Difference between content of stream and the string `str()` returns?

Question

I'm using a small piece of code that generates PDF files, which I found on the internet, and tried to (softly) optimize it as the creation of a file would take ages. After profiling, I narrowed it down to the following piece of code : ``` std::ostringstream tmp; tmp << std::hex << std::uppercase << std::setfill('0') << std::setw(2) << r << " " << std::setw(2) << g << " " << std::setw(2) << b; out << tmp.str(); ``` found in a tight loop, with `out` being a `ostringstream` that contains basically the whole PDF content before it's written into the file. I found that `tmp.str()` was the line that took the most time in that loop, and saw when looking up the C++ reference that `str()` would return a copy of the stream's underlying string. Then, I thought removing that copy and using directly `out` would be faster. So I dumped `tmp` and directly did: ``` out << std::hex << std::uppercase << std::setfill('0') << std::setw(2) << r << " " << std::setw(2) << g << " " << std::setw(2) << b; ``` Bu…

Accepted Answer

The thing to consider is that the io manipulators (for example `std::hex`) are persistent from that point forward on the stream.

So once you insert the `std::hex` manipulator, all integral values are printed out in hex format from this point forward.

Your previous approach did not have this problem as the manipulator was on a transient stream. You could try inserting the `std::dec` manipulator once you are done...

Pulling up from Jan Hudec's comment, Boost IO State Savers are the nice way forward to handle this cleanly.

Difference between content of stream and the string `str()` returns?

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