C++ 迭代器->秒是什么意思？
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What does iterator->second mean?
提问by Noich
In C++, what is the type of a std::map<>::iterator?
在 C++ 中， a 的类型是std::map<>::iterator什么？
We know that an object itof type std::map<A,B>::iteratorhas an overloaded operator ->which returns a std::pair<A,B>*, and that the std::pair<>has a firstand secondmember.
我们知道一个it类型的对象std::map<A,B>::iterator有一个重载operator ->，它返回 a std::pair<A,B>*，并且std::pair<>有 afirst和second成员。
But, what do these two members correspond to, and why do we have to access the value stored in the map as it->second?
但是，这两个成员对应什么，为什么我们必须访问存储在地图中的值it->second？
回答by Joseph Mansfield
I'm sure you know that a std::vector<X>stores a whole bunch of Xobjects, right? But if you have a std::map<X, Y>, what it actually stores is a whole bunch of std::pair<const X, Y>s. That's exactly what a map is - it pairs together the keys and the associated values.
我确定您知道 astd::vector<X>存储了一大堆X对象，对吗？但是如果你有一个std::map<X, Y>，它实际上存储的是一大堆std::pair<const X, Y>s。这正是映射的含义——它将键和关联的值配对在一起。
When you iterate over a std::map, you're iterating over all of these std::pairs. When you dereference one of these iterators, you get a std::paircontaining the key and its associated value.
当您迭代 a 时std::map，您就是在迭代所有这些std::pairs。当您取消引用这些迭代器之一时，您会得到一个std::pair包含键及其关联值的对象。
std::map<std::string, int> m = /* fill it */;
auto it = m.begin();
Here, if you now do *it, you will get the the std::pairfor the first element in the map.
在这里，如果您现在执行*it，您将获得std::pair地图中第一个元素的 。
Now the type std::pairgives you access to its elements through two members: firstand second. So if you have a std::pair<X, Y>called p, p.firstis an Xobject and p.secondis a Yobject.
现在，该类型std::pair使您可以通过两个成员访问其元素：first和second。因此，如果您有一个std::pair<X, Y>被调用的p,p.first是一个X对象并且p.second是一个Y对象。
So now you know that dereferencing a std::mapiterator gives you a std::pair, you can then access its elements with firstand second. For example, (*it).firstwill give you the key and (*it).secondwill give you the value. These are equivalent to it->firstand it->second.
所以现在您知道取消引用std::map迭代器会给您 a std::pair，然后您可以使用firstand访问其元素second。例如，(*it).first会给你钥匙，(*it).second会给你价值。这些等价于it->first和it->second。
回答by Andy Prowl
The type of the elements of an std::map(which is also the type of an expression obtained by dereferencing an iterator of that map) whose key is Kand value is Vis std::pair<const K, V>- the key is constto prevent you from interfering with the internal sorting of map values.
an 的元素类型std::map（也是通过解引用该映射的迭代器获得的表达式的类型），其键是K，值是Vis std::pair<const K, V>- 关键是const防止您干扰映射值的内部排序。
std::pair<>has two members named firstand second(see here), with quite an intuitive meaning. Thus, given an iterator ito a certain map, the expression:
std::pair<>有两个名为firstand second（见这里）的成员，具有相当直观的含义。因此，给定i某个映射的迭代器，表达式：
i->first
Which is equivalent to:
这相当于：
(*i).first
Refers to the first(const) element of the pairobject pointed to by the iterator - i.e. it refers to a keyin the map. Instead, the expression:
指的是迭代器指向的对象的第一个( const) 元素pair——即它指的是映射中的一个键。相反，表达式：
i->second
Which is equivalent to:
这相当于：
(*i).second
Refers to the secondelement of the pair- i.e. to the corresponding valuein the map.
指的是-的第二个元素，pair即映射中的对应值。
