Can modern C++ compilers inline functions that are defined in a cpp file

c++ compiler-construction inline
Martin picture Martin · Dec 9, 2011 · Viewed 8k times · Source

I am aware that the keyword inline has useful properties e.g. for keeping template specializations inside a header file. On the other hand I have often read that inline is almost useless as hint for the compiler to actually inline functions. Further the keyword cannot be used inside a cpp file since the compiler wants to inspect functions marked with the inline keyword whenever they are called.

Hence I am a little confused about the "automatic" inlining capabilities of modern compilers (namely gcc 4.43). When I define a function inside a cpp, can the compiler inline it anyway if it deems that inlining makes sense for the function or do I rob him of some optimization capabilities ? (Which would be fine for the majority of functions, but important to know for small ones called very often)

Answer

Martin York picture Martin York · Dec 9, 2011

Within the compilation unit the compiler will have no problem inline functions (even if they are not marked as inline). Across compilation units it is harder but modern compilers can do it.

Use of the inline tag has little affect on 'modern' compilers and whether it actually inlines functions (it has better heuristics than the human mind) (unless you specify flags to force it one way or the other (which is usually a bad idea as humans are bad at making this decision)).

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