RAII doesn't imply allocating.

My guess is that you're assuming all user defined types, and maybe even all non-trivial built-in types too, are boxed, meaning they're allocated on the heap when we create them.

That's not the case in C++ (the language in question here) and it's rarely the case in other modern languages because it has terrible performance qualities.

Open a file in the constructor, close it in the destructor. RAII with 0 allocations.

std::vector<int> allocated and freed on the stack will allocate an array for its int’s on the heap…

I've heard that MSVC does (did?) that, but if so that's an MSVC problem. gcc and clang don't do that.

https://godbolt.org/z/nasoWeq5M

WDYM? Vector is an abstraction over dynamically sized arrays so sure it does use heap to store its elements.

I think usefulcat interpreted "std::vector<int> allocated and freed on the stack" as creating a default std::vector<int> and then destroying it without pushing elements to it. That's what their godbolt link shows, at least, though to be fair MSVC seems to match the described GCC/Clang behavior these days.

Sure, but my point was that RAII doesn't need to involve the heap. Another example would be acquiring abd releasing a mutex.

Stack "allocations" are basically free.

No. And they're unsafe. Avoid them at all costs.

RAII doesn't necessarily require allocation?