The inline and __inline keywords allow the compiler to insert a copy of the function body into each place the function is called. The insertion occurs only if the compiler's cost/benefit analysis show it to be profitable. The __forceinline keyword overrides the cost/benefit analysis and relies on the judgement of the programmer instead. Exercise caution when using __forceinline. Indiscriminate use of __forceinline can result in larger code with only marginal performance gains or, in some cases, even performance losses (due to increased paging of a larger executable, for example).
You cannot force the compiler to inline a function when conditions other than cost/benefit analysis prevent it. You cannot inline a function if:
The function or its caller is compiled with /Ob0 (the default option for debug builds).
The function and the caller use different types of exception handling (C++ exception handling in one, structured exception handling in the other).
The function has a variable argument list.
The function uses inline assembly and is not compiled with /Og, /Ox, /O1, or /O2).
Function returns an unwindable object by value and is not compiled with /GX, /EHs, or /EHa).
The function receives a copy-constructed object passed by value, when compiled with /GX, /EHs,, or /EHa.
The function is recursive and is not accompanied by #pragma(inline_recursion, on). With the pragma, recursive functions can be inlined to a default depth of eight calls. To change the inlining depth, use #pragma(inline_depth, n).
If the compiler cannot inline a function declared __forceinline, it generates a level 1 warning (4714).
The inline keyword is available only in C++. The __inline and __forceinline keywords are available in both C and C++. For compatibility with previous versions, _inline is a synonym for __inline.
Using inline functions can make your program faster because they eliminate the overhead associated with function calls. Functions expanded inline are subject to code optimizations not available to normal functions.
The /Ob compiler optimization option determines whether inline function expansion actually occurs
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