Question

I have the following function which creates a std::vector of iterators into another container:

template <typename T,
          template <typename, typename = std::allocator<T>> class Con>
std::vector<typename Con<T>::iterator> make_itervec(Con<T>& v)
{
    std::vector<typename Con<T>::iterator> itervec;
    for (auto i = v.begin(); i != v.end(); ++i)
    {
        itervec.push_back(i);
    }
    return itervec;
}

What I want to do is this:

template <typename T,
          template <typename, typename = std::allocator<T>> class Con>
auto make_itervec(Con<T>& v) -> decltype(x) // This line
{
    std::vector<typename Con<T>::iterator> itervec;
    for (auto i = v.begin(); i != v.end(); ++i)
    {
        itervec.push_back(i);
    }
    return itervec;
}

What do I put for x to get this to work?

Tried but failed attempts:

decltype(std::vector<typename Con<T>::iterator>)
decltype(std::vector<decltype(v)::iterator>)

(Also, I’m not an expert on this, so any other suggestions, comments are welcome! Thanks.)

Answer 1

You could use std::vector<decltype(v)::iterator> or std::vector<decltype(v.begin())>. Note that there’s no decltype around the std::vector because that one already is a type, not a variable or expression. decltype is used only to get the type of an expression.