Complicated copying with std::transform

You might have noticed, way back when we presented the implementation of std::copy, that the value_type of the two iterator type parameters were not constrained to be the same. This is a feature, not a bug! It means that we can write code that relies on implicit conversions and it will just Do The Right Thing:

    std::vector<const char *> input = {"hello", "world"};
    std::vector<std::string> output(2);

    std::copy(input.begin(), input.end(), output.begin());

    assert(output[0] == "hello");
    assert(output[1] == "world");

Looks trivial, right? Look closely! Deep within our instantiation of std::copy is a call to the implicit constructor that converts const char * (the type of *input.begin()) to std::string (the type of *output.begin()). So for the umpteenth time, we're seeing an example of generic code that does surprisingly complicated operations simply by virtue of being given certain iterator types.

But sometimes you want to...