g++ allows this construction of an istream_iterator from an ifstream instance:
std::ifstream ifstr("test.txt");
std::istream_iterator<std::string> iter1(ifstr);
...but it doesn't allow the same construction with an unnamed temporary:
std::istream_iterator<std::string> iter2(std::ifstream("test.txt"));
This gives:
error: no matching function for call to ‘std::istream_iterator, ptrdiff_t>::istream_iterator(std::ifstream)’
Does anyone know why this doesn't work? - thanks!
It doesn't, because istream_iterator's constructor parameter is a non-const reference, but you provide a temporary. You cannot provide temporaries (that are rvalues) to non-const references.
But aside, even if it would take a const reference, it would still not work, because ifstream is not copyable. Curiously, C++ requires an accessible copy constructor to bind an rvalue to a non-const reference.
The stream is passed by non-const reference, but a temporary can only be passed by const-reference.
Streams are essentially always passed by non-const reference, because almost anything you do with a stream can/will modify the stream's state.