multiple definition error including c++ header file with inline code from multiple sources

Question

I have a c++ header file containing a class. I want to use this class in several projects, bu I don\'t want to create a separate library for it, so I\'m putting both methods

Answer 1

Your first code snippet is falling foul of C++'s "One Definition Rule" - see here for a link to a Wikipedia article describing ODR. You're actually falling foul of point #2 because every time the compiler includes the header file into a source file, you run into the risk of the compiler generating a globally visible definition of test_ns::TestClass::testMethod(). And of course by the time you get to link the code, the linker will have kittens because it will find the same symbol in multiple object files.

The second snippet works because you've inlined the definition of the function, which means that even if the compiler doesn't generate any inline code for the function (say, you've got inlining turned off or the compiler decides the function is too big to inline), the code generated for the function definition will be visible in the translation unit only, as if you'd stuck it in an anonymous namespace. Hence you get multiple copies of the function in the generated object code that the linker may or may not optimize away depending on how smart it is.

You could achieve a similar effect in your first code snippet by prefixing TestClass::testMethod() with inline.