Re: [boost] std::launder and its usage in Boost

On 2/18/22 18:15, Andrea Bocci wrote:

Hi Andrey, now I'm curious... is

  alignas(int) unsigned char storage[sizeof(int)];   int* p1 = new (storage) int(1);   int* p3 = new (storage) int(3);   std::printf("%d\n", *p1);

different from

    int* p1 = new int(1);     int* p3 = new (p1) int(3);     std::printf("%d\n", *p1);

or even

    int p1 = 1;     int* p3 = new (&p1) int(3);     std::printf("%d\n", p1);

?

In the examples above, you're reusing storage of the object with a different object of the same type, that is the old object is "transparently replaceable" by the new one. See here: https://en.cppreference.com/w/cpp/language/lifetime#Storage_reuse In this case, p1 remains valid and can be used to access the new object.