C++11中的模板类template <class T> class enable_shared_from_this功能：允许从std::enable_shared_from_this派生的类T的对象创建指向自身的shared_ptr实例并与现有的shared_ptr对象共享所有权。T需要为完整类型，通常是final的。如果使用this创建一个新的shared_ptr，则其与现有的shared_ptr所有者不同，从而会导致无效引用或导致此对象被删除多次。
std::enable_shared_from_this能让其一个对象(假设其名为t，且已被一个std::shared_ptr对象pt管理)安全地生成其它额外的std::shared_ptr实例(假设名为pt1, pt2, …)，它们与pt共享对象t的所有权。
若一个类T继承std::enable_shared_from_this<T>，则会为该类T提供成员函数：shared_from_this。当T类型对象t被一个名为pt的std::shared_ptr<T>类对象管理时，调用T::shared_from_this成员函数，将会返回一个新的std::shared_ptr<T>对象，它与pt共享t的所有权。
以下为测试代码：
namespace {
struct C : std::enable_shared_from_this<C> {};
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.
#ifdef _MSC_VER
static std::shared_ptr<Best> create() // nodiscard need compiler flags: "/std:c++17"
#else
[[nodiscard]] static std::shared_ptr<Best> create()
#endif
{
// Not using std::make_shared<Best> because the c'tor is private.
return std::shared_ptr<Best>(new Best());
}
private:
Best() = default;
};
class Y: public std::enable_shared_from_this<Y> {
public:
std::shared_ptr<Y> f()
{
return shared_from_this(); // returns a valid shared_ptr, even though it had no member instance
//return std::shared_ptr<Y>(this); // error
}
};
class Foo : public std::enable_shared_from_this<Foo> {
public:
Foo() = default;
~Foo() {}
void set_name(std::string name) { name_ = name; }
void set_age(int age) { age_ = age; }
const std::string& get_name() const { return name_; }
int get_age() const { return age_; }
private:
std::string name_;
int age_;
};
void modify_foo(std::shared_ptr<Foo> foo)
{
foo->set_name("Tom");
foo->set_age(10);
fprintf(stdout, "modify_foo use count: %d\n", foo.use_count()); // 2
}
} // namespace
int test_enable_shared_from_this_1()
{
// reference: http://www.cplusplus.com/reference/memory/enable_shared_from_this/
std::shared_ptr<C> foo, bar;
foo = std::make_shared<C>();
bar = foo->shared_from_this();
if (!foo.owner_before(bar) && !bar.owner_before(foo))
fprintf(stdout, "foo and bar share ownership\n");
return 0;
}
int test_enable_shared_from_this_2()
{
// reference: https://en.cppreference.com/w/cpp/memory/enable_shared_from_this
// Best: Same but can't stack-allocate it:
std::shared_ptr<Best> best0 = Best::create();
std::shared_ptr<Best> best1 = best0->getptr();
fprintf(stdout, "best0.use_count: %d, best1.use_count: %d\n", best0.use_count(), best1.use_count()); // 2, 2
//Best stackBest; // <- Will not compile because Best::Best() is private.
return 0;
}
int test_enable_shared_from_this_3()
{
// reference: https://stackoverflow.com/questions/712279/what-is-the-usefulness-of-enable-shared-from-this
// It enables you to get a valid shared_ptr instance to this, when all you have is this.
// Without it, you would have no way of getting a shared_ptr to this, unless you already had one as a member.
std::shared_ptr<Y> p(new Y);
std::shared_ptr<Y> q = p->f();
if (!(p == q)) {
fprintf(stderr, "p should be equal to q\n");
return -1;
}
if (p < q || q < p) { // p and q must share ownership
fprintf(stderr, "p and q must share ownership\n");
return -1;
}
return 0;
}
int test_enable_shared_from_this_4()
{
std::shared_ptr<Foo> foo = std::make_shared<Foo>();
foo->set_name("Take");
foo->set_age(20);
fprintf(stdout, "foo use count: %d, name: %s, age: %d\n", foo.use_count(), foo->get_name().c_str(), foo->get_age()); // 1, Take, 20
modify_foo(foo->shared_from_this());
fprintf(stdout, "foo use count: %d, name: %s, age: %d\n", foo.use_count(), foo->get_name().c_str(), foo->get_age()); // 1, Tom, 10
return 0;
}
GitHub：https://github.com/fengbingchun/Messy_Test
