Isn't C++'s inline totally optional?

Question

I have a class that had an inline member, but I later decided that I wanted to remove the implementation from the headers so I moved the members body of the functions out to

Answer 1

Regarding harshath.jr's answer, a method need not be declared inline if its definition has the "inline" keyword, and that definition is available in the same header, i.e.:

class foo
{
  void bar();
};

inline void foo::bar()
{
  ...
}

This is useful for conditionally inlining a method depending on whether or not the build is "debug" or "release" like so:

// Header - foo.h

class foo
{
  void bar();  // Conditionally inlined.
};

#ifndef FOO_DEBUG
# include "foo.inl"
#endif

The "inline" file could look like:

// Inline Functions/Methods - foo.inl
#ifndef FOO_DEBUG
# define FOO_INLINE inline
#else
# define FOO_INLINE
#endif

FOO_INLINE void foo::bar()
{
  ...
}

and the implementation could like the following:

// Implementation file - foo.cpp
#ifdef FOO_DEBUG
# include "foo.inl"
#endif

...

It's not exactly pretty but it has it's uses when aggressive inline becomes a debugging headache.