“Multiple definition of” C++ compiler error

问题

I can't seem to get rid of these seemingly random compiles errors in one of my classes. I get about 4 errors such as:

multiple definition of `draw_line(float, float, float, float)'

and

multiple definition of `near_far_clip(float, float, float*, float*, float*, float*, float*, float*)'

that are flagged in the middle of the method.

I also consistently get multiple definition of `stack' in the middle of another method. stack is a global variable in a totally different file. It isn't even mentioned in the file I'm getting the error in.

I tried separating the error prone file into .h and .cpp files (was originally just a .cpp) and nothing about the error changed...

I don't have duplicate methods. I only have one #include of lines.h and there is an #ifndef clause at the beginning. All these errors appear in the .cpp file.

Any ideas what it could be?

Alright, I got the code up:

lines.cpp
ThreeD.cpp
ThreeD.h
makefile

The lines.cpp is a converted .c file I received from my instructor. I included the makefile just in case, because I always have problems with it. I also annotated exactly where the errors were flagged in the file, but they seem pretty random so I don't know if it's particularly important. I abandoned the .h file because it wasn't solving anything or helping. I believe it will be easier to find the mistake without it.

Here is the requested main.cpp file (there is no .h).

I remade the lines.h file due to and I'm still receiving the:

multiple definition of `draw_line(float, float, float, float)'

and

multiple definition of `near_far_clip(float, float, float*, float*, float*, float*, float*, float*)'

errors in the lines.cpp file, but the multiple definition of `stack' error is now in a random place in the ThreeD.cpp file (and is marked by a comment now). Update: This error has been fixed and the files have been revised to show this:

lines.h
lines.cpp
ThreeD.cpp
ThreeD.h

I messed around with labeling some the global variables extern, but it didn't seem to affect anything.

回答1:

Why do you #include lines.cpp in ThreeD.cpp? This is very unusual.

Your makefile wants lines.o, so you're going to compile lines.cpp. Anything defined in lines.cpp will be in lines.o and also in ThreeD.o.

There is an intriguing comment in lines.cpp:

Don't forget to put declarations in your .h files.

I think the instructor wants you to break lines.cpp into a .h and a .cpp.

Excerpt from lines.cpp:

/* These go in your .h file or in lines.h */
/*

Line drawing header.

*/


void draw_line(float, float, float, float);
int near_far_clip(float, float, float *, float *, float *, float *,
                  float *, float *);

I suspect that these two declarations are the only thing that should be in lines.h.

回答2:

You are likely including the function definitions in a header file. Include the inline keyword so they are not exported by each object file, or put them in their own .cpp file.

For your global variable, you need to use the extern keyword in your header file. Otherwise, each object file exports their own variable and the linker gets confused as to which is the correct one to use.

回答3:

Please post some code snippets. Maybe you are defining your methods in both the class declaration and outside?

class X {
    void foo();    // No definition, just declaration
    void bar() {}  // Declaration + definition
};

void X::foo() {}    // First Definition, OK
void X::bar() {}    // Already defined, ERROR

回答4:

As has been said - not enough information to properly diagnose here.

But if draw_line et al are defined in a header file rather than in a source (cpp) file, you may have methods which are supposedly declared as inline in the header file, which aren't actually properly inlining. In that case, each .cpp file that includes the header will generate their own definition of the draw_line function, and generate warnings at link time.

This can happen if you use a #defined INLINE macro which comes from a system header that has been forgotten or removed, and for whatever reason the INLINE gets pre-processed away into nothing.

E.g.:

//Lines.h

#define GCCC //Note the typo

#if defined(GCC)
#define INLINE inline
#elif defined (MSVC)
#define INLINE __inline
#else

#define INLINE //Due to the typo, INLINE will be turned into nothing
#endif

INLINE void draw_line(float x1, float y1, float x2, float y2)
{
   //Draw the line
}

//File1.cpp
#include "lines.h" //will define draw_line

//File2.cpp
#include "lines.h" //will also define draw_line

And linking File1.cpp and File2.cpp will generate multiple declaration errors

回答5:

Check include guards on misspelling.
Check your make options, maybe someone compiled into multiple object files.
Try to exclude parts of files and code until not find cause of errors.

EDITED:
fix include *.cpp files. They are should be linked.

回答6:

#ifndef THREED_H_
#define THREED_H_
#endif /* THREED_H_ */

delete or comment these lines and this worked for me

回答7:

Multiple definition errors are linker errors, but without more information it's difficult to diagnose them. Check the same file does not appear twice in the linker command and that your header files contain only declarations, no definitions.

回答8:

Without seeing the code there's really no helping you. The compiler clearly claims the opposite of you (there is at least one duplicate definition).

Try reproducing the error with a minimum example. Have you tried compiling the code outside Eclipse in the command line? With what result?

回答9:

Hi, What I see here that the basic rule applies for ll. I will explain how the things work in these kind of errors and wht those comes : Let's take one example of "Making a global variable"

Dont confuse the header file, for a cpp file. The header is only there to allow multiple file projects to have a common interface before linking.

1.List item

You should give info about the global variable in header:

extern int Var_Global;

List item

while keeping the Actual code in the
cpp :

int Var_Global;

And make sure you 1. include the header, and 2. link the code.

Link the code once, and once only, and there is only one copy of the data, howver you can include headers all day long, this tells the other code that there will be a copy of the data at link time.

I see a lot of people putting code in header files, while that may work in some cases, it is a bad habit to get into, and will cause problems sooner or later.

Regards,

Prashanta

来源：https://stackoverflow.com/questions/685439/multiple-definition-of-c-compiler-error

标签

c++

linker