C ++预处理器条件参数 [英] C++ preprocessor conditional parameter

问题描述

请注意C ++ 03！任何C ++ 11解决方案都不适合我，但只是为了知识而发布它们。

Please note C++03! any C++11 solutions are not good for me, but do post them just for knowledge sake.

我知道预处理器可以做以下事情：

I know the preprocessor can do things like:

#define FOO 4
#if FOO == 4
    cout<<"hi"<<endl;
#endif

我需要的是：

#define BAR(X)\
    #if X == 4\
       cout<<"hi"<<endl;\
    #endif

main.cpp

BAR(4)

我不喜欢看看为什么在预处理器时间内无法获得所有需要的信息。

I don't see why all the needed information wouldn't be available in preprocessor time.

所以，请告诉我如何实现这种行为。

So, Please tell me how to achieve this kind of behavior.

编辑1：
正常if条件对我的情况不起作用，因为我也做了类似的事情：

edit 1: A normal if condition won't work for my case, because I also do things like:

#define BAR(X)\
    #if X == 4\
       int poop;
    #elif
       double poop;
    #endif

推荐答案

这里的一些答案更好相对于其它的。
我接受的那个是由Christian Kiewiet在评论中发布的，但这对我来说是最准确的。
以下是扩展版本：

Some answers here were better than others. The one I accepted was posted by Christian Kiewiet in a comment, but it was the most accurate for my purpose. Here is the expanded version:

useCases.h

useCases.h

enum UseCases{
    useCase1=0,
    useCase2,
    useCaseNumber//always last for iterations
} 

specializer.h

specializer.h

#include "useCases.h"
<template UseCases theCase>
struct StaticCase{
    //empty, thus accidents calling from this can't happen
}

//specialization
template<>
class StaticCase<UseCases::useCase1>{
     typedef int T;
     static foo(T arg){cout<<"case1";};
}


template<>
class StaticCase<UseCases::useCase2>{
     typedef double T;
     static foo(){cout<<"case2";};
}

现在，我可以做到

#define BAR1(useCase) StaticCase<useCase>::foo(); 

或

#define BAR2(useCase) StaticCase<useCase>::T var;

和电话：

BAR1(UseCases::useCase1)//output - case1
BAR1(UseCases::useCase2)//output - case2

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