I've just noticed that the pointers passed to delete can be const qualified while those passed to free cannot. That is really a surprise to me.
And in C++ an overload to operator delete should have a signature like:
void operator delete(void* p);
But adding a const to the parameter pointer is invalid:
void operator delete(void const* p);
May anyone tell me why delete is designed this way?
In C++, undefined behavior is predicated only on the constness of the object as instantiated, not the presence of a const qualifier along an access path thereto. Thus, const_cast can be used to modify objects behind a pointer of const-qualified target type, so long as the objects were not instantiated const.
Because malloc() instantiates non-const primitive data, it would be completely legal for free() to take a pointer to const, then cast it away and modify the memory as necessary. But because free() predates the const keyword, it does not qualify as it should.
C++ pogrammers may cast away const in order to free malloc'd memory.