I'd like to clear out and reuse an ostringstream (and the underlying buffer) so that my app doesn't have to do as many allocations. How do I reset the object to its initial state?
Worth pointing out that this won't re-use the underlying buffer from the ostringstream - it just assigns a new buffer. So while you're reusing the ostringstream object, you're still allocating two buffers. I don't think ostringstream is designed for reuse in the manner you intend.
sgreeve
2009-03-08 21:09:19
It also doesn't clear out the state, which is what .clear() does. I agree, it really isn't meant to be used like this. Just create a new one to be sure. Only if you profile will you find out if it makes any difference.
Brian Neal
2009-03-09 02:30:16
sgreeve, Brian, that's right. Note, however, how the litb's method above requires of the usage of std::ends. It reuses the buffer, but makes you code differently as usual with stringstreams (normally you don't use std::ends).
Diego Sevilla
2009-03-09 08:21:17
A:
You don't. Use two differently named streams for clarity and let the optimizing compiler figure out that it can reuse the old one.
Sebastian Ganslandt
2009-03-08 21:04:01
+26
A:
I've used a sequence of clear and str in the past:
// clear, because eof or other bits may be still set.
s.clear();
s.str("");
Which has done the thing for both input and output stringstreams. Alternatively, you can manually clear, then seek the appropriate sequence to the begin:
s.clear();
s.seekp(0); // for outputs: seek put ptr to start
s.seekg(0); // for inputs: seek get ptr to start
That will prevent some reallocations done by str by overwriting whatever is in the output buffer currently instead. Results are like this:
std::ostringstream s;
s << "hello";
s.seekp(0);
s << "b";
assert(s.str() == "bello");
If you want to use the string for c-functions, you can use std::ends, putting a terminating null like this:
std::ostringstream s;
s << "hello";
s.seekp(0);
s << "b" << std::ends;
assert(s.str().size() == 5 && std::strlen(s.str().data()) == 1);
std::ends is a relict of the deprecated std::strstream, which was able to write directly to a char array you allocated on the stack. You had to insert a terminating null manually. However, std::ends is not deprecated, i think because it's still useful as in the above cases.
Johannes Schaub - litb
2009-03-08 21:08:38