C++ 如何向下转换 std::shared_ptr ？
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How does one downcast a std::shared_ptr?
提问by Billy ONeal
Consider:
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.
}
}
(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)
（请注意，我不能简单地执行 a std::shared_ptr<ChildOne>(static_cast<ChildOne*>(ptr.get()))，因为那样引用计数不会在两个shared_ptrs之间共享）
回答by mwigdahl
This ought to work:
这应该有效：
if (ptr->IsChildOne())
{
SomeClientExpectingAChildOne(std::static_pointer_cast<ChildOne>(ptr));
}
回答by Joel
The shared_ptrequivalent of static_castis static_pointer_cast, and the shared_ptrequivalent of dynamic_castis dynamic_pointer_cast.
该shared_ptr当量static_cast是static_pointer_cast，与shared_ptr等效的dynamic_cast是dynamic_pointer_cast。
回答by jotik
Starting from C++11, §20.10.2.2.9 ([util.smartptr.shared.cast]) of the C++ standard specifies the equivalents of static_cast, const_castand dynamic_castfor std::shared_ptrto be as follows:
从 C++11 开始，C++ 标准的 §20.10.2.2.9 ( [util.smartptr.shared.cast]) 指定static_cast,const_cast和dynamic_castfor的等效项std::shared_ptr如下：
std::static_pointer_cast:
std::static_pointer_cast：
template <class T, class U>
shared_ptr<T> static_pointer_cast(shared_ptr<U> const & r) noexcept;
static_pointer_castrequires static_cast<T *>(r.get())to be well formed. If ris empty, an empty shared_ptr<T>is returned, otherwise returns a pointer wsharing ownership with rwhere w.get() == static_cast<T *>(r.get())and w.use_count() == r.use_count().
static_pointer_cast需要static_cast<T *>(r.get())很好地形成。如果r为空，shared_ptr<T>则返回空，否则返回w与rwherew.get() == static_cast<T *>(r.get())和共享所有权的指针w.use_count() == r.use_count()。
std::const_pointer_cast:
std::const_pointer_cast：
template <class T, class U>
shared_ptr<T> const_pointer_cast(shared_ptr<U> const & r) noexcept;
const_pointer_casthas similar requirements and semantics to static_pointer_cast, except that const_castis used instead of static_cast.
const_pointer_cast与 具有相似的要求和语义static_pointer_cast，除了const_cast使用 代替static_cast。
std::dynamic_pointer_cast:
std::dynamic_pointer_cast：
template <class T, class U>
shared_ptr<T> dynamic_pointer_cast(shared_ptr<U> const & r) noexcept;
dynamic_pointer_castis a bit different as it requires dynamic_cast<T *>(r.get())to be well formed and have well defined semantics. If dynamic_cast<T *>(r.get())is a non-zero value, returns a pointer wsharing ownership with rwhere w.get() == dynamic_cast<T *>(r.get())and w.use_count() == r.use_count(), otherwise an empty shared_ptr<T>is returned.
dynamic_pointer_cast有点不同，因为它需要dynamic_cast<T *>(r.get())格式良好并具有明确定义的语义。如果dynamic_cast<T *>(r.get())是非零值，则返回w与rwherew.get() == dynamic_cast<T *>(r.get())和共享所有权的指针w.use_count() == r.use_count()，否则shared_ptr<T>返回空值。
std::reinterpret_pointer_cast:
std::reinterpret_pointer_cast：
For C++17, N3920(adopted into Library Fundamentals TS in February 2014) also proposed a std::reinterpret_pointer_castsimilar to the above, which would only require reinterpret_cast<T *>((U *) 0)to be well formed and returns shared_ptr<T>(r, reinterpret_cast<typename shared_ptr<T>::element_type *>(r.get())). Note N3920 also changed the wording for the other shared_ptrcasts and extended shared_ptrto support arrays.
对于 C++17，N3920（2014 年 2 月被采纳到 Library Fundamentals TS中）也提出了std::reinterpret_pointer_cast与上述类似的方法，只需要reinterpret_cast<T *>((U *) 0)格式良好并返回shared_ptr<T>(r, reinterpret_cast<typename shared_ptr<T>::element_type *>(r.get()))。注意 N3920 还更改了其他类型shared_ptr转换的措辞并扩展shared_ptr为支持数组。
