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

This is more commonly known as 'placement new' and is discussed pretty well by the C++ FAQ (in the 'Destructors' area):

• http://www.parashift.com/c++-faq-lite/dtors.html#faq-11.10

It allows you to construct objects in raw memory, which can be useful in certain specialized situations, such as when you might want to allocate an array for a large number of possible objects, but want to construct then as needed because you often might not need anywhere near the maximum, or because you want or need to use a custom memory allocator.

The short answer is that your code constructs an object in the space pointed to by y. The long answer is best covered by the C++ FAQ.

I'm rusty on this one but it allows you to write the object to a memory block you have already allocated. It also needs a reciprocal delete statement to clear it from memory.

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);