Can any one help me to understand below code?

Question

char* ptr = new char[sizeof(T)]; // allocate memory
T* tptr = new(ptr) T; // construct in allocated storage ("place")
tptr->~T(); // destruct
delete[] ptr; // deallocate memory

Answer 1

Yes it works, but new(&mem[number... it's not safe.

I think you should use something like [ sizeof(base)*index] instead of a number.. you won't know the size of an object, because if you want to add functionalities to your class later, you can have serious troubles.

Answer 2

it is tricky!

T * tptr = new(ptr) T doesn't allocate memory,
it constructs the T obj at the given pointer (ptr).

the ptr pointer have been allocated previously whit the correct size.. new char[sizeof(T)]

now the tricky part is that ptr is a pointer of type char[] ,it holds an array of chars.
the size of memory allocated is T size but it cannot hold a T class type..
so the T * tptr is a new pointer that holds a T type and contains the same address that ptr contains.

the difference is between the address of tptr and the address of pre.

tptr==ptr but &ptr != &tptr

hope it helps and tell me if you need a better explain

Answer 3

hey @AZTECCO is this placement new operator.
I have used it as below. Is it?
https://code.sololearn.com/cl2LB21ckOnM/?ref=app