在 C/C++ 中定义 (XXXX) 宏？
defined(XXXX) macro in C/C++?
我正在浏览 BSON 源代码，发现了一些我以前从未见过的东西。
#if !defined(BSON_INSIDE) && !defined(BSON_COMPILATION)
#error "Only <bson.h> can be included directly."
#endif
上面的defined(XXXX)宏是什么？我可以猜到它的作用，但我似乎找不到任何关于它的文档。它特定于某些编译器吗？它在 Microsoft Visual C++ 上给我一个 W4 警告（我正试图在我的项目中解决）。
来自6.10.1
The expression that controls conditional inclusion shall be an integer
constant expression except that: identifiers (including those
lexically identical to keywords) are interpreted as described
below;166) and it may contain unary operator expressions of the form
defined identifier
or
defined ( identifier )
which evaluate to 1 if the identifier is currently defined as a macro
name (that is, if it is predefined or if it has been the subject of a
#define preprocessing directive without an intervening #undef directive with the same subject identifier), 0 if it is not.
它不是 macro - 它是一个运算符。
我正在浏览 BSON 源代码，发现了一些我以前从未见过的东西。
#if !defined(BSON_INSIDE) && !defined(BSON_COMPILATION)
#error "Only <bson.h> can be included directly."
#endif
上面的defined(XXXX)宏是什么？我可以猜到它的作用，但我似乎找不到任何关于它的文档。它特定于某些编译器吗？它在 Microsoft Visual C++ 上给我一个 W4 警告（我正试图在我的项目中解决）。
来自6.10.1
The expression that controls conditional inclusion shall be an integer constant expression except that: identifiers (including those lexically identical to keywords) are interpreted as described below;166) and it may contain unary operator expressions of the form
defined identifieror
defined ( identifier )which evaluate to 1 if the identifier is currently defined as a macro name (that is, if it is predefined or if it has been the subject of a
#definepreprocessing directive without an intervening#undefdirective with the same subject identifier), 0 if it is not.
它不是 macro - 它是一个运算符。