C++: What is std::shared_ptr and reference counting?

1#include <memory>
2
3auto sp1 = std::make_shared<int>(42);
4std::cout << sp1.use_count();  // 1
5
6{
7    auto sp2 = sp1;  // Share ownership
8    std::cout << sp1.use_count();  // 2
9}  // sp2 destroyed, count = 1
10
11std::cout << sp1.use_count();  // 1
12
13// Control block layout:
14// [weak_count | shared_count | deleter | data]
15
16// make_shared allocates in one block (more efficient)
17auto efficient = std::make_shared<MyClass>(args...);
18
19// Two allocations (less efficient)
20std::shared_ptr<MyClass> two_alloc(new MyClass(args...));