可变宏中的参数计数无效 [英] Invalid argument count in a variadic macro

问题描述

我试图创建一个宏，它将迭代一个定义的术语列表，并为每个调用另一个宏，可能有额外的参数列表。这是我得到的：

  #define ITERATE_OVER_TERMS（MACRO，...）\ 
 MACRO ，__VA_ARGS__）\ 
 MACRO（Term2，__VA_ARGS__）\ 
 MACRO（Term3，__VA_ARGS__）\ 
 ...等等
  
 
 
 
但是，当我试图在Visual Studio 2015中使用它时，会出现错误
 警告C4003：宏'BODY'的实际参数不足
  
其中 BODY 是作为 MACRO 参数传递的宏的名称。 
 
 
 
为了缩小错误范围，我将我的示例缩减为以下内容：
 p> 
 
 
  #include< iostream> 
 
 #define ITERATE（MACRO，...）\ 
 MACRO（1，__VA_ARGS__）MACRO（2，__VA_ARGS__）
 
 #define BODY（IterationArg， Arg1，Arg2）\ 
 std :: cout<< IterationArg<< Arg1<< Arg2< std :: endl; 
 
 int main（）{
 ITERATE（BODY，8，9）
 return 0; 
} 
  
它给我的错误如上所示，而我期望它编译成功并产生输出
  189 
 289 
  
它似乎用g ++编译，但不是Visual Studio。 
我缺少什么？ 
解决方案
问题是Visual Studio扩展 __ VA_ARGS __ ，它们被传递到后续的宏，而不是之前。这也导致过去的问题，例如在这里 -  
 为什么这个可变参数计数宏与VC ++失败？ 
 
 
 
在你的情况下， / p> 
 
 
  #include< iostream> 
 
 #define ITERATE（MACRO，...）\ 
 MACRO（1，__VA_ARGS__）MACRO（2，__VA_ARGS__）
 
 #define BODY（IterationArg， Arg1，Arg2）\ 
 std :: cout<< ＃Arg1<< std :: endl; 
 
 int main（）{
 ITERATE（BODY，8，9）
 return 0; 
} 
  
参数＃Arg1 是字符串，显示我们在输出中的内容：
  8，9 
 8，9 
  
不是我们预期的，是吗？
 
 
 
与链接问题相同：通过虚拟 EXPAND 宏强制扩展：
  #define EXPAND（x）x 
 
 #define ITERATE（MACRO，...）\ 
 EXPAND（MACRO（1，__VA_ARGS__））EXPAND（MACRO __VA_ARGS__））
  
这将在VS和gcc中提供所需的结果。
 
I am trying to create a macro that would iterate over a defined list of terms, and for each invoke another macro, possibly with additional argument list. Here is what I have got:
#define ITERATE_OVER_TERMS(MACRO, ...) \
MACRO(Term1, __VA_ARGS__) \
MACRO(Term2, __VA_ARGS__) \
MACRO(Term3, __VA_ARGS__) \
... and so on
However, when I was trying to use it with Visual Studio 2015, i get an error
warning C4003: not enough actual parameters for macro 'BODY'
where BODY is the name of the macro passed as the MACRO argument. While technically a warning, it shows that something has gone wrong with the expansion.

In an attempt to narrow down the error, I reduced my example to the following:
#include <iostream>

#define ITERATE(MACRO, ...) \
MACRO(1, __VA_ARGS__) MACRO(2, __VA_ARGS__)

#define BODY(IterationArg, Arg1, Arg2) \
std::cout << IterationArg << Arg1 << Arg2 << std::endl;

int main() {
  ITERATE(BODY, 8, 9)
    return 0;
}
It gives me the error as shown above, while I expected it to compile succesfully and produce the output
189
289
It does seem to compile with g++, but not Visual Studio.
What am I missing? Is there some walkaround for this to work?
解决方案
The problem is that Visual Studio expands __VA_ARGS__ after they are being passed into a subsequent macro, and not before. This has caused problems in the past as well, for example here --
Why does this variadic argument count macro fail with VC++?

In your case, consider a simple change in the code:
#include <iostream>

#define ITERATE(MACRO, ...) \
MACRO(1, __VA_ARGS__) MACRO(2, __VA_ARGS__)

#define BODY(IterationArg, Arg1, Arg2) \
std::cout << #Arg1 << std::endl;

int main() {
  ITERATE(BODY, 8, 9)
    return 0;
}
The argument #Arg1 is stringified, showing us its contents in the output:
8, 9
8, 9
Not what we expected, huh?

A solution is the same as in the linked question: force an expansion through a dummy EXPAND macro:
#define EXPAND(x) x

#define ITERATE(MACRO, ...) \
EXPAND(MACRO(1, __VA_ARGS__)) EXPAND(MACRO(2, __VA_ARGS__))
This gives you the desired result both in VS and gcc.

                        
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