C++ – “Multiple definition of” C++ compiler error

c++linker

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:

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:

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

Best Solution

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.