如何从STL容器实例中获取元素类型？

Question

I know about value_type, key_type... but they operate on types, not on instances. I tried stuff like :

std::set<uint64_t> mySet;   

decltype (mySet)::value_type pos;

But it doesnt work.

EDIT: I use VS 2010.

EDIT2: the prupose of this code was to get a type to give it to boost::lexical_cast<> is there a workaround that enables that ? I want something like this:

   mySet.insert(boost::lexical_cast<decltype(mySet)::value_type>(*it));
  // it is a iterator in vector of strings

EDIT3 : this works:

mySet.insert(boost::lexical_cast<decltype(mySet)::value_type>(*it));

Answer 1

decltype (mySet)::value_type is correct. Make sure you have C++11 mode enabled in your compiler. If you have, then it's a compiler bug.

A possible workaround involves using the identity metafunction:

template <typename T>
struct identity { typedef T type; };

identity<decltype(mySet)>::type::value_type pos;

Answer 2

I'd do it the other way around:

typedef std::set<uint_least64_t> set_type;
set_type mySet;
set_type::value_type pos;

Answer 3

Here is a simple example of print method that prints elements of a priority queue:

template<typename T> void print_queue(T& queue) {
    while (!queue.empty()) {
        std::cout << queue.top() << " ";
        queue.pop();
    }
    std::cout << '\n';
}

The problem is, that after printing of all elements the queue is empty. To restore the queue to original state we add vector container. The type of queue elements is deduced from the queue:

template<typename T> void print_queue(T& queue) {
    std::vector<T::value_type> vec;
    while (!queue.empty()) {
        std::cout << queue.top() << " ";
        vec.push_back(queue.top());
        queue.pop();
    }
    std::cout << '\n';
    for (auto & v : vec) {
        queue.push(v);
    }
}