Construct container with initializer list of iterators

Question

It's possible to construct a vector with an iterator range, like this:

std::vector<std::string> vec(std::istream_iterator<std::string>{std::cin},
                             std::istream_iterator<std::string>{});

But I can also compile and run code using C++11 uniform initialization syntax (note the bracers), like this:

std::vector<std::string> vec{std::istream_iterator<std::string>{std::cin},
                             std::istream_iterator<std::string>{}};

What's really going on here?

I know that a constructor taking an initializer list gets priority over other forms of construction. Shouldn't the compiler resolve to the constructor taking an initializer list containing 2 elements of std::istream_iterator? This should be an error as a std::istream_iterator can't be converted to the vectors value type std::string, right?

Answer 1

From §13.3.2/1 ([over.match.list])

When objects of non-aggregate class type T are list-initialized (8.5.4), overload resolution selects the constructor in two phases:

— Initially, the candidate functions are the initializer-list constructors (8.5.4) of the class T and the argument list consists of the initializer list as a single argument.

— If no viable initializer-list constructor is found, overload resolution is performed again, where the candidate functions are all the constructors of the class T and the argument list consists of the elements of the initializer list.

In your case the initializer list constructor is deemed non-viable (because std::istream_iterator<std::string> is not convertible to std::string), and the second condition applies. This results in the constructor taking 2 iterators to be selected.