如何写`is_complete`模板？ [英] How to write `is_complete` template?

问题描述

回答这个问题后，我试图找到在Boost库中的is_complete 模板，我意识到在Boost.TypeTraits中没有这样的模板。为什么Boost库中没有这样的模板？它应该是什么样子？

  //！检查类型是否完整
模板< typename T> 
 struct is_complete 
 {
 static const bool value =（sizeof（T）> 0）; 
}; 
 
 ... 
 
 //所以我可以这样使用它
 BOOST_STATIC_ASSERT（boost :: is_complete< T> :: value）; 
  

上述代码不正确，因为应用 sizeof 为不完整类型。

---

那么，什么是好的解决方案？ ODR规则的情况下解决，但是有一个特定的平台解决方案，适用于我。

 
 
 code> namespace 
 {
 template< class T，int discriminator> 
 struct is_complete {
 static T& getT（）; 
 static char（& pass（T））[2]; 
 static char pass（...）; 
 static const bool value = sizeof（pass（getT（）））== 2; 
}; 
} 
 #define IS_COMPLETE（X）is_complete< X，__ COUNTER __> :: value 
  

很遗憾， COUNTER 预定义宏不是标准的一部分，因此不会在每个编译器上工作。

After answering this question I was trying to find is_complete template in Boost library and I realized that there is no such template in Boost.TypeTraits. Why there is no such template in Boost library? How it should look like?

//! Check whether type complete
template<typename T>
struct is_complete
{   
  static const bool value = ( sizeof(T) > 0 );
};

...

// so I could use it in such a way
BOOST_STATIC_ASSERT( boost::is_complete<T>::value );

The code above is not correct, because it is illegal to apply sizeof to an incomplete type. What will be a good solution? Is it possible to apply SFINAE in this case somehow?

---

Well, this problem couldn't be solved in general without violating the ODR rule, but there is there a platform specific solution which works for me.

解决方案

The answer given by Alexey Malistov can be used on MSVC with a minor modification:

namespace 
{
    template<class T, int discriminator>
    struct is_complete {  
      static T & getT();   
      static char (& pass(T))[2]; 
      static char pass(...);   
      static const bool value = sizeof(pass(getT()))==2;
    };
}
#define IS_COMPLETE(X) is_complete<X,__COUNTER__>::value

Unfortunately, the COUNTER predefined macro is not part of the standard, so it would not work on every compiler.

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