Question

C++11

There should be a one-line version of the last two lines.

        typedef std::pair<T1, T2> impl_node;
        std::vector<impl_node> impl;
        /* do stuff with impl */
        std::vector<T1> retval(impl.size());
        std::transform(impl.cbegin(), impl.cend(), retval.begin(),
                         [](const impl_node& in) { return *in.first; });

I tried writing some sort of custom iterator adapter, and the types are getting hairy. What’s the “right” solution? (And it probably generalizes to all sorts of other adapters.)

Answer 1

This is still two lines, but less typing (in both senses):

std::vector<T1> retval(impl.size());
for (const auto& p : impl) retval.push_back(p.first); 

Actually, now that I look at it, I’d prefer three lines:

std::vector<T1> retval;
retval.reserve(impl.size());
for (const auto& p : impl) retval.push_back(p.first); 

(Edited to remove move because there’s no evidence that it’s appropriate)