How can I keep track of the number of bytes written to an std::ostream object?

Question

I want to write a bunch of data to an ostream object and return the number of bytes written. For example:

using namespace std; size_t writeStuffToStream(ostream &stream) { stream << some_string << some_integer << some_other_arbitrary_object << endl; return number_of_bytes_written; }

There is the obvious workaround of writing everything to a stringstream and getting the byte count out of that, and then writing the stringstream to the stream, but that takes extra time and memory.

I also realize that if if all the data I wanted to write were preexisting strings, then there would be no problem. It's some_integer and some_other_arbitrary_object that are the problem.

you can chain streambuf objects, so you can derive a counting streambuf from std::streambuf. in it, delegate operations to the original streambuf after recording the counter info. There's a nice tutorial here: mr-edd.co.uk/blog/beginners_guide_streambuf — Richard Hodges
– Richard Hodges, Commented Dec 15, 2016 at 21:50

Nicolas Defranoux · Accepted Answer · 2016-12-15 21:20:14Z

Use the ostream tellp() method.

Note that this might fail if the provided ostream does not support positions. In that case you can create a temporary ostringstream to format your data, then extract the string, get its length and send it to the input ostream.

You can probably also write a custom ostream that send to another ostream and count emitted characters. I expected to find a virtual method to override in ostream to write just characters, but I did not find it :( You can re-use the stringstream code and replace the buffer writes to writes to an other ostream. string-stream.cc is about 500 lines long, so that's not this bad.

Yeah, I'm a little put off by the fact that tellp() is not guaranteed to work for all objects. And I already mentioned that I don't want to have to create an intermediate stringstream. How would I create a custom ostream that can count the bytes written, if I can't count the bytes written in the first place?

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