带字符串文字前缀的预处理器字符串化运算符 [英] Preprocessor Stringizing Operator with String Literal Prefixes

问题描述

所以我想做传统的事情，与宏中的字符串化运算符有关:

So I want to do the traditional thing to do with the stringizing operator in a macro:

#define FOO(x) foo(#x, (x))

但是我需要使用字符串文字前缀: http://en.cppreference. com/w/cpp/language/string_literal
这是一个问题，因为如果我需要UTF-32字符串文字，我会尝试这样做:

However I need to use a string literal prefix: http://en.cppreference.com/w/cpp/language/string_literal
Which is a problem because if I need a UTF-32 string literal I try to do this:

#define FOO(x) foo(U#x, (x))

但是gcc 4.9.2抱怨:

But gcc 4.9.2 complains:

错误:未在此范围内声明"U"

error: 'U' was not declared in this scope

有没有办法使编译器将U当作字符串化宏变量的前缀?

Is there a way to make the compiler treat the U as a prefix to the stringized macro variable?

推荐答案

是的，使用串联:

#define CONCAT2(x, y) x ## y
#define CONCAT(x, y) CONCAT2(x, y)
#define STRINGIZE(x) #x

#define FOO(x) foo(CONCAT(U, STRINGIZE(x)), (x))

由于N3936§16.3.2[cpp.stringize]/2，它表示:需要"额外的间接寻址(如果您传递应该首先进行评估的宏是好的)，

The extra indirection, besides being good if you pass a macro that should be evaluated first, is "needed" because of N3936 §16.3.2 [cpp.stringize]/2, which says:

#和##运算符的求值顺序未指定.

The order of evaluation of # and ## operators is unspecified.

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