如何测试表达式是否是临时的？ [英] How to test whether expression is a temporary?

问题描述

使用以下宏：

  #define ASSERT_IF_TEMP（expr）static_assert 
  

我应该为问号放什么？

解决方案

首先我们应该澄清一下：临时是什么意思？

。从技术上讲， int（）不是一个临时的，但大多数人会将它们包含在自己对该术语的意义上。技术上，给定 std :: string s; ，然后 move（s）可能想把它作为一个与你的宏。

我上面提到的第一种临时是真正的prvalue表达式。这些是 std :: string（foo）或 int（） code> move（s）以及（当然）不是 s 。 decltype 运算符产生了上面讨论的第一种临时的非引用类型。对于第二种类型， move（s），它们是xvalue，它将产生一个右值引用。对于非临时性，即 s 情况，它将产生一个左值引用。

总而言之，我将定义三个精确的宏，您可以从中选择

  #define IS_LVALUE（...）std :: is_lvalue_reference< decltype（（__ VA_ARGS __））> :: value 
 #define IS_XVALUE（...）std :: is_rvalue_reference< decltype __VA_ARGS __））> :: value 
 #define IS_PRVALUE（...）！std :: is_reference< decltype（（__ VA_ARGS __））> :: value 
  pre> 
With the following macro:
#define ASSERT_IF_TEMP(expr) static_assert(?, "Is temporary!");
What should I put for question mark?
解决方案
First we should clarify: What do you mean by "temporary"?

Many people mean different things when they say temporary. Technically, int() is not a temporary, but most people will include them into their own meaning of that term. Technically, given std::string s;, then move(s) isn't a temporary either, but you may want to treat it as one with your macro. 

The first kind of "temporaries" I mentioned above are really "prvalue expressions". Those are the std::string("foo") or int() kind of things, but not the move(s) and also (for sure) not the s kind of things. The decltype operator yields a non-reference type for the first kind of "temporaries" I talked about above. For the second kind, move(s), which are xvalues, it will yield an rvalue reference. And for the "non-temporaries", i.e the s cases, it will yield an lvalue reference. 

So to summarize, I will define three precise macros, and you can choose from them
#define IS_LVALUE(...) std::is_lvalue_reference<decltype((__VA_ARGS__))>::value
#define IS_XVALUE(...) std::is_rvalue_reference<decltype((__VA_ARGS__))>::value
#define IS_PRVALUE(...) !std::is_reference<decltype((__VA_ARGS__))>::value


                        
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