c++ - Moving inline methods from a header file to a .cpp files

Question

I have the following class defined in a foo.h header file

class Foo {

public:
    inline int Method();

};

inline int Foo::Method() { // Implementation }

I would like now to move the implementation to a foo.cpp file. To this end, I have to remove the inline keyword and move the implementation of the method to a foo.cpp file like this

#include `foo.h`

inline int Foo::Method() { // Implementation }

I have two questions:

1. Is my statement about the removal of the inline keyword correct? Should it be necessarily removed?
2. How typically the removal of the inline keyword affect the performance (practically all my methods are inlined)?

Thank you very much in advance.

Answer 1

If you moved the function definition from a header to a cpp file, you MUST remove the inline keyword all all locations for that function. With older linkers it might make things slightly slower, but with modern linkers you should notice no real difference in performance.^*

There are certain cases where a public member function can be inline, but that's just a bad idea. Don't do it. Arguments can be made for marking certain private member functions as inline, but in reality what you really want in those to be __attribute__((always_inline)) or __forceinline

^{*In extremely rare cases it will make a difference, but 99% of the time it won't, and 99.9% of what's left you don't care. If measurements show you hit that one-in-ten-thousand, you can use the aformentioned __forceinline.}