unique_ptr 的动态转换
和 Boost 一样，C++11 提供了一些用于转换 shared_ptr 的函数:
As it was the case in Boost, C++11 provides some functions for casting shared_ptr:
std::static_pointer_cast
std::dynamic_pointer_cast
std::const_pointer_cast
然而，我想知道为什么 unique_ptr 没有等效函数.
I am wondering, however, why there are no equivalents functions for unique_ptr.
考虑以下简单示例:
class A { virtual ~A(); ... }
class B : public A { ... }
unique_ptr<A> pA(new B(...));
unique_ptr<A> qA = std::move(pA); // This is legal since there is no casting
unique_ptr<B> pB = std::move(pA); // This is not legal
// I would like to do something like:
// (Of course, it is not valid, but that would be the idea)
unique_ptr<B> pB = std::move(std::dynamic_pointer_cast<B>(pA));
是否有任何理由不鼓励这种使用模式，因此没有为 unique_ptr 提供与 shared_ptr 中存在的功能等效的功能?
Is there any reason why this usage pattern is discouraged, and thus, equivalent functions to the ones present in shared_ptr are not provided for unique_ptr?
推荐答案
您引用的每个函数都会对指针进行复制.由于您无法复制 unique_ptr，因此为其提供这些功能是没有意义的.
The functions you refer to each make a copy of the pointer. Since you can't make a copy of a unique_ptr it doesn't make sense to provide those functions for it.
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