inline vs __inline vs __inline__ vs __forceinline?

Question

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

inline, __inline, __inline__, __forceinline.

Solution 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.

Solution 2

__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.

Solution 3

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

Comments

[Martin York]

You should also ask the community what the recommendation for usage of inline are.

[Joris Timmermans]

__forceinline is a more forceful hint than inline, but still just a hint (msdn.microsoft.com/en-us/library/z8y1yy88%28VS.80%29.aspx).

[Dan]

Maybe consider changing the "Microsoft-specific" language to "vendor-specific" or something like that. Many of the embedded cross-development toolsets I use also support __inline and __forceinline. The world is bigger than MSOFT ;-)

[osirisgothra]

All __forceinline does is skip the analysis of the overhead and benefit factors of inlining and 'force' it to happen anyways. Unless you really need to to be inlined you are better off to just stick with inline when using C++ or __inline when using C. As far as i know, __forceinline originates from MSVC but it gets used and defined other places, notably by GCC (as stated above) in the attribute expression. But of course, you have to use the inline headers or flags for it to be used. Some IDEs will disable this by default, others enable it by default. I'd steer clear if youre a cross developer!

[thomasrutter]

gcc suggests __inline__ as an alternative to inline for pre-C99