C/C++プリプロセッサにおいて、一意な識別子名生成に利用できる__COUNTER__マクロについて。非標準機能だが主要コンパイラで一通りサポートされている。
#define CAT_IMPL(s1, s2) s1##s2 #define CAT(s1, s2) CAT_IMPL(s1, s2) #ifdef __COUNTER__ #define GEN_ID(str) CAT(str, __COUNTER__) #else #define GEN_ID(str) CAT(str, __LINE__) #endif // 識別子生成: 変数"foo1"などが生成される auto GEN_ID(foo) = /*...*/;
Visual C++
Visual Studio .NET 2003(MSVC7.1)以降で利用可能。
Expands to an integer starting with 0 and incrementing by 1 every time it is used in a compiland.
http://msdn.microsoft.com/ja-jp/library/b0084kay%28v=vs.71%29.aspx__COUNTER__remembers its state when using precompiled headers. If the last__COUNTER__value was 4 after building a precompiled header (PCH), it will start with 5 on each PCH use.
__COUNTER__lets you generate unique variable names. You can use token pasting with a prefix to make a unique name.
gcc
gcc 4.3以降で利用可能。
C family
http://gcc.gnu.org/gcc-4.3/changes.html
- A new predefined macro
__COUNTER__has been added. It expands to sequential integral values starting from 0. In conjunction with the##operator, this provides a convenient means to generate unique identifiers.
Clang
http://clang.llvm.org/docs/LanguageExtensions.html#builtinmacros
__COUNTER__
Defined to an integer value that starts at zero and is incremented each time the__COUNTER__macro is expanded.
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