将预处理器宏扩展为另一个预处理器指令

问题描述：

最初，我以为我需要这个，但最终我避免了.但是，我的好奇心(以及对知识的渴望，嗡嗡声)使我问:

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

例如可以在

#include "MyClass.h"

INSTANTIATE_FOO_TEMPLATE_CLASS(MyClass)

扩展到另一个包含项，例如

expand to another include, like in

#include "MyClass.h"

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

答

我认为无法完成，这是因为预处理程序是单遍.因此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，根据C99标准(6.10.3.4第3段):

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

3结果完全宏替换的预处理令牌序列不作为预处理指令，即使它类似于一个，...

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

足够有趣，这就是为什么将一元_Pragma运算符添加到c99的原因.因为#pragma不能由宏发出，但是_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.