Inline keyword in member function definition

Question

Why inline keyword should used in the definition of member function. and Not in declaration?

Answer 1

inline has some pre-historic use, but nowadays it's best to remember it as saying: "this definition is going to be defined multiple times, and that's okay."

That is, normally the one-definition rule prohibits multiple definitions of a function. This:

// foo.hpp
void foo() { /* body */ }

// a.cpp
#include "foo.hpp"

// b.cpp
#include "foo.hpp"

results in an error, as foo is defined in two translation units. You can declare things as often as you want. This:

// foo.hpp
void foo();

// foo.cpp
void foo()
{
    /* body */
}

// a.cpp
#include "foo.hpp"

// b.cpp
#include "foo.hpp"

is fine, as foo is defined once, and declared multiple times. What inline does is allow this:

// foo.hpp
inline void foo() { /* body */ }

// a.cpp
#include "foo.hpp"

// b.cpp
#include "foo.hpp"

to work. It says "if you see foo more than once, just assume they are the same and be okay with it".

Answer 2

No, it can be used in the member function declaration also. Though msdn documentation isn't standard, it is mentioned MSDN inline. See the note part in it.

But, I learnt that it is up to the modern compilers to make a function inline or not despite explicitly mentioning inline.

class foo
{
    inline void methodOne();
};

void foo::methodOne()
{
}

IdeOne Results

Also, one can specify it both declaration and definition to have the same effect.

class foo
{
    inline void methodOne();
};

inline void foo::methodOne()  // Here keyword inline is optional. Needs to be mentioned if method declaration isn't declared inline.
{
}

IdeOne Results

Both of the above will have the same effect.