NAME¶

std::enable_shared_from_this - std::enable_shared_from_this

Synopsis¶

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, ... 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¶

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

Member objects¶

Member name Definition
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 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: __cpp_lib_enable_shared_from_this

Example¶

// Run this code

#include <memory>
#include <iostream>

struct Good : std::enable_shared_from_this<Good> // note: public inheritance
{
std::shared_ptr<Good> getptr() {
return shared_from_this();
}
};

struct Best : std::enable_shared_from_this<Best> // note: public inheritance
{
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 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

See also¶

shared_ptr smart pointer with shared object ownership semantics
(C++11) (class template)