__inline keyword tells the compiler to substitute the code within the function definition for every instance of a function call. However, substitution occurs only at the compiler's discretion. For example, the compiler does not inline a function if its address is taken or if it is too large to inline.
For a function to be considered as a candidate for inlining, it must use the new-style function definition.
Use this form to specify an inline function:
__inline typeopt function-definition
The use of inline functions generates faster code and can sometimes generate smaller code than the equivalent function call generates for the following reasons:
It saves the time required to execute function calls.
Small inline functions, perhaps three lines or less, create less code than the equivalent function call because the compiler doesn't generate code to handle arguments and a return value.
Functions generated inline are subject to code optimizations not available to normal functions because the compiler does not perform interprocedural optimizations.
__inline should not be confused with inline assembler code. See Inline Assembler for more information.
END Microsoft Specific