Calls to small and quick functions can be smaller and quicker when expanded inline than when called normally. Conversely, calls to large or slow functions can be larger and slower when expanded inline than when branched to. Furthermore, all calls to an inline function must be recompiled whenever the function definition changes. Consequently, the decision to use inline functions requires considerable care.
Do not use inline functions when you anticipate changes to the function definition and recompiling all callers is expensive. Otherwise, use inline functions when the code to expand the function inline is smaller than the code to call the function or the application performs significantly faster with the function inline.
The compiler cannot inline all function calls, so making the most effective use of function inlining may require some source changes. Use the +w option to learn when function inlining does not occur. In the following situations, the compiler will not inline the function:
The function contains difficult control constructs, such as loops, switch statements, and try/catch statements. Many times these functions execute the difficult control constructs infrequently. To inline such a function, split the function into two parts: an inner part that contains the difficult control constructs and an outer part that decides when to call the inner part. This technique of separating the infrequent part from the frequent part of a function can improve performance even when the compiler can inline the full function.
The inline function body is large or complicated. Apparently simple function bodies may be complicated because of calls to other inline functions within the body, or because of implicit constructor and destructor calls (as often occurs in constructors and destructors for derived classes). For such functions, inline expansion rarely provides significant performance improvement, and the function is best left uninlined.
The arguments to an inline function call are large or complicated. The compiler is particularly sensitive when the object for an inline member function call is itself the result of an inline function call. To inline functions with complicated arguments, simply compute the function arguments into local variables and then pass the variables to the function.