C++ Smart Pointers Getting a shared_ptr referring to this
Example
enable_shared_from_this enables you to get a valid shared_ptr instance to this.
By deriving your class from the class template enable_shared_from_this, you inherit a method shared_from_this that returns a shared_ptr instance to this.
Note that the object must be created as a shared_ptr in first place:
#include <memory>
class A: public enable_shared_from_this<A> {
};
A* ap1 =new A();
shared_ptr<A> ap2(ap1); // First prepare a shared pointer to the object and hold it!
// Then get a shared pointer to the object from the object itself
shared_ptr<A> ap3 = ap1->shared_from_this();
int c3 =ap3.use_count(); // =2: pointing to the same object
Note(2) you cannot call enable_shared_from_this inside the constructor.
#include <memory> // enable_shared_from_this
class Widget : public std::enable_shared_from_this< Widget >
{
public:
void DoSomething()
{
std::shared_ptr< Widget > self = shared_from_this();
someEvent -> Register( self );
}
private:
...
};
int main()
{
...
auto w = std::make_shared< Widget >();
w -> DoSomething();
...
}
If you use shared_from_this() on an object not owned by a shared_ptr, such as a local automatic object or a global object, then the behavior is undefined. Since C++17 it throws std::bad_alloc instead.
Using shared_from_this() from a constructor is equivalent to using it on an object not owned by a shared_ptr, because the objects is possessed by the shared_ptr after the constructor returns.
