C++11中模板类std::enable_shared_from_this的使用

Posted 2022-04-18 fengbingchun

      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