具有可访问构造函数的std :: vector <T> :: iterator

Question

g++ implementation of STL allows me in my client code to construct an iterator of std::vector<T>::iterator type with arbitrary address value:

int* i_ptr;
std::vector< int >::iterator it(i_ptr);

Is it standard ability, or just rudiment of implementation?

Where it designed to be used?

Answer 1

This is just an implementation detail, it is not even guaranteed to compile by the standard. It is also not listed in gcc's implementation defined behavior nor in the GNU C++ extensions, so your snippet may break at any version change.

Don't rely on this "working" in any sense in your code.

Answer 2

In Visual Studio 2017 RC this does not compile:

main.cpp(8): error C2664: 'std::_Vector_iterator<std::_Vector_val<std::_Simple_types<int>>>::
_Vector_iterator(std::_Vector_iterator<std::_Vector_val<std::_Simple_types<int>>> &&)':
cannot convert argument 1 from 'int *' to
'const std::_Vector_iterator<std::_Vector_val<std::_Simple_types<int>>> &'

Answer 3

I do not think that it is a standard ability. Just an implementation defined feature.

In many implementations vector iterators are just simple pointers. So if you have a vector< T > then iterator is simply a pointer to type T . It is done for some optimization purposes.

In such implementations it is possible to create iterators initialized from the pointers to type T because you can initialize a pointer to type T by another pointer of such type.

But it is implementation defined, of course, if an iterator defined as a pointer to type T or not.