Error Lnk2005, Already Defined

error LNK2005, already defined?

Why this error?

You broke the one definition rule and hence the linking error.

Suggested Solutions:


If you need the same named variable in the two cpp files then You need to use Nameless namespace(Anonymous Namespace) to avoid the error.

namespace 
{
int k;
}

If you need to share the same variable across multiple files then you need to use extern.

A.h

extern int k;

A.cpp

#include "A.h"
int k = 0;

B.cpp

#include "A.h"

//Use `k` anywhere in the file

What is causing the error LNK2005: already defined in .obj s errors in my code?

In your project you are compiling ObjectHandler.cpp as an independent translation unit. And at the same time you also including ObjectHandler.cpp into Main.cpp where it gets compiled again as part of Main.cpp.

Thus, everything defined in ObjectHandler.cpp gets defined twice in your program. Hence the errors.

The question is: why are you including ObjectHandler.cpp into Main.cpp? What did you try to achieve by that?

And if you, for some unorthodox reason, really want/have to include ObjectHandler.cpp into Main.cpp and compile it as part of Main.cpp, then stop compiling ObjectHandler.cpp as an independent translation unit at the same time.

Meanwhile, a more traditional approach in this case would be to write a proper ObjectHandler.h file for your ObjectHandler.cpp. Something like

#pragma once
#include "ComplexTypes.h"
#include <string>
#include <map>
#include <vector>

extern std::vector<pixel> pixels;
extern std::map<std::string, sprite> sprites;

void pixelsInit();
void spritesInit();
void pixelsUpdate();
void spritesUpdate();

and include it into Main.cpp instead.

Your ObjectHandler.cpp might look as follows

#include "ObjectHandler.h"

std::vector<pixel> pixels;
std::map<std::string, sprite> sprites;

void pixelsInit(){}
void spritesInit(){}
void pixelsUpdate(){}
void spritesUpdate(){}

Note the important detail: the .h file contains declarations only (functions have no bodies and objects are declared with extern), while .cpp file contains definitions (functions have bodies and objects are declared without extern)

Error lnk2005 already defined in .obj

Your project has two definitions of function WorldObjects2 : one is in the compilation unit test.cpp and other is in the compilation unit main.cpp because the header where the function is defined is included in these two cpp files.

Either use function specifier inline

inline void WorldObjects2(unsigned int mask)
{
.......
}

Or move the function definition to some cpp file leaving only a function declaration (without its definition) in the header.

Another approach is to make the function as having the internal linkage. For example you could add keyword static

static void WorldObjects2(unsigned int mask)
{
.......
}

or could place the function in an unnamed namespace within the given namespace.

namespace World
{
// ...
namespace
{
void WorldObjects2(unsigned int mask)
{
.......
}
}
}

C++ LNK 2005 Error already defined in .obj

When including game.cpp, you actually implement the contructor Game::Game() twice, i.e. one time in game.cpp (which is a separate translation unit) and one time in main.cpp (where you include the constructor implementation code).
Hence, you get a linker error, not a compiler error.

To solve the problem, remove the #include "game.cpp".

Error LNK2005: function already defined in MyForm.obj

If you declare and implement a function in a header file (Header.h) and if this file gets included twice, then you'll most likely will get a function already defined error at some point.

This can be fixed by either:

  • Moving function implementation to a source (cpp) file and only keep it's declaration in the header (h) file

  • Make the function inline (if it is acceptable), that will remove the error

C++ LNK2005 already defined in B_calculating.obj

There is a clue in the boost/json/src.hpp header:

This file is meant to be included once, in a translation unit of the program.

You should only include boost/json/src.hpp in one cpp file. You should remove it from B_calculating.h and only include it in main.cpp.

LNK2005 already defined error on inclusion of C-type header file in C++ project [MSVC12]

This is a common problem with the header files, that contain implementaton. When you use #include directive, compiler simply inserts .h file content instead of it. So when you use this header in different places of your project, you get several identical implementations of its methods and global variables. Since it has #ifdef or #pragma once compiler guard, it compiles just fine. But when linker is trying to unite all compiled obj files to one executable module, it gets several identical implementations. Since it could not know which one should be used, you get LNK2005 error. To solve this problem you could move implementations and globals into the separate cpp file and include it in the project. Other way would be to mark all header functions as inline, or use __declspec(selectany)



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