std::enable_shared_from_this

Defined in header <memory>.

Since C++11

template< class T > class enable_shared_from_this;

std::enable_shared_from_this allows an object t that is currently managed by a std::shared_ptr named pt to safely generate additional std::shared_ptr instances pt1, pt2, ... that all share ownership of t with pt.

Publicly inheriting from std::enable_shared_from_this<T> provides the type T with a member function shared_from_this. If an object t of type T is managed by a std::shared_ptr<T> named pt, then calling T::shared_from_this will return a new std::shared_ptr<T> that shares ownership of t with pt.

Member functions

pub	(constructor)	constructs an enable_shared_from_this object (protected member function)
pub	(destructor)	destroys an enable_shared_from_this object (protected member function)
pub	operator=	returns a reference to this (protected member function)
pub	shared_from_this	returns a shared_ptr which shares ownership of *this (public member function)
pub	weak_from_this(C++17)	returns the weak_ptr which shares ownership of *this (public member function)

Member objects

pub

weak_this (private)(C++17)

std::weak_ptr object tracking the control block of the first shared owner of *this. Exposition only

Notes

A common implementation for enable_shared_from_this is to hold a weak reference (such as std::weak_ptr) to this.

The constructors of std::shared_ptr detect the presence of an unambiguous and accessible (ie. public inheritance is mandatory) (since C++17) enable_shared_from_this base and assign the newly created std::shared_ptr to the internally stored weak reference if not already owned by a live std::shared_ptr (since C++17). Constructing a std::shared_ptr for an object that is already managed by another std::shared_ptr will not consult the internally stored weak reference and thus will lead to undefined behavior.

It is permitted to call shared_from_this only on a previously shared object, i.e. on an object managed by std::shared_ptr<T>. Otherwise the behavior is undefined (until C++17) std::bad_weak_ptr is thrown (by the std::shared_ptr constructor from a default-constructed weak_this) (since C++17).

enable_shared_from_this provides the safe alternative to an expression like std::shared_ptr<T>(this), which is likely to result in this being destructed more than once by multiple owners that are unaware of each other (see example below).

Feature-test macro	Value	Std	Comment
__cpp_lib_enable_shared_from_this	201603L	(C++17)	More precise specification of std::enable_shared_from_this

Example

#include <memory>
#include <iostream>
 
class Good : public std::enable_shared_from_this<Good>
{
public:
    std::shared_ptr<Good> getptr() {
        return shared_from_this();
    }
};
 
class Best : public std::enable_shared_from_this<Best>
{
public:
    std::shared_ptr<Best> getptr() {
        return shared_from_this();
    }
    // No public constructor, only a factory function,
    // so there's no way to have getptr return nullptr.
    [[nodiscard]] static std::shared_ptr<Best> create() {
        // Not using std::make_shared<Best> because the c'tor is private.
        return std::shared_ptr<Best>(new Best());
    }
private:
    Best() = default;
};
 
 
struct Bad
{
    std::shared_ptr<Bad> getptr() {
        return std::shared_ptr<Bad>(this);
    }
    ~Bad() { std::cout << "Bad::~Bad() called\n"; }
};
 
void testGood()
{
    // Good: the two shared_ptr's share the same object
    std::shared_ptr<Good> good0 = std::make_shared<Good>();
    std::shared_ptr<Good> good1 = good0->getptr();
    std::cout << "good1.use_count() = " << good1.use_count() << '\n';
}
 
 
void misuseGood()
{
    // Bad: shared_from_this is called without having std::shared_ptr owning the caller
    try {
        Good not_so_good;
        std::shared_ptr<Good> gp1 = not_so_good.getptr();
    } catch(std::bad_weak_ptr& e) {
        // undefined behavior (until C++17) and std::bad_weak_ptr thrown (since C++17)
        std::cout << e.what() << '\n';
    }
}
 
 
void testBest()
{
    // Best: Same but can't stack-allocate it:
    std::shared_ptr<Best> best0 = Best::create();
    std::shared_ptr<Best> best1 = best0->getptr();
    std::cout << "best1.use_count() = " << best1.use_count() << '\n';
 
    // Best stackBest; // <- Will not compile because Best::Best() is private.
}
 
 
void testBad()
{
    // Bad, each shared_ptr thinks it's the only owner of the object
    std::shared_ptr<Bad> bad0 = std::make_shared<Bad>();
    std::shared_ptr<Bad> bad1 = bad0->getptr();
    std::cout << "bad1.use_count() = " << bad1.use_count() << '\n';
} // UB: double-delete of Bad
 
 
int main()
{
    testGood();
    misuseGood();
 
    testBest();
 
    testBad();
}

Possible Result

good1.use_count() = 2
bad_weak_ptr
best1.use_count() = 2
bad1.use_count() = 1
Bad::~Bad() called
Bad::~Bad() called
*** glibc detected *** ./test: double free or corruption