std::enable_shared_from_this
Defined in header <memory>
.
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();
}
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