Thread: returning a stream from a function

Originally Posted by killme

So one example of this would be this function:

The problem here is that input is a local variable. When you return it, you have a problem, because once control returns from the function, the stream object is destroyed, yet you are returning a reference to it. Consequently, the caller would have a reference to an object that no longer exists, which is a Bad Thing.

Now, if input is going to be a member variable instead, then one solution is to get rid of that local variable (which you obviously will do in such a case) and use the open member function of the stream instead. Then, no_cc_msg will have void return type. Note that objects of this class will be not be copyable since input stream objects are not copyable, so you might want to declare the copy constructor and copy assignment operator as private to make that clear.

If you're using C++11, you could return a smart pointer to a new allocated stream object.

Code:

std::unique_ptr< std::istream > makeStream( /* various argument here ....... */ )
{
    return std::move( std::unique_ptr< std::istream >( new std::ifstream( /* input file path, etc. */ ) ) );
}

P.S. But your solution is probably better since it involves no superfluous mucking around with the freestore.

Originally Posted by laserlight

input stream objects are not copyable

They can't be copied, but under C++11, can they be moved?

Originally Posted by brewbuck

They can't be copied, but under C++11, can they be moved?

Yes:

Code:

#include <fstream>
#include <string>
#include <iostream>

std::ifstream test() { return std::ifstream("C:\\test.txt"); }

int main ()
{
	std::ifstream test2 = test();
	std::string txt;
	std::getline(test2, txt);
	std::cout << txt << std::endl;
}

To bring the point home, if you add

std::ifstream test3 = test2;

then you will get a compile error because you cannot copy the object.
Also, antred, your solution is silly. You don't need to move a unique_ptr. Furthermore, it is also silly to use a smart pointer in the first case since the streams are movable.