问题
What are the differences between these four inline (key)words?
inline, __inline, __inline__, __forceinline.
回答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.
回答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.
回答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
来源：https://stackoverflow.com/questions/2765164/inline-vs-inline-vs-inline-vs-forceinline