What is this second new?

Question

What is the second line? (Seen while answering another question.)

int * x = new int [1] ;
int * y = new (x) int;

After the second line x and y have the same value (point to a same place). What's the difference between y = x and the second line? Is it like a constructor or something?

Answer 1

It's placement new. It constructs a new int in the memory pointed to by x.

If you try:

int * x = new int [1];
*x = 5;
std::cout << *x << std::endl;
int * y = new (x) int;
*y = 7;
std::cout << *x << std::endl;

the output will be:

5
7

Answer 2

This is called placement new. It allows you to construct an object in memory already allocated.

This earlier thread discusses where and how it is useful for.