Question

I’m pretty new to C++, but I know you can’t just use memory willy nilly like the std::string class seems to let you do. For instance:

std::string f = "asdf";
f += "fdsa";

How does the string class handle getting larger and smaller? I assume it allocates a default amount of memory and if it needs more, it news a larger block of memory and copies itself over to that. But wouldn’t that be pretty inefficient to have to copy the whole string every time it needed to resize? I can’t really think of another way it could be done (but obviously somebody did).

And for that matter, how do all the stdlib classes like vector, queue, stack, etc handle growing and shrinking so transparently?

Answer 1

Usually, there’s a doubling algorithm. In other words, when it fills the current buffer, it allocates a new buffer that’s twice as big, and then copies the current data over. This results in fewer allocate/copy operations than the alternative of growing by a single allocation block.