilation units. The first one is the compilation unit main.cpp and the second one is the compilation unit test.cpp.

Functions by default have external linkage. This means that functions with the same name and signature denote the same function in different compilation units. They shall be defined once. However in your program the definition of the function test2 is found in two compilation units and the linker does not know what definition of the function to use.

You could declare the function as an inline function. For example

inline void test2() { std::cerr << "test2" << std::endl; }

In this case it may be defined in each compilation unit.

Or you can place in the header only the function declaration as you made with the function test and define it for example in test.cpp.

Another way is to declare the function as having internal linkage. To do this you can define the function in the header either with the keyword static

static void test2() { std::cerr << "test2" << std::endl; }

or place it in unnamed name space

namespace
{
    void test2() { std::cerr << "test2" << std::endl; }
}

In this case each compilation unit will have its own function test2.

Why am I getting a 'multiple definition' error? how do i fix it?

4 Answers