问题描述

为什么编译失败?

char programDate[] = "("__DATE__")";

但这编译得很好(见空间):

But this compiles fine (see space):

char programDate[] = "(" __DATE__")";

我知道 VC2015 现在支持文字运算符.但这不应该在编译阶段吗?__DATE__ 应该已经被预处理器处理了.这是怎么回事?

I do know VC2015 now supports literal-operators. But shouldn't that be in compilation phase? __DATE__ should have been processed by the pre-processor. What is going on here?

我想到了一些 Unicode/非 Unicode 构建的混合匹配问题 - 但它没有帮助.这不仅是预定义宏的问题，还有用户定义的问题:

I thought of some mix-match issue with Unicode/non-Unicode build - but it doesn't help. It's not just issue with pre-defined macros, but with user defined also:

#define MACRO "abc"
char data[] = "("MACRO")";

Error C3688 invalid literal suffix '__DATE__'; literal operator or literal operator template 'operator ""__DATE__' not found

推荐答案

从 C++11 开始，用户定义的文字就存在并且是预处理的一部分.语法是:

Since C++11, user-defined literals exist and are part of preprocessing. The grammar is:

preprocessing-token:
    user-defined-string-literal
    // other stuff...

user-defined-string-literal:
    string_literal ud-suffix

ud-suffix:
    identifier

所以 "("__DATE__ 匹配 preprocessing-token，但 "(" __DATE__ 不匹配 (那是两个单独的预处理标记).

So "("__DATE__ matches preprocessing-token, but "(" __DATE__ doesn't (that is two separate preprocessing tokens).

宏替换发生在标记化之后.由于您的第一个示例中没有标记 __DATE__，因此没有替换.

Macro replacement happens after tokenization. Since there is no token __DATE__ in your first example, there is no replacement.

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