make_shared x new

Using new, counters are allocated separatelly, them the object and memory is dealocated with de last shared_ptr remaning only weakpointer counters.

#include <memory>

using namespace std;

struct X
{
    int b[1000];
};

int main()
{
    {
        weak_ptr<X> wp; //lives longer than shared_ptr
        {
            shared_ptr<X> sp(new X());
            wp = sp;
        }
    }   
}

makeshared makes a single alloc with the object plus counters. The object destructor is called with the last sharedptr, however the memory used by the object is dealocated only with the last weak_ptr.

From the standard:

Remarks: Implementations are encouraged, but not required, to perform no more than one memory
allocation.  Note: this provides efficiency equivalent to an intrusive smart pointer. end note 

#include <memory>

using namespace std;

struct X
{
    int b[1000];
};

int main()
{
    {
        weak_ptr<X> wp; //lives longer than shared_ptr
        {
            shared_ptr<X> sp = make_shared<X>();
            wp = sp;
        }
    }   
}