When does std::vector's storage automatically contract?

Question

https://en.cppreference.com/w/cpp/container/vector says:

The storage of the vector is handled automatically, being expanded and contracted as needed.

But I can't find a case where contraction would happen "as needed". None of the shrinking operations contract std::vector's capacity:

clear():

Leaves the capacity() of the vector unchanged

resize():

Vector capacity is never reduced when resizing to smaller size because that would invalidate all iterators

erase():

Invalidates iterators and references at or after the point of the erase, including the end() iterator.

This implies that iterators and references before the point of the erase don't get invalidated, which would not be the case if the capacity changed.

pop_back():

Iterators and references to the last element, as well as the end() iterator, are invalidated.

The same applies here.

shrink_to_fit() does contract the std::vector, but it was added in C++11 and the sentence above ("and contracted") isn't marked as only applying to C++11 and up. Also, an explicit call to shrink_to_fit() is not automatic ("The storage of the vector is handled automatically") and doesn't happen "as needed".

So when does std::vector automatically contract its storage?

Answer 1

After looking at the source code of the libstdc++, i didn't find any situation where the memory is freed.

It looks like the standard opens the door to complex memory management implementation but the widely known ones chose to never do that.