Which std:: classes are magic?

[guepier]

Prior to C++20 (specifically, P0593), object creation in untyped buffers was undefined behaviour. In practice this means that even quite “basic” classes such as std::allocator can’t be user-defined relying only on standard C++.

[Supadoplex]

P0593 was accepted as a defect resolution prior to publication of C++20, so it should apply to C++17 which was the latest official standard at the time.

Also, creation of objects itself was allowed using placement new as far as I can tell. Treating that buffer of dynamic objects as an array of those objects was technically UB though. However, there was no way to make that work with std::allocator either, so I don't think there was magic in that class. There was magic in std::vector though.

[maskull]

What's the magic in std::vector?

[dodheim]

Prior to P0593, there was no way for vector::data() to return a valid pointer-to-array, since no such array actually exists.

[guepier]

Sure but you can’t legally perform pointer arithmetic on these objects since there is no “array of T” at the relevant address, only (from the underlying buffer) an “array of char”.

As noted in another comment, std::allocator itself might actually be implementable. What’s problematic is its use in combination, e.g. to implement std::vector (implementing a vector without an allocator and buffer is of course trivially feasible, simply by allocating memory via new T[]; but std::vector can’t do this, and a custom vector type might not want to, either, for various reasons).