使用宏中的模板参数的问题 [英] Trouble with template parameters used in macros

问题描述

我试图编译下面的代码，我得到一个专门的std :: vector行的错误，似乎一个参数被传入在某种程度上被认为是两个参数。

有没有特殊的方法/机制，通过这样的参数可以正确地传递给宏？

  #include< vector> 
 
 template< typename A> 
 struct AClass {}; 
 
 #define specialize_AClass（X）\ 
 template<> struct AClass< X> {X a; }; 
 
 
 specialize_AClass（int）// ok 
 
 specialize_AClass（s​​td :: vector< int，std :: allocator< int>>）// error 
 
 int main（）
 {
 return 0; 
} 
  

我得到的错误如下：

1行55：错误：宏specialize_AClass传递2个参数，但只需1
行15：错误：expected constructor，destructor，或'int'之前的类型转换
3由于-Wfatal-errors而导致编译终止。


链接： http： /codepad.org/qIiKsw4l

解决方案

您有两个选项。其中一个已经提到：使用 __ VA_ARGS __ 。然而这有一个缺点，它不工作在严格的C ++ 03，但需要一个足够的C99 / C ++ 0x兼容的预处理器。

另一个选项是对类型名称加括号。但不像另一个答案声称，它不像简单的括号类型名称一样简单。编写如下的专业化是错误的

  //错误，无效！ 
模板<> struct AClass<（int）> {Xa; }; 
 

我已经解决了这个问题（并且可能使用相同的底层）传递类型名称

  template< typename T> struct get_first_param; 
 template< typename R，typename P1> struct get_first_param< R（P1）> {
 typedef P1 type; 
}; 
  

这样， get_first_param< void（X）> :: type 表示类型 X 。现在您可以将宏重写为

  #define specialize_AClass（X）\ 
 template< struct AClass< get_first_param< void X> :: type> {
 get_first_param< void X> :: type a; 
}; 
  

你只需要传递括号中的类型。

I'm trying to compile the following piece of code, I get an error on the line which specializes std::vector, it seems the one parameter being passed-in is somehow being assumed to be two parameters. Is it perhaps something to do with angle-brackets?

Is there a special way/mechanism where by such parameters can be correctly passed to the macro?

#include <vector>

template<typename A>
struct AClass {};

#define specialize_AClass(X)\
template<> struct AClass<X> { X a; };


specialize_AClass(int) //ok

specialize_AClass(std::vector<int,std::allocator<int> >) //error

int main()
{
   return 0;
}

The error that I get is as follows:

1 Line 55: error: macro "specialize_AClass" passed 2 arguments, but takes just 1
2 Line 15: error: expected constructor, destructor, or type conversion before 'int'
3 compilation terminated due to -Wfatal-errors.

Link: http://codepad.org/qIiKsw4l

解决方案

You have two options. One of which was mentioned already: Using __VA_ARGS__. This however has the disadvantage that it doesn't work in strict C++03, but requires a sufficiently C99/C++0x compatible preprocessor.

The other option is to parenthesize the type-name. But unlike another answer claims, it's not as easy as just parenthesizing the type name. Writing a specialization as follows is ill-formed

// error, NOT valid!
template<> struct AClass<(int)> { X a; };

I have worked around this (and boost probably uses the same under the hood) by passing the type name in parentheses, and then building up a function type out of it

template<typename T> struct get_first_param;
template<typename R, typename P1> struct get_first_param<R(P1)> {
  typedef P1 type;
};

With that, get_first_param<void(X)>::type denotes the type X. Now you can rewrite your macro to

#define specialize_AClass(X) \
template<> struct AClass<get_first_param<void X>::type> { 
  get_first_param<void X>::type a; 
};

And you just need to pass the type wrapped in parentheses.

这篇关于使用宏中的模板参数的问题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IT屋！