Header guard / translation unit problem

Question

I was under the impression that header guards solve the problem of redefinition. I\'m getting linker errors that say there are redefinitions in the .obj files. This is the h

Answer 1

Header guards only prevent the guarded portion of the header file from being included twice. The result is passed to the compiler, so the compiler does not know anything of the header guards.

Consequently it will emit these symbols for every translation unit that includes the header (since it can not know that there was somewhere another unrelated translation unit compiled).

Also the linker can not know that you didn't wanted this to happen.

To solve the problem, declare the variables extern in the header, and define them in one single translation unit.

Answer 2

Header guards solve the problem of including the same header twice or hidden recursion of includes, not double definition.

If you include the same header in different translation units, header guards won't help.

The solution is to never declare variables in header files. If you need to share variables, use the extern keyword in the header, and declare the actual variables in one of the translation units.