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Thinking in C++
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There are two situations in which the compiler cannot perform inlining. In these cases, it simply reverts to the ordinary form of a function by taking the inline definition and creating storage for the function just as it does for a non-inline. If it must do this in multiple translation units (which would normally cause a multiple definition error), the linker is told to ignore the multiple definitions.

The compiler cannot perform inlining if the function is too complicated. This depends upon the particular compiler, but at the point most compilers give up, the inline probably wouldn’t gain you any efficiency. In general, any sort of looping is considered too complicated to expand as an inline, and if you think about it, looping probably entails much more time inside the function than what is required for the function call overhead. If the function is just a collection of simple statements, the compiler probably won’t have any trouble inlining it, but if there are a lot of statements, the overhead of the function call will be much less than the cost of executing the body. And remember, every time you call a big inline function, the entire function body is inserted in place of each call, so you can easily get code bloat without any noticeable performance improvement. (Note that some of the examples in this book may exceed reasonable inline sizes in favor of conserving screen real estate.)

The compiler also cannot perform inlining if the address of the function is taken implicitly or explicitly. If the compiler must produce an address, then it will allocate storage for the function code and use the resulting address. However, where an address is not required, the compiler will probably still inline the code.

It is important to understand that an inline is just a suggestion to the compiler; the compiler is not forced to inline anything at all. A good compiler will inline small, simple functions while intelligently ignoring inlines that are too complicated. This will give you the results you want – the true semantics of a function call with the efficiency of a macro.

Thinking in C++
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire