将enable_shared_from_this与多个继承一起使用 [英] Use of enable_shared_from_this with multiple inheritance

问题描述

BI正在我的代码中使用enable_shared_from_this，但我不确定其用法是否正确.这是代码:

BI am using enable_shared_from_this in my code, and I am not sure if its usage is correct. This is the code:

class A: public std::enable_shared_from_this<A>
{
public:
    void foo1()
    {
        auto ptr = shared_from_this(); 
    }
};

class B:public std::enable_shared_from_this<B>
{
public:
    void foo2()
    {
        auto ptr = shared_from_this(); 
    }
};

class C:public std::enable_shared_from_this<C>
{
public:
    void foo3()
    {
        auto ptr = shared_from_this(); 
    }
};

class D: public A, public B, public C
{
public:
    void foo()
    {
        auto ptr = A::shared_from_this(); 
    }
};

假设总是通过D的shared_ptr调用make_shared_from_this()的这些用法正确吗?

Are these usage of make_shared_from_this() correct, assuming that they are always being called through shared_ptr of D?

推荐答案

确实是您做错了.如果您具有简单的继承，则只需从基类中的enable_shared_from this继承，派生类即可免费获得它. (当然，您需要向下转换结果)

Indeed you are doing it wrong. If you have simple inheritance, just inherit from enable_shared_from this in the base class, and derived class get it for free. (of course you'll need to downcast the result)

如果您有多个继承(看起来像)，则必须使用此处:

If you have multiple inheritance (like it seems), you must use the trick described here and also here :

/* Trick to allow multiple inheritance of objects
 * inheriting shared_from_this.
 * cf. https://stackoverflow.com/a/12793989/587407
 */

/* First a common base class
 * of course, one should always virtually inherit from it.
 */
class MultipleInheritableEnableSharedFromThis: public std::enable_shared_from_this<MultipleInheritableEnableSharedFromThis>
{
public:
  virtual ~MultipleInheritableEnableSharedFromThis()
  {}
};

template <class T>
class inheritable_enable_shared_from_this : virtual public MultipleInheritableEnableSharedFromThis
{
public:
  std::shared_ptr<T> shared_from_this() {
    return std::dynamic_pointer_cast<T>(MultipleInheritableEnableSharedFromThis::shared_from_this());
  }
  /* Utility method to easily downcast.
   * Useful when a child doesn't inherit directly from enable_shared_from_this
   * but wants to use the feature.
   */
  template <class Down>
  std::shared_ptr<Down> downcasted_shared_from_this() {
    return std::dynamic_pointer_cast<Down>(MultipleInheritableEnableSharedFromThis::shared_from_this());
  }
};

然后您的代码变为:

class A: public inheritable_enable_shared_from_this<A>
{
public:
    void foo1()
    {
        auto ptr = shared_from_this(); 
    }
};

class B: public inheritable_enable_shared_from_this<B>
{
public:
    void foo2()
    {
        auto ptr = shared_from_this(); 
    }
};

class C: public inheritable_enable_shared_from_this<C>
{
public:
    void foo3()
    {
        auto ptr = shared_from_this(); 
    }
};

class D: public A, public B, public C
{
public:
    void foo()
    {
        auto ptr = A::downcasted_shared_from_this<D>(); 
    }
};

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