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std::enable_shared_from_this

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std::enable_shared_from_this
 
Defined in header <memory>
template< class T >
class enable_shared_from_this;
 (since C++11)

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 etc. 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.

Contents

[edit] Data members

Member Description
mutable std::weak_ptr<T> weak_this the object tracking the control block of the first shared owner of *this
(exposition-only member object*)

[edit] Member functions

 constructs an enable_shared_from_this object
(protected member function)
destroys an enable_shared_from_this object
(protected member function)
returns a reference to *this
(protected member function)
returns a std::shared_ptr which shares ownership of *this
(public member function)
(C++17)
 returns a std::weak_ptr which shares ownership of *this
(public member function)

[edit] Notes

The constructors of std::shared_ptr detect the presence of an unambiguous and accessible (i.e. public inheritance is mandatory) enable_shared_from_this base and assign the newly created std::shared_ptr to weak_this if not already owned by a live std::shared_ptr. Constructing a std::shared_ptr for an object that is already managed by another std::shared_ptr will not consult weak_this 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, std::bad_weak_ptr is thrown (by the std::shared_ptr constructor from a default-constructed weak_this ).

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).

[edit] Example

Run this code
#include <iostream>
#include <memory>
 
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>
{
    struct Private{ explicit Private() = default; };
 
public:
    // Constructor is only usable by this class
    Best(Private) {}
 
    // Everyone else has to use this factory function
    // Hence all Best objects will be contained in shared_ptr
    static std::shared_ptr<Best> create()
    {
        return std::make_shared<Best>(Private());
    }
 
    std::shared_ptr<Best> getptr()
    {
        return shared_from_this();
    }
};
 
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 cannot 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 is 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 output: 

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

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 2179
(P0033R1) 		C++11 given type T derived from enable_shared_from_this, the behavior of
constructing two std::shared_ptr<T>s from the same T* object was unclear 		the behavior is
undefined in this case
LWG 2529
(P0033R1) 		C++11 it was unclear how the underlying std::weak_ptr is updated made clear

[edit] See also

(C++11)
 smart pointer with shared object ownership semantics
(class template) [edit]
(C++20)
 creates a shared pointer that manages a new object
(function template) [edit]
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