non-copying istringstream

Question

So istringstream copies the contents of a string when initialised, e.g

string moo("one two three four");
istringstream iss(moo.c_str());

I was wondering if there's a way to make std::istringstream use the given c_str as its buffer without copying things. This way, it won't have to copy large bits of memory before passing the std::istringstream& to functions that take istream& as an argument.

What I've been trying to do is convert some functions which only take std::ifstream& arguments (they're mostly parsers) into taking istream& as well. Would I have to make my own istream subclass for this?

Answer 1

Using istringstream is not a satisfactory solution, because this copies the entire buffer.

A previous answer suggests the deprecated istrstream, but as this generates warnings and may be removed in future, a better solution is to use boost::iostreams:

boost::iostreams::stream<boost::iostreams::array_source> stream(moo.c_str(), moo.size());

This avoids copying the buffer in the same way istrstream did, and saves you having to write your own stream class.

Answer 2

It's fairly trivial to write a basic std::streambuf class that reads from a given memory area. You can then construct an istream from this and read from that.

initializing a C++ std::istringstream from an in memory buffer?

Note that the lifetime of the buffer pointed to be c_str() is very limited, though, and there's no guarantee that a call to c_str() want cause some copying although I don't know of any implementations where it does.