Uses for the capacity value of a string

Question

In the C++ Standard Library, std::string has a public member function capacity() which returns the size of the internal allocated storage, a value greater than or equal to the number of characters in the string (according to here). What can this value be used for? Does it have something to do with custom allocators?

Answer 1

You are more likely to use the reserve() member function, which sets the capacity to at least the supplied value.

The capacity() member function itself might be used to avoid allocating memory. For instance, you could recycle used strings through a pool, and put each one in a different size bucket based on its capacity. A client of the pool could then ask for a string that already has some minimum capacity.

Answer 2

The string::capacity() function return the number of total characters the std::string may contain before it has to reallocate memory, which is quite an expensive operation.

A std::vector works in the same way so I suggest you look up std::vector here for an in detail explanation of the difference between allocated and initialized memory.

Update

Hmm I can see I misread the question, actually I think I've never used capacity() myself on either std::string or std::vector, seems to seldomely be of any reason as you have to call reserve anyway.