Can a temporary stringstream object be used?

Question

This works:

stringstream temp;
temp << i;
result_stream << transform(temp.str());

(transform is a function that takes a string and returns a string; i is an int). However, my attempt to let C++11 create a temporary object without a name didn't work:

result_stream << transform((stringstream() << i).str());

I thought it would work, since the second << should just return the first argument and I'd be able to use str() on that. But I get this error:

error: 'class std::basic_ostream<char>' has no member named 'str'

I'm using g++ 4.8.1 (MinGW-W64).

Is there a way to accomplish this (i.e. write code like this using an unnamed temporary)? (The above code is a bit simplified, and the actual code involves using << on arguments other than int.)

Answer 1

This doesn't work because the second << is std::ostream &operator<<(std::ostream &, int); and so the return type is ostream& which has no member str().

You would have to write:

result_stream << transform( static_cast<stringstream &>(stringstream() << i).str() );

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Update (2019): According to LWG 1203 the standard may be changed in future (and one major implementation already has) so that this code no longer works, and a simpler code works instead. See this question for detail.

In the interim period, apparently the following works on both old and new:

result_stream << transform( static_cast<stringstream &>(stringstream().flush() << i).str() );
//                                                                    ^^^^^^^^

This should not be a performance penalty since flushing an empty stream has no effect...

Answer 2

operator<<() returns a reference to the base class std::ostream contained within the std::stringstream. The base class doesn't contain the str() method. You can cast it back down to a std::stringstream&:

result_stream << transform(static_cast<std::stringstream&>(std::stringstream() << i).str());