c++ problems with temporary ostream objects

Question

I thought to transform this working code:

ofstream outfile("my_file.txt");
copy(v.begin(), v.end(), ostream_iterator<int>(outfile));

into this:

copy(v.begin(), v.end(), ostream_iterator<int>(ofstream("my_file.txt")));

In other words, I use an "anonymous", or unnamed, version of the ofstream object.

Two questions:

(1) Why does the second attempt fail?

(2) Is the second attempt even good stylistically, or is it better in C++ to keep everything explicitly named? I'm coming from a Python background, where objects are created on the fly all the time.

Thanks!!

Answer 1

The ostream_iterator<T> constructor takes a non-const reference to a stream object, while temporaries can be passed at most as const references (at least in C++03); the rationale for this is well explained in this question.

By the way, here there wasn't much choice about how to pass the stream: a const reference wouldn't have made sense (the ostream_iterator has to modify the stream), and a plain ostream isn't acceptable because it is non-copyable (slicing would kill polymorphism).

In Python you can get away with constructing/passing stuff on the fly because you are always dealing with references to reference-counted (and garbage-collected) objects.

The C++ object semantic is radically different - an object is an object, not a reference; to obtain a semantic similar to Python's you would have to allocate every object dynamically with new and pass them around wrapped in shared_ptr<T>.

is it better in C++ to keep everything explicitly named

Not necessarily - it's perfectly normal to create temporaries, but you must be aware of what you can and cannot do with them, how references affect their lifetime, etc.