c++ - std::vector resize downward

Question

The C++ standard seems to make no statement regarding side-effects on capacity by either resize(n), with n < size(), or clear().

It does make a statement about amortized cost of push_back and pop_back - O(1)

I can envision an implementation that does the usual sort of capacity changes ala CLRS Algorithms (e.g. double when enlarging, halve when decreasing size to < capacity()/4). (Cormen Lieserson Rivest Stein)

Does anyone have a reference for any implementation restrictions?

Answer 1

Calling resize() with a smaller size has no effect on the capacity of a vector. It will not free memory.

The standard idiom for freeing memory from a vector is to swap() it with an empty temporary vector: std::vector<T>().swap(vec);. If you want to resize downwards you'd need to copy from your original vector into a new local temporary vector and then swap the resulting vector with your original.

Updated: C++11 added a member function shrink_to_fit() for this purpose, it's a non-binding request to reduce capacity() to size().