My C++ class is having trouble linking to a function in another .cpp file

Question

I asked the same question yesterday and the answer was not applicable.

https://stackoverflow.com/questions/6194578/breaking-up-class-functions-into-multiple-cpp-files

Answer 1

inline means that the function effectively has internal linkage (that is, it must exist inside the translation unit where it is used). Either move the definition of the function into the header, or remove inline.

(inline for modern compilers really means "use internal linkage" -- compilers will inline where it makes sense to by themselves, and they typically make better decisions than humans)

EDIT: Technically speaking, the language the standard uses here says that inline functions have external linkage; however, it also says An inline function shall be defined in every translation unit in which it is used. in section 3.2 paragraph #3 of the standard, and also There can be more than one definition of a ... inline function with external linkage (7.1.2) ... Given such an entity named D defined in more than one translation unit ... each definition of D shall consist of the same sequence of tokens in paragraph 5. So while technically speaking the name you declared inline is accessible from outside a given translation unit, to do so is to cause undefined behavior from C++.

Answer 2

This has nothing to do with includes. What you have there is a linker error. Includes are all about compilation, which requires the declarations of identifiers referred to.
The error you have means that the linker can't find the definition of a function in any of the object files passed to it which is called by one or more of them. (See this answer for what's a declaration and what's a definition and what they are needed for.)

You need to pass to the linker the object files created from compiling all of your .cpp files. If you're using some sort of an IDE, it should do this for you if you add all .cpp files to your project. If you're using a makefile, list all .cpp files as dependencies of your target. If you compile by invoking the compiler manually (which then calls the linker), pass all .cpp files to it in the same invocation.