what is the meaning for &t for expression new (&t) T(t) in c++?

Question

what is the meaning for &t for expression new (&t) T(t) in c++?

as titled.

T t;
new (&t) T(t);

Answer 1

This is referred to as the 'placement new' syntax. The T value is constructed in the address that is specified by &t.

This sample is a bit off since it's creating a new T in the exact location of an existing T using the copy constructor. I think it's easier to explain this concept with an explicit address. Here is a variation of this code.

T t;
void* pAddress = malloc(sizeof(T));
new (pAddress) T(t);

// Or just creating a T without a copy ctor
new (pAddress) T();

Answer 2

This is the Placement Syntax; This statment:

new (&t) T(t);

Constructs an object at the adress of t (&t) but it doesn't allocate memory.