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What I'd like to do is use glUseProgram(); which is an OpenGL function however too use it returns problems....

Why?

Because apparently even when declaring a seperate header file with an declared extern variable then using it in my main.cpp file and user-defined OpenGL Engine structure definition gives me errors with MSVC 2013

Here is a test case using only generic c++:

variable_header.hpp

 extern int ProgramID = whateverfunctionyouwant();

OGL_Engine.hpp

 #ifndef __OGL_ENGINE_HPP__ #define __OGL_ENGINE_HPP__ struct OGL_Engine { int Setup(); int ShaderSetup(); }; #endif

OGL_Engine.cpp

 #ifndef __OGL_ENGINE_CPP__ #define __OGL_Engine_CPP__ int OGL_Engine::Setup(){ int whatever = 0; } int OGL_Engine::ShaderSetup(){ int readfile = 0; afunctionyouwant(ProgramID); } #endif

The errors I get are :

Error 3 error LNK1169: one or more multiply defined symbols found

This happens in my original files, not test case but it should say this I hope.

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  • typically, extern variables are declared in headers and defined in source files. Try moving the definition and initialization to a source file. Commented Dec 31, 2015 at 1:43
  • I tried that with main.cpp and OGL_Engine.cpp both returning an error however I'll try it again... Commented Dec 31, 2015 at 1:45
  • Yep undeclared indentifier ... Commented Dec 31, 2015 at 1:46
  • It doesn't look like you have any include guards in variable_header.hpp. Do you have them in the original? Please include a minimal, complete, verifiable example along with the exact error message(s) you get when you try to build the MCVE. Commented Dec 31, 2015 at 1:48
  • I can't edit too add the main.cpp unforchunately ... -__- But I've added the header guards too variable_header.hpp going to see if that works probably won't. Commented Dec 31, 2015 at 1:52

2 Answers 2

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Two things to keep in mind here:

  1. When you include a header file, you are literally including its text into your C file. This means that including a header with the line extern int ProgramID = whateverfunctionyouwant(); will declare and define the variable ProgramID in each C file.
  2. An extern variable needs to be declared wherever it is used, but only defined once. The definition must take the form of a declaration without the extern keyword. That will tell the compiler how much storage to allocate for the variable, etc.

To use this information, change your variable_header.hpp to only declare an extern, not assign it (be sure to include header guards):

#ifndef __VARIABLE_HEADER_HPP__ #define __VARIABLE_HEADER_HPP__ extern int ProgramID; #endif

and modify exactly one source file (say OGL_Engine.cpp) to declare the variable globally (in the file but outside of any function, class, struct or namespace):

int ProgramID = whateverfunctionyouwant();

What your example is missing is a #include "variable_header.hpp" in either the C file or the matching header. Assuming that the include is there, the compiler will know that ProgramID refers to a name declared elsewhere and will match it to the correct memory location during the linking phase. When the compiler gets to the file that defines the variable, will mark it with the correct symbol regardless of whether it is extern or not in that file. This happens to any variable that is declared globally.

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You should not define a variable in header file, or you will get the above error if you include such header file in multiple source files.

extern int ProgramID = whateverfunctionyouwant();

Just use extern to declare it

extern int ProgramID;

and then define it in one of source file

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10 Comments

Jacky I'm getting this error message : Error 4 error LNK1120: 1 unresolved externals I wanna assume that this is due too the way I defined it if for example I do [variable header.hpp] extern int programID; [ogl_engine.cpp header] programID = whateverfunctiontouseprogramID(); but apparently that's not good enough... hmm
You still have not defined it. Since your included header declares it as extern, the assignment does not actually allocate the variable.
Okay got it too work somewhat had to do the type name and value not just the name and value in the cpp files that needed to use it.
Correct. The type is what makes a declaration as opposed to an assignment.
@DominicHughes Define it in a source file like int ProgramID = 89;
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