Question

What is an in-place constructor in C++?

e.g. Datatype *x = new(y) Datatype();

Answer 1

This is called the placement new operator. It allows you to supply the memory the data will be allocated in without having the new operator allocate it. For example:

Foo * f = new Foo();

The above will allocate memory for you.

void * fm = malloc(sizeof(Foo));
Foo *f = new (fm) Foo(); 

The above will use the memory allocated by the call to malloc. new will not allocate any more. You are not, however, limited to classes. You can use a placement new operator for any type you would allocate with a call to new.

A ‘gotcha’ for placement new is that you should not release the memory allocated by a call to the placement new operator using the delete keyword. You will destroy the object by calling the destructor directly.

f->~Foo();

After the destructor is manually called, the memory can then be freed as expected.

free(fm);