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Inline functions

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The inline keyword is a function specifier that tells the compiler to substitute the code within the function definition for every instance of a function call.

Remarks

Inline code substitution occurs only at the compiler's discretion. For example, the compiler won't inline a function if its address is taken or if it's too large to inline. When the compiler doesn't inline a function defined in a header file, it's marked for the linker to avoid one-definition rule (ODR) violations.

For a function to be considered as a candidate for inlining, it must use the new-style function definition with a signature that declares the return type and any parameter types.

Use this form to specify an inline function:

inline function-definition

Inline functions generate faster and sometimes smaller code than the equivalent function call:

  • Inline functions save the time required to prepare the stack for arguments and a return value, and the time to execute the jump and return of a function call.

  • Even when repeated several times, small inline functions of 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.

  • The compiler can optimize functions generated inline in ways that aren't available to normal functions. The compiler doesn't usually perform optimizations between different procedures.

Don't confuse functions that use inline with inline assembler code. For more information about inline assembler, see Inline assembler.

Microsoft specific

Microsoft also supports __inline and __forceinline keywords to tell the compiler to substitute the code within the function definition for every instance of a function call. The __inline keyword is a synonym for inline. The __forceinline keyword tells the compiler to relax the heuristics on whether to inline the function, though it doesn't guarantee a function will be inlined.

For compatibility with previous versions, _inline and _forceinline are synonyms for __inline and __forceinline, respectively, unless compiler option /Za (Disable language extensions) is specified.

END Microsoft specific

See also

inline, __inline, __forceinline