预处理器宏扩展为另一个预处理器指令

Preprocessor macro expansion to another preprocessor directive(预处理器宏扩展为另一个预处理器指令)

问题描述

最初我认为我需要这个，但我最终避免了它.然而，我的好奇心(以及对知识的渴望，哼)让我问:

Initially I thought I needed this, but I eventually avoided it. However, my curiosity (and appetite for knowledge, hum) make me ask:

可以一个预处理器宏，例如

Can a preprocessor macro, for instance in

#include "MyClass.h"

INSTANTIATE_FOO_TEMPLATE_CLASS(MyClass)

扩展到另一个包含，如 in

expand to another include, like in

#include "MyClass.h"

#include "FooTemplate.h"
template class FooTemplate<MyClass>;

?

推荐答案

相信做不到，这是因为预处理器是single pass.所以它不能发出其他预处理器指令.

I believe that cannot be done, this is because the pre-processor is single pass. So it cannot emit other preprocessor directives.

具体来说，来自 C99 标准(6.10.3.4 第 3 段):

Specifically, from the C99 Standard (6.10.3.4 paragraph 3):

3 结果完全宏替换预处理令牌序列不被处理为预处理指令，即使它像一个，...

3 The resulting completely macro-replaced preprocessing token sequence is not processed as a preprocessing directive even if it resembles one, ...

有趣的是，这就是为什么将一元 _Pragma 运算符添加到 c99 中的原因.因为 #pragma 不能由宏发出，但 _Pragma 可以.

Interestingly enough, This is why the unary _Pragma operator was added to c99. Because #pragma could not be emited by macros, but _Pragma can.

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