c++ - 根据 C++ 标准在宏定义中推送/弹出宏时的行为是什么

Question

我刚才回答了一个关于附加到 C 宏的问题在Can I append to a preprocessor macro?

正如答案所说，以下内容在 clang 和 g++ 中都有效，但在 msvc 中无效

#define pushfoo _Pragma("push_macro(\"foo\")") //for convenience
#define popfoo _Pragma("pop_macro(\"foo\")") //I tried __pragma and __Pragma for msvc as well

#define foo 1

pushfoo                           //push the old value
#undef foo                        //so you don't get a warning on the next line
#define foo popfoo foo , 2        //append to the previous value of foo

pushfoo
#undef foo
#define foo popfoo foo , 3

pushfoo
#undef foo
#define foo popfoo foo , 4


foo //this whole list will expand to something like popfoo foo popfoo foo popfoo foo , 4
    //which will in turn expand to 1 , 2 , 3 , 4

foo //the second time this will expand to just 1

虽然这看起来确实有效，但我想知道这是否恰好适用于 gcc和 clang，但 msvc 也是(或仅)正确的，或者如果它们表现出的行为是强制的行为由语言规范。如果 C 和 C++ 有所不同，我对它们都感兴趣(主要是对最新版本，写作时的 C18 和 C++17)

Answer 1

What is the behavior when pushing/popping a macro inside that macro definition according to c++ standard?

在 C 和 C++ 中，#pragma 和 _Pragma 的结果是实现定义的，因此标准不管理它们的行为。参见 C11 6.10.6p1 :

A preprocessing directive of the form

 # pragma pp-tokensopt new-line

where the preprocessing token STDC does not immediately follow pragma in the directive (prior to any macro replacement)174) causes the implementation to behave in an implementation-defined manner. The behavior might cause translation to fail or cause the translator or the resulting program to behave in a non-conforming manner. Any such pragma that is not recognized by the implementation is ignored.

和6.10.9涵盖 _Pragma。

和最新的 C++ 草案 [cpp.pragma] :

A preprocessing directive of the form

# pragma pp-tokensopt new-line

causes the implementation to behave in an implementation-defined manner.* The behavior might cause translation to fail or cause the translator or the resulting program to behave in a non-conforming manner. Any pragma that is not recognized by the implementation is ignored.

另见 [cpp.pragma.op]

gcc 中没有关于这些 pragam 的文档或 MSVC似乎很详细。所以他们没有提供太多指导。