inline vs __inline vs __inline__ vs __forceinline?

Question

What are the differences between these four inline (key)words?

inline, __inline, __inline__, __forceinline.

Answer 1

inline is the keyword, in C++ and C99.

__inline is a vendor-specific keyword (e.g. MSVC) for inline function in C, since C89 doesn't have it.

__inline__ is similar to __inline but is from another set of compilers.

__forceinline is another vendor-specific (mainly MSVC) keyword, which will apply more force to inline the function than the __inline hint (e.g. inline even if it result in worse code).

There's also __attribute__((always_inline)) in GCC and clang.

Answer 2

For the Visual Studio compiler it means:

• inline - suggestion to the compiler to inline your code

• __forceinline - overrides the builtin compiler optimization and generates inline code

For more details see: http://msdn.microsoft.com/en-us/library/z8y1yy88%28VS.71%29.aspx

Answer 3

__inline, __inline__ and __forceinline are all implementation specific. Because of the double underscore they are all identifiers reserved for the implementation so shouldn't conflict with identifiers used in applications.

inline is the only C++ keyword.