通过访问器函数使用重载的operator [] [英] Using overloaded operator[] via an accessor function

问题描述

我有一个访问器函数，返回一个类型的引用（std :: map）... ...

I have an accessor function that returns a const reference to a type (std::map) ... ...

myMap_t const& getMap() const {return paramMap;} 

类型具有重载 ] 运算符。然后使用 [] 运算符直接从getter函数中以下面的方式，但实际上是工作的语法。

The type has an overloaded [] operator. What is the syntax to then use the [] operator directly from the getter function in a way like the following, but that actually works.

parameter = contextObj.getMap()[key];

错误讯息是：

context.cpp:35: error: passing   
  'const std::map<
     std::basic_string<char, std::char_traits<char>, std::allocator<char> >, 
     float, 
     std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, 
     std::allocator<std::pair<
       const std::basic_string<char, std::char_traits<char>, std::allocator<char> >,
       float> > >'
as 'this' argument of 
  '_Tp& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const _Key&)  
with 
  _Key = std::basic_string<char, std::char_traits<char>, std::allocator<char> >,
  _Tp = float, 
  _Compare = std::less<std::basic_string<char, std::char_traits<char>,  td::allocator<char> > >, 
  _Alloc = std::allocator<std::pair<
    const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, 
    float> >]' 
discards qualifiers

推荐答案

问题是在地图中的 operator [] 是一个变异操作，你不能在对地图的const引用上调用它。它是变异的原因是它必须返回一个值的引用，并且为此，如果键在容器中不存在，它将插入一个新的默认构造值由该键并返回它的引用。

The problem is that operator[] in a map is a mutating operation, and you cannot call it on a const reference to a map. The reason that it is mutating is that it must return a reference to a value, and for that, if the key was not already present in the container, it will insert a new default constructed value addressed by that key and return the reference to it.

如果你有一个const引用，并且你想确定一个元素是否存在（或访问它），你必须使用 std： ：map<> :: find ，它将返回一个迭代器。如果元素不存在，迭代器的值为 m.end（）

If you have a const reference, and you want to determine whether an element is present (or access it), you must use std::map<>::find that will return an iterator. If the element is not present the value of the iterator would be m.end()

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