在C或C ++中使用逗号作为宏名称 [英] Using comma as macro name in C or C++

问题描述

我想做以下事情：

#define , 
#define MAX 10,000,000
// ...
#undef ,

有什么窍门吗？

编辑：我知道C ++ 14中'的数字分隔符。我正在寻找一种对不兼容的编译器执行相同操作的技巧。

编辑2：请考虑 Variadic宏。

I know about the ' digit separator in C++14. I'm looking for a trick to do the same for uncompliant compilers.
Please consider Variadic Macros.

推荐答案

警告，黑魔法即将来临。

Warning, black magic ahead.

确实可以使用宏，尽管预设数量为论点。该数字可以是任意的，但每个数字都必须用手写的：

Macros can indeed be used, albeit with a preset number of arguments. This number can be arbitrary, but each must be written by hand:

#include <stdio.h>
#include <stdlib.h>

#define MERGE_EXPAND( a , b )     a##b
#define MERGE( a , b )            MERGE_EXPAND( a , b )

#define COUNT_PICK( a , b , c , pick , ... )  pick

#define COUNT( ... )    COUNT_PICK( __VA_ARGS__ , 3 , 2 , 1 , 0 )

#define JOIN_1( a )           a
#define JOIN_2( a , b )       a##b
#define JOIN_3( a , b , c )   a##b##c

#define JOIN( ... ) MERGE( JOIN_ , COUNT( __VA_ARGS__ ) )( __VA_ARGS__ )

int main( void )
{
    printf( "%d\n" , JOIN( 12345 ) ) ;
    printf( "%d\n" , JOIN( 100,44 ) ) ;
    printf( "%d\n" , JOIN( -10,44,9999 ) ) ;  

    return EXIT_SUCCESS ;
}

宏COUNT计算传递给它的参数数量。这是通过将参数传递给辅助宏COUNT_PICK并添加其他参数（它们是相反顺序的连续数字）来完成的。然后，传递给COUNT的原始参数数将操纵COUNT_PICK的参数，因此将选择其中一个数字。

The macro COUNT count the number of arguments passed to it. This is done by passing arguments to the helper macro COUNT_PICK, and adding additional argument which are consecutive numbers in reverse order. The number of original arguments passed to COUNT then manipulates the arguments of COUNT_PICK, so that one of the numbers is chosen.

然后将所选的数字与JOIN合并，从而得到JOIN_1，JOIN_2或JOIN_3。然后将选定的宏与原始参数一起使用，并将它们简单地合并为一个整数文字。

That chosen number is then merged wtih JOIN, resulting in either JOIN_1, JOIN_2, or JOIN_3. The chosen macro is then used with original arguments and simply merges them into a single integer literal.

可以通过手动定义更多的JOIN_X宏（其中X是连续的）来扩展此示例数。同时必须更改宏COUNT和COUNT_PICK。

This example can be expanded by manually defining more JOIN_X macros where X is a consecutive number. Simultaneously the macros COUNT and COUNT_PICK, must be altered as well.

另一个好处是，传递无效参数，例如：

As an additional benefit, passing invalid arguments, like:

JOIN( 10,+44 );
JOIN( 10,-44 );
JOIN( 10,*44 );
JOIN( 10,/44 );
JOIN( /10,44 );
//etc...

会产生编译时间警告，但仍然允许将导致有效整数常量的参数。

will yield a compile time warning, but still allows for arguments that will result in a valid integer constant.

要与经过SVC14（Microsoft Visual Studio 2015 Update 3）测试的Microsoft编译器一起使用，必须对代码进行修改。 COUNT_PICK和MERGE宏必须使用其他扩展宏进行包装：

To be used with a Microsoft compiler, tested with SVC14 (Microsoft Visual Studio 2015 Update 3), the code must be amended. Macros COUNT_PICK and MERGE must be wrapped with an additional expand macro:

#define EXPAND(...)   __VA_ARGS__

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