const和非const键有什么区别？ [英] What is difference between const and non const key?

问题描述

以下两行之间有什么区别？

  map< int，float& map_data; 
 map< const int，float> map_data; 
  

解决方案

• code> int 和 const int 是两种不同的类型。



• p> std :: map< int，float> 和 std :: map&




std :: map之间的区别< const int，float> ; 和 std :: map< int，float> 是一个程度，类似于之间的区别，例如 std :: map< int，float> 和 std :: map< std :: string，float>





在非 - const ，内部键类型 仍然是非< - code> const int ：

  std :: map< const int，float> :: key_type => const int 
 std :: map< int，float> :: key_type => int 
  

但是，映射键在语义上是不可变的，允许直接访问键（例如，取消引用迭代器，它产生 value_type ） const ify key_type ：

  std :: map< const int，float> :: value_type => ; std :: pair< const int，float> 
 std :: map< int，float> :: value_type => std :: pair< const int，float> 
  

因此，对于您而言，

虽然：标准正式要求 您的键类型是可复制和可移动的，和一些实施重用地图节点;在这些实现下，尝试使用 const 键将无法工作。

What is the difference between the following two lines?

map<int, float> map_data;
map<const int, float> map_data;

解决方案

• int and const int are two distinct types.

• std::map<int, float> and std::map<const int, float> are, similarly, different types.

The difference between std::map<const int, float> and std::map<int, float> is, to a degree, analogous to the difference between, say, std::map<int, float> and std::map<std::string, float>; you get a fresh map type for each.

In the non-const case, the internal key type is still non-const int:

std::map<const int, float>::key_type       => const int
std::map<int, float>::key_type             => int

However, map keys are semantically immutable, and all map operations that allow direct access to keys (for example, dereferencing iterators, which yields value_type) does constify the key_type:

std::map<const int, float>::value_type => std::pair<const int, float>
std::map<int, float>::value_type       => std::pair<const int, float>

So the difference may be largely invisible to you in every way that matters, if your implementation allows it.

That's not always the case, though: the standard officially requires your key type to be copyable and moveable, and some implementations re-use map nodes; under those implementations, attempting to use a const key simply won't work.

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