What's the difference between istringstream, ostringstream and stringstream? / Why not use stringstream in every case?

Personally, I find it very rare that I want to perform streaming into and out of the same string stream.

Usually I want to either initialize a stream from a string and then parse it; or stream things to a string stream and then extract the result and store it.

If you're streaming to and from the same stream, you have to be very careful with the stream state and stream positions.

Using 'just' istringstream or ostringstream better expresses your intent and gives you some checking against silly mistakes such as accidental use of << vs >>.

There might be some performance improvement but I wouldn't be looking at that first.

There's nothing wrong with what you've written. If you find it doesn't perform well enough, then you could profile other approaches, otherwise stick with what's clearest. Personally, I'd just go for:

std::string stHehe( "Hello stackoverflow.com!" );

A stringstream is somewhat larger, and might have slightly lower performance -- multiple inheritance can require an adjustment to the vtable pointer. The main difference is (at least in theory) better expressing your intent, and preventing you from accidentally using >> where you intended << (or vice versa). OTOH, the difference is sufficiently small that especially for quick bits of demonstration code and such, I'm lazy and just use stringstream. I can't quite remember the last time I accidentally used << when I intended >>, so to me that bit of safety seems mostly theoretical (especially since if you do make such a mistake, it'll almost always be really obvious almost immediately).

Nothing at all wrong with just using a string, as long as it accomplishes what you want. If you're just putting strings together, it's easy and works fine. If you want to format other kinds of data though, a stringstream will support that, and a string mostly won't.

In most cases, you won't find yourself needing both input and output on the same stringstream, so using std::ostringstream and std::istringstream explicitly makes your intention clear. It also prevents you from accidentally typing the wrong operator (<< vs >>).

When you need to do both operations on the same stream you would obviously use the general purpose version.

Performance issues would be the least of your concerns here, clarity is the main advantage.

Finally there's nothing wrong with using string append as you have to construct pure strings. You just can't use that to combine numbers like you can in languages such as perl.