I believe std::vector traditionally was forced to copy or move-construct instead of realloc() because there was no concept of "trivially movable" or "trivially copyable". Has that changed in recent standards?
But regardless, realloc() can't be used to implement the optimisation efficiently because of this line from the C man page:
> The realloc() function returns a pointer to the newly allocated memory, which is suitably aligned for any kind of variable *and may be different from ptr*
Basically, if the implementation can't extend the memory allocation it can fallback to malloc(), memcpy() and free() - so by the time C++ gets control back, it's too late to call the appropriate C++ functions.
The std::is_trivially_copyable type-trait can be used to determine if a type is trivially copyable. It's been in the standard since C++11. I imagine an implementation of std::vector could make use of it to determine if it's safe to use realloc.
