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In the C and C++ programming languages, an inline function is one qualified with the keyword inline; this serves two purposes. Firstly, it serves as a compiler directive that suggests (but does not require) that the compiler substitute the body of the function inline by performing inline expansion, i.e.: by inserting the function code at the address of each function call, thereby saving the overhead of a function call. In this respect it is analogous to the register storage class specifier, which similarly provides an optimization hint.The second purpose of inline is to change linkage behavior; the details of this are complicated. This is necessary due to the C/C++ separate compilation + linkage model, specifically because the definition (body) of the function must be duplicated in all translation units where it is used, to allow inlining during ''compiling'', which, if the function has external linkage, causes a collision during ''linking'' (it violates uniqueness of external symbols; in C++, the One Definition Rule). C and C++ (and dialects such as GNU C and Visual C++) resolve this in different ways.〔==Problems== Besides the problems with inline expansion in general, inline functions as a language feature may not be as valuable as they appear, for a number of reasons: * Often, a compiler is in a better position than a human to decide whether a particular function should be inlined. Sometimes the compiler may not be able to inline as many functions as the programmer indicates. * An important point to note is that the code (of the inline function) gets exposed to its client (the calling function). * As functions evolve, they may become suitable for inlining where they were not before, or no longer suitable for inlining where they were before. While inlining or un-inlining a function is easier than converting to and from macros, it still requires extra maintenance which typically yields relatively little benefit. * Inline functions used in proliferation in native C-based compilation systems can increase compilation time, since the intermediate representation of their bodies is copied into each call site where they are * The specification of inline in C99 requires exactly one additional external definition of a function in another compilation unit, when the corresponding inline definition, that can occur multiple times in different compilation units, if that function is used somewhere. That can easily lead to linker errors because such a definition wasn't provided by the programmer. For this reason, inline in C99 often is used together with static, which gives the function internal linkage. * In C++, it is necessary to define an inline function in every module (compilation unit) that uses it, whereas an ordinary function must be defined in only a single module. Otherwise it would not be possible to compile a single module independently of all other modules. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「inline function」の詳細全文を読む スポンサード リンク
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