如何测试预处理器符号是否已#define'd 但没有值?

问题描述

使用 C++ 预处理器指令，是否可以测试预处理器符号是否已定义但没有值?类似的东西:

Using C++ preprocessor directives, is it possible to test if a preprocessor symbol has been defined but has no value? Something like that:

#define MYVARIABLE
#if !defined(MYVARIABLE) || #MYVARIABLE == ""
... blablabla ...
#endif

我这样做的原因是因为我正在处理的项目应该通过 /DMYSTR=$(MYENVSTR)/DMYSTR=$(MYENVSTR)，并且此字符串可能为空.如果用户忘记定义这个字符串，我想确保项目无法编译.

The reason why I am doing it is because the project I'm working on is supposed to take a string from the environment through /DMYSTR=$(MYENVSTR), and this string might be empty. I want to make sure that the project fails to compile if user forgot to define this string.

推荐答案

Soma 宏魔法:

#define DO_EXPAND(VAL)  VAL ## 1
#define EXPAND(VAL)     DO_EXPAND(VAL)

#if !defined(MYVARIABLE) || (EXPAND(MYVARIABLE) == 1)

Only here if MYVARIABLE is not defined
OR MYVARIABLE is the empty string

#endif

请注意，如果您在命令行中定义了 MYVARIABLE，则默认值为 1:

Note if you define MYVARIABLE on the command line the default value is 1:

g++ -DMYVARIABLE <file>

这里 MYVARIABLE 的值是空字符串:

Here the value of MYVARIABLE is the empty string:

g++ -DMYVARIABLE= <file>

引用问题解决了:

#define DO_QUOTE(X)        #X
#define QUOTE(X)           DO_QUOTE(X)

#define MY_QUOTED_VAR      QUOTE(MYVARIABLE)

std::string x = MY_QUOTED_VAR;
std::string p = QUOTE(MYVARIABLE);

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