Should I preallocate std::stringstream?

Question

I use std::stringstream extensively to construct strings and error messages in my application. The stringstreams are usually very short life automatic variables.

Will such usage cause heap reallocation for every variable? Should I switch from temporary to class-member stringstream variable?

In latter case, how can I reserve stringstream buffer? (Should I initialize it with a large enough string or is there a more elegant method?)

Answer 1

Have you profiled your execution, and found them to be a source of slow down?

Consider their usage. Are they mostly for error messages outside the normal flow of your code?

As far as reserving space...

Some implementations probably reserve a small buffer before any allocation takes place for the stringstream. Many implementations of std::string do this.

Another option might be (untested!)

std::string str; str.reserve(50); std::stringstream sstr(str); 

You might find some more ideas in this gamedev thread.

edit:

Mucking around with the stringstream's rdbuf might also be a solution. This approach is probably Very Easy To Get Wrong though, so please be sure it's absolutely necessary. Definitely not elegant or concise.

Answer 2

Although "mucking around with the stringstream's rdbuf...is probably Very Easy To Get Wrong", I went ahead and hacked together a proof-of-concept anyway for fun, as it has always bugged me that there is no easy way to reserve storage for stringstream. Again, as @luke said, you are probably better off optimizing what your profiler tells you needs optimizing, so this is just to address "What if I want to do it anyway?".

Instead of mucking around with stringstream's rdbuf, I made my own, which does pretty much the same thing. It implements only the minimum, and uses a string as a buffer. Don't ask me why I called it a VECTOR_output_stream. This is just a quickly-hacked-together thing.

constexpr auto preallocated_size = 256; auto stream = vector_output_stream(preallocated_size); stream << "My parrot ate " << 3 << " cookies."; cout << stream.str() << endl;