map::const_iterator 映射类型不是 const

Question

I'm trying to write an iterator adaptor for a class built on top of a map. I'm having issue when trying to get the mapped type from the iterator type. Basically I'd like to get:

• map<Key,Val>::iterator --> Val
• map<Key,Val>::const_iterator --> Val const

Here is a minimal repro.

#include <map>
#include <type_traits>
    
template <typename BaseIteratorT>
using ValueType = typename BaseIteratorT::value_type::second_type;
    
// Passes
static_assert(
    std::is_same<ValueType<std::map<double, int>::iterator>, int>::value,
    "bad type for mutable iterator");

// Fails
static_assert(
    std::is_same<ValueType<std::map<double, int>::const_iterator>, const int>::value,
    "bad type for const iterator");

How can I achieve that (C++14)?

Answer 1

The value_type of a std::map is std::pair<const Key, T> . The second_type of that pair is just T , not const T , not const T& , just T . Dereferencing a map::const_iterator will return a const pair& reference, but that will not change that pair's second_type to const . If you want that, detect whether the iterator's value_type is const or not, such as with std::is_const , and if so then apply const to its second_type , such as with std::add_const .

Answer 2

For almost all practical purposes, types you might want to deduce is tied to some expression, in which case decltype is always the answer

template<typename T>
using mapped_type = std::remove_reference_t<decltype((std::declval<T>()->second))>;

using m = std::map<char, int>;
using i = mapped_type<m::iterator>  // int
using ci = mapped_type<m::const_iterator>  // const int