C++ ifdef 中的布尔值：“#ifdef A && B”是否与“#if defined(A) && defined(B)”相同？
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Boolean in ifdef: is "#ifdef A && B" the same as "#if defined(A) && defined(B)"?
提问by criddell
In C++, is this:
在 C++ 中，是这样的：
#ifdef A && B
the same as:
等同于：
#if defined(A) && defined(B)
?
?
I was thinking it wasn't, but I haven't been able to find a difference with my compiler (VS2005).
我认为它不是，但我无法找到与我的编译器（VS2005）的区别。
回答by Evan Teran
They are not the same. The first one doesn't work (I tested in gcc 4.4.1). Error message was:
她们不一样。第一个不起作用（我在 gcc 4.4.1 中测试过）。错误消息是：
test.cc:1:15: warning: extra tokens at end of #ifdef directive
test.cc:1:15: 警告：#ifdef 指令末尾的额外标记
If you want to check if multiple things are defined, use the second one.
如果要检查是否定义了多个事物，请使用第二个。
回答by Svetlozar Angelov
You can use the defined operator in the #if directive to use expressions that evaluate to 0 or 1 within a preprocessor line. This saves you from using nested preprocessing directives. The parentheses around the identifier are optional. For example:
#if defined (MAX) && ! defined (MIN)Without using the defined operator, you would have to include the following two directives to perform the above example:
#ifdef max #ifndef min
您可以在 #if 指令中使用已定义的运算符来使用预处理器行中计算结果为 0 或 1 的表达式。这使您无需使用嵌套的预处理指令。标识符周围的括号是可选的。例如：
#if defined (MAX) && ! defined (MIN)如果不使用定义的运算符，则必须包含以下两个指令才能执行上述示例：
#ifdef max #ifndef min
回答by way good
The following results are the same:
以下结果相同：
1.
1.
#define A
#define B
#if(defined A && defined B)
printf("define test");
#endif
2.
2.
#ifdef A
#ifdef B
printf("define test");
#endif
#endif
回答by MikeB
For those that might be looking for example (UNIX/g++) that is a little different from the OP, this may help:
对于那些可能正在寻找与 OP 略有不同的示例 (UNIX/g++) 的人来说，这可能会有所帮助：
`
`
#if(defined A && defined B && defined C)
const string foo = "xyz";
#else
#if(defined A && defined B)
const string foo = "xy";
#else
#if(defined A && defined C)
const string foo = "xz";
#else
#ifdef A
const string foo = "x";
#endif
#endif
#endif
#endif
