强制键类型的一个std :: map不是const [英] force key type of a std::map not to be const

问题描述

C ++引用告诉我们std :: map

  typedef pair< const Key，T& value_type; 
  

是否可以强制键类型不是const？
我需要在一个模板方法中这样做

  template< class T& // T代表一个地图（std :: map，boost :: unordered_map或者其他..）
 void foo（const T& m）
 {
 typename T :: value_type： ：first_type x; 
 x = 0; //错误，因为x是const ... 
} 
  

解决方案>

不，不是。

这是因为map基于键执行其内部排序。

你应该使用提供的API函数;如果你可以自己修改密钥，其中使用一个结果改变一个键值（实际上我不认为任何做），适当的内部重新排序可能发生。

想想

---

但是，你可以写下这个：

  template< class T> 
 void foo（const T& m）
 {
 typename T :: key_type x; 
 x = 0; 
} 
  

---

std :: map type aliases

  key_type Key 
 mapped_type T 
 value_type pair< const Key，T> 
 key_compare比较
 value_compare用于比较元素的嵌套类
 allocator_type分配器
引用Allocator :: reference 
 const_reference Allocator :: const_reference 
迭代器双向迭代器
 const_iterator常量双向迭代器
 size_type无符号整数类型（通常与size_t相同）
 difference_type有符号整数类型（通常与ptrdiff_t相同）
指针Allocator :: pointer 
 const_pointer Allocator :: const_pointer 
 reverse_iterator reverse_iterator< iterator> 
 const_reverse_iterator reverse_iterator< const_iterator> 
  

C++ references tells us for a std::map

typedef pair<const Key, T> value_type;

Is it possible to force the Key Type not to be const ? I need to do this in a template method like

template<class T> // T represent a map in general (std::map, boost::unordered_map or whatever..)
void foo(const T& m)
{
  typename T::value_type::first_type x;
  x=0; // Wrong because x is const ...
}

解决方案

No, it's not.

This is because map performs its internal ordering based on key. If you could modify the key yourself, willy-nilly, all hell would break loose.

You should use the provided API functions; where the use of one results in changing a Key value (actually I don't think that any do), the appropriate internal re-ordering may take place.

Think of getters and setters, and their use in providing an alternative to messy/dangerous direct member access.

---

However, you could write this:

template<class T>
void foo(const T& m)
{
   typename T::key_type x;
   x = 0;
}

---

std::map type aliases

key_type                Key
mapped_type             T
value_type              pair<const Key,T>
key_compare             Compare
value_compare           Nested class to compare elements
allocator_type          Allocator
reference               Allocator::reference
const_reference         Allocator::const_reference
iterator                Bidirectional iterator
const_iterator          Constant bidirectional iterator
size_type               Unsigned integral type (usually same as size_t)
difference_type         Signed integral type (usually same as ptrdiff_t)
pointer                 Allocator::pointer
const_pointer           Allocator::const_pointer
reverse_iterator        reverse_iterator<iterator>
const_reverse_iterator  reverse_iterator<const_iterator>

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