How does delete[] “know” the size of the operand array?

Question

Foo* set = new Foo[100];
// ...
delete [] set;

You don\'t pass the array\'s boundaries to delete[]. But where is that information stor

Answer 1

It's defined in the C++ standard to be compiler specific. Which means compiler magic. It can break with non-trivial alignment restrictions on at least one major platform.

You can think about possible implementations by realizing that delete[] is only defined for pointers returned by new[], which may not be the same pointer as returned by operator new[]. One implementation in the wild is to store the array count in the first int returned by operator new[], and have new[] return a pointer offset past that. (This is why non-trivial alignments can break new[].)

Keep in mind that operator new[]/operator delete[]!=new[]/delete[].

Plus, this is orthogonal to how C knows the size of memory allocated by malloc.