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What are the differences between these four inline (key)words?
inline, __inline, __inline__, __forceinline.
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The __forceinline keyword forces the compiler to compile a C or C++ function inline. The semantics of __forceinline are exactly the same as those of the C++ inline keyword.
As compilers became better at optimising, this functionality has receded, and using inline as a suggestion to inline a function is indeed obsolete. The compiler will happily ignore it and inline something else entirely if it finds that's a better optimisation.
The inline specifier, when used in a decl-specifier-seq of a variable with static storage duration (static class member or namespace-scope variable), declares the variable to be an inline variable. A static member variable (but not a namespace-scope variable) declared constexpr is implicitly an inline variable.
Inline functions provide following advantages: 1) Function call overhead doesn't occur. 2) It also saves the overhead of push/pop variables on the stack when function is called. 3) It also saves overhead of a return call from a function.
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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__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.
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