如何一个std :: shared_ptr downcast？ [英] How does one downcast a std::shared_ptr?

问题描述

请考虑：

struct SomethingThatsABase
{
    virtual bool IsChildOne() const { return false; }
    virtual bool IsChildTwo() const { return false; }
};

struct ChildOne : public SomethingThatsABase
{
    virtual bool IsChildOne() const { return true; }
};

struct ChildTwo : public SomethingThatsABase
{
    virtual bool IsChildTwo() const { return true; }
};

void SomeClientExpectingAChildOne(std::shared_ptr<ChildOne> const& ptrOne)
{
    //Does stuff
}

void SomeClient(std::shared_ptr<SomethingThatsABase> const& ptr)
{
    if (ptr->IsChildOne())
    {
        SomeClientExpectingAChildOne(ptr); //Oops.
        //Hmm.. can't static_cast here, because we need a `shared_ptr` out of it.
    }
}

（注意，我不能简单地做一个 std :: shared_ptr< ChildOne>（static_cast< ChildOne *>（ptr.get（））），因为那么引用计数不会在两个 shared_ptr s）

(Note that I can't simply do a std::shared_ptr<ChildOne>(static_cast<ChildOne*>(ptr.get())), because then the reference counts don't get shared between the two shared_ptrs)

推荐答案

这应该可以工作：

if (ptr->IsChildOne())
{
    SomeClientExpectingAChildOne(std::static_pointer_cast<ChildOne>(ptr));
}

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