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The compliant solution is to use std::enable_shared_from_this::shared_from_this() to get a shared pointer from S that is related to an existing std::shared_ptr object. A common implementation strategy is for the std::shared_ptr constructors to detect the presence of a pointer that inherits from std::enable_shared_from_this, and automatically update the internal bookkeeping required for std::enable_shared_from_this::shared_from_this() to work. Note that std::enable_shared_from_this::shared_from_this() requires an existing std::shared_ptr instance that manages the pointer value pointed to by this. Failure to meet this requirement results in undefined behavior, as it would result in a smart pointer attempting to manage the lifetime of an object that itself does not have lifetime management semantics.

#include <memory>

struct S : std::enable_shared_from_this<S> {
  std::shared_ptr<S> g() { return shared_from_this(); }    
};

void f() {
  std::shared_ptr<S> s1 = std::make_shared<S>();
  std::shared_ptr<S> s2 = s1->g();
}

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