To be fair grow-but-don't-initialize is a pretty fundamental part of the API, the reserve() method.

But already the basic premise that you should push back without thinking is wrong. You will suffer reallocations and invalisations when you least expected them, and frankly you have to architect around that fact which is a terrible restriction. You can work around by pre reserving but at that point it's just a basic fixed heap allocated array but worse because the type gives you a weird look all the time, "I'll realloc as soon as you don't pay attention, harhar"!

std::vector lacks what I call the "bifurcated reservation API". It has Rust's Vec::reserve_exact but not Vec::reserve -- these APIs serve subtly different purposes, the former (which C++ calls just "reserve") says "Here's a hint for exactly how big this container will ever grow" while the latter says "Here's a hint for how big this container will get in my immediate future, but it might grow further later".

The implementation always tries to grow (if necessary) to the exact size chosen for Vec::reserve_exact, but for plain Vec::reserve if growth is needed it always grows exponentially, not to the exact size, preserving the O(1) push cost.

For a typical "doubling" growable array type, if we're pushing groups of ten items, reserve_exact or C++ grows like 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 ... which is much worse than O(1) whereas the correct reserve grows 10, 20, 40, 80, 160 preserving O(1)

For trivial types you can work around this in C++ with a little work, and for the non-trivial types you can work around it with a bunch more extra code, but you probably won't.

Bjarne among other people teaching C++ recommend just not using the reservation API as a reservation API because of this problem†, and the resulting teaching definitely leaks into CS graduates and even into languages which have the correct API and so you have to un-teach the bad lesson.

In applications where you actually can't afford to pay for growth (or at least in some cases can't afford this) I also like Vec::push_within_capacity which I believe comes from Rust for Linux where the kernel legitimately needs this "If there's room push, otherwise I have a plan B" approach.

† To Bjarne this API is instead conceived of as a way to preserve reference validity. Since it won't grow, our references will still work.