使用[]的C ++地图插入

Question

I see that the code in subscript operator is like this for a std::map when the key is not found. But here they add a default value.

__i = insert(__i, value_type(__k, mapped_type()));

Now if I do

std::map<std::string,int> mapped;
mapped["current"] = 5;

I wanted to know how is this pair added to the map using the above statement.

Answer 1

operator[] inserts the element of the map if it was not found, and then returns it.

Therefore, the parameters key (__i) and value (__k) are added to the map.

This is one STL implementation:

_Tp& operator[](const key_type& k) { // k is the parameter you passed

    iterator i = lower_bound(k);

    // if i -> first >= k then insert a place for the new element.
   if (i == end() || key_comp()(k, (*i).first))
       i = insert(i, value_type(k, _Tp()));

    // return a reference to the element mapped to by the key
    return i -> second;
 }

Since a reference is returned, this acts the exact same way as accessing an array does:

You can assign ray[5] = 10 and access int i = ray[5] .