Extern for class method - c++

Question

I want to use extern keyword for class method in C++. I'm using Visual Studio 2019.
I want to force method inline, using __forceinline(without extern I will get unresolved external symbol). I don't want to put method implementation in this header file, I want put implementation in other .cpp file.
I noticed that method may be inlined automaticly without extern and __forceinline and when method's implementation in .cpp file. How can I force it?
Example code:
class Window {
public:
void Show();
};
That's ok!
class Window {
public:
__forceinline void Show();
};
Gets unresolved external symbol when I calls this function from other CPP.
With extern I gets:
screenshot
For simple functions extern __forceinline works fine:
screenshot

Answer 1

You cannot specify extern for a member function.
The standard clarifies this in [dcl.stc]/5:
The extern specifier can be applied only to the names of variables and functions. The extern specifier cannot be used in the declaration of class members or function parameters.
When you use extern, you say to the compiler that you declare a variable or a function, but without defining it. In the case of member functions, this is not needed: they can always be declared in a class body without being defined:
class A {
public:
void f(); // you can define it in which ever compilation unit (cpp) you want
};
...
// Else where...may be in another cpp
void A::f() {...}
The linker will resolve the symbols when needed.
The inline is not so useful
The standard in [dcl.inline]/2 says that inline is not guaranteed:
The inline specifier indicates to the implementation that inline
substitution of the function body at the point of call is to be
preferred to the usual function call mechanism. An implementation is
not required to perform this inline substitution at the point of call;
Furthermore, [dcl.inline]/6 explains that inline and defined eleswhere is not something meant to work well:
An inline function or variable shall be defined in every translation unit in which it is odr-used and shall have exactly the same definition in every case.
(odr says that all the definitions must be exactly the same definition each time)
(in every translation unit means in every cpp or the headers therein included)
On the other side, the global optimizer of some modern compilers is able to achieve similar performance effects than inlined functions, and this accross compilation units (cpp).
Implementation specific forceinline
MSVC 2019 uses inline carefully, making a cost/benefit analysis to decide whether or not to inline.
forceinline allows you to overrule the compiler's judgement. But it comes with all the requirements of the normal inline: so you need to have a definition of the function in the same compilation unit to get it work.

Answer 2

Two things:
You cannot declare a member function as extern. These already have external linkage.
For the compiler to inline a function or method, it has to be defined (as opposed to declared) in the header file. This is so that the compiler can see it when compiling code that calls the function.
So, just put the function body in the header file and drop the extern.

Answer 3

I don't want to put method implementation in this header file, I want put implementation in other .cpp file.
Sounds to me like you just want to have all declarations in one file like this:
This is your window.h- file:
class Window
{
public:
void Show();
};
To have the implementation in another file, you have to use the scope operator.
This is the other window.cpp- file, in which you have the actual implementation of the function:
#include "window.h"
Window::Show()
{
// Your implementation goes here
}
I hope I understood your question right and this helps.