问题描述

所以我有一个在 GCC 中运行良好的宏，但在 Microsoft 的 C++ 编译器中不起作用.我希望有人可能知道一种解决方法，或者可以向我解释为什么它会这样.

So I've got a macro that works nicely in GCC, but not in Microsoft's C++ Compiler. I'm hoping somebody might know of a workaround, or perhaps can explain to me why it behaves this way.

我确定这个宏并不完全是标准的"，但它确实对我有帮助.

I'm sure this macro isn't exactly "standard", but it would really help me out.

这是宏的功能示例:

#define VA_NARGS_IMPL(_1, _2, _3, _4, _5, N, ...) N
#define VA_NARGS(...) VA_NARGS_IMPL(__VA_ARGS__, 5, 4, 3, 2, 1)

#define FULLY_EXPANDED(count, ...) 
  MAC ## count (__VA_ARGS__)

#define SEMI_EXPANDED(count, ...) FULLY_EXPANDED(count, __VA_ARGS__)

#define EXPAND_THESE(...) SEMI_EXPANDED(VA_NARGS(__VA_ARGS__), __VA_ARGS__)

#define ACTUAL_MACRO(x) parent->GetProperty<x>();
#define MAC1(a) ACTUAL_MACRO(a)
#define MAC2(a,b) MAC1(a) ACTUAL_MACRO(b)
#define MAC3(a,b,c) MAC2(a,b) ACTUAL_MACRO(c)
#define MAC4(a,b,c,d) MAC3(a,b,c) ACTUAL_MACRO(d)
#define MAC5(a,b,c,d,e) MAC4(a,b,c,d) ACTUAL_MACRO(e)

这里是我可以使用这个宏的方法:

Here is how I might use this macro:

struct MyStructure
{
  void Foo()
  {
    EXPAND_THESE(Property1, Property2, Property3, Property4)
  }

  Base * parent;
}

以下是 GCC 如何扩展上述内容:

Here's how GCC expands the above:

struct MyStructure
{
  void Foo()
  {
    parent->GetProperty<Property1>(); 
    parent->GetProperty<Property2>(); 
    parent->GetProperty<Property3>(); 
    parent->GetProperty<Property4>();
  }

  Base * parent;
}

但微软出于某种原因将我所有的 __VA_ARGS__ 扩展为一个参数:

But Microsoft for some reason expands all my __VA_ARGS__ as one argument:

struct MyStructure
{
  void Foo()
  {
    parent->GetProperty<Property1, Property2, Property3, Property4>();
  }

  Base * parent;
}

有人知道这是为什么吗?有什么技巧可以让微软像 GCC 一样扩展它吗?也许多加几对括号?

Does anybody know why this is? Is there some trick I can pull to get Microsoft to expand this like GCC? Maybe toss in a couple extra pairs of parentheses?

像这样的宏确实可以帮助我替换一堆胶水"代码，但是由于这个问题，我无法将其移动到我的 VS 项目中.任何帮助将不胜感激！

Macros like this could really help me out in replacing a bunch of "glue" code, but because of this problem, I can't move it into my VS project. Any help would be greatly appreciated!

谢谢.

推荐答案

巧合的是，我今天刚好遇到了这个问题，经过足够的努力，我想我已经找到了适合自己的解决方案.错误是 MSVC 将 __VA_ARGS__ 视为参数列表中的单个标记.但是您可以通过不在宏调用参数列表中直接使用它来解决此问题.此评论 建议开始回答您的问题:

Coincidentally, I happened to run into this problem just today, and after enough effort I think I've found a solution for my own purposes. The bug is MSVC treats __VA_ARGS__ as a single token in argument lists. But you can work around this by not using it directly within a macro call argument list. This comment suggests the start of an answer to your problems:

#define VA_NARGS(...) VA_NUM_ARGS_IMPL_((__VA_ARGS__, 5,4,3,2,1))
#define VA_NARGS_IMPL_(tuple) VA_NUM_ARGS_IMPL tuple
#define VA_NARGS_IMPL(_1,_2,_3,_4,_5,N,...) N

但是我怀疑您可能会遇到确保完全扩展到您想要的实际N"的问题，而不是 VA_NARGS_IMPL (arg1, arg2, 5, 4, 3, 2, 1) ，比如说.我发现我的代码(看起来像你的)必须更改以将 MAC##code 全部扩展为一个单元，然后必须将其与参数列表分开组合.这是我发现对我有用的代码:

But then I suspect you'll likely run into the issue of making sure that gets fully expanded to the actual "N" you want, and not to VA_NARGS_IMPL (arg1, arg2, 5, 4, 3, 2, 1), say. I found that my code (which looked like yours) had to change to expand MAC##code all as one unit, and then that had to be separately combined with the argument list. Here's the code that I found worked for me:

#define ASSERT_HELPER1(expr) singleArgumentExpansion(expr)
#define ASSERT_HELPER2(expr, explain) 
   twoArgumentExpansion(expr, explain)

/*
 * Count the number of arguments passed to ASSERT, very carefully
 * tiptoeing around an MSVC bug where it improperly expands __VA_ARGS__ as a
 * single token in argument lists.  See these URLs for details:
 *
 *   http://connect.microsoft.com/VisualStudio/feedback/details/380090/variadic-macro-replacement
 *   http://cplusplus.co.il/2010/07/17/variadic-macro-to-count-number-of-arguments/#comment-644
 */
#define COUNT_ASSERT_ARGS_IMPL2(_1, _2, count, ...) 
   count
#define COUNT_ASSERT_ARGS_IMPL(args) 
   COUNT_ASSERT_ARGS_IMPL2 args
#define COUNT_ASSERT_ARGS(...) 
   COUNT_ASSERT_ARGS_IMPL((__VA_ARGS__, 2, 1, 0))
 /* Pick the right helper macro to invoke. */
#define ASSERT_CHOOSE_HELPER2(count) ASSERT_HELPER##count
#define ASSERT_CHOOSE_HELPER1(count) ASSERT_CHOOSE_HELPER2(count)
#define ASSERT_CHOOSE_HELPER(count) ASSERT_CHOOSE_HELPER1(count)
 /* The actual macro. */
#define ASSERT_GLUE(x, y) x y
#define ASSERT(...) 
   ASSERT_GLUE(ASSERT_CHOOSE_HELPER(COUNT_ASSERT_ARGS(__VA_ARGS__)), 
               (__VA_ARGS__))

int foo()
{
  ASSERT(one); // singleArgumentExpansion(one)
  ASSERT(two, "foopy"); // twoArgumentExpansion(two, "foopy")
}

在解决了我自己的问题几个小时后，我的思绪太混乱了，无法完全解决你的问题，我很抱歉地说.:-) 但我认为这足以让你做一些有用的事情，只需做一点工作.

My mind is too much mush after a few hours solving my own issues to then go and completely solve yours, I'm sorry to say. :-) But I think this is enough to get you to something that works, with a little work.

这篇关于MSVC++ 可变参数宏扩展的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持跟版网！