inline vs __inline vs __inline__ vs __forceinline?

c++ inline keyword
Xavier Ho picture Xavier Ho · May 4, 2010 · Viewed 29.5k times · Source

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

inline, __inline, __inline__, __forceinline.

Answer

kennytm picture kennytm · May 4, 2010

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.

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