为什么需要将对插入地图？

Question

Though I do not dislike it, but find it inconvinient to declare a pair<X,Y> object, or make a call to make_pair , in order to call map::insert . Why insert doesn't take two arguments instead to specify Key and Value respectively.

While I understand it is for compatibility with other STL containers, that exhibit value_type . But find method takes key_type which breaks this compatibility assertion. map has both key_type and mapped_type , so why cant map have:

iterator insert(const key_type&, const mapped_type&);

Yes, there are overloads of insert taking iterator(s). But this two-argument insert could have been mixed well.

Just one advantage I see is: less call stack usage.

EDIT : Just found out that insert is the only method that takes value_type , that is pair<X,Y> . Many other methods like find , erase , at , count , equal_range , lower_bound , upper_bound and operator[] take key_type .

Answer 1

All standard library containers define a value_type member type, and their interfaces generically operate in terms of this value_type : insert , push_back , push_front . The new interface emplace adds a way of constructing a value_type object as if by:

value_type(std::forward<Args>(args)...)

Basically, there is no special interface provided for the satellite-data associative containers (ie maps) which is aware of the special structure of value_type (which is defined, not perfectly well-known, to be pair<const key_type, mapped_type> ), with the exception of find and erase and operator[] , which take key_type arguments.

It is perhaps an oversight of the standard, or perhaps it was never deemed to be a problem, since you can always use make_pair , make_tuple or forward_as_tuple , or emplace , to create map value types.

(There is one problem with insert and move-only mapped types that has surfaced and is subject of this recent proposal .)