How does C++ free the memory when a constructor throws an exception and a custom new is used

Question

I see the following constructs:

• new X will free the memory if X constructor throws.

• operator new() can be

Answer 1

When the construction of the object being constructed as a part of the new-expression fails, a corresponding deallocation function -- if there is one -- will be called. For instance

new X;

will use the following pair of allocation/deallocation functions.

void * operator new(std::size_t);
void operator delete(void *);

Similarly, for a placement new of the form

new(&a) X;

the placement versions of operator new and operator delete function will be used.

void * operator new(std::size_t, void *);
void operator delete(void *, void *);

Note that the last function intentionally performs no action.