C++11 unique_ptr 和 shared_ptr 是否能够相互转换为彼此的类型？

Question

C++11 标准库是否提供任何实用工具来将 std::shared_ptr 转换为 std::unique_ptr，反之亦然？这样操作安全吗？

Answer 1

std::unique_ptr is the C++11 way to express exclusive ownership, but one of its most attractive features is that it easily and efficiently converts to a std::shared_ptr.

This is a key part of why std::unique_ptr is so well suited as a factory function return type. Factory functions can’t know whether callers will want to use exclusive ownership semantics for the object they return or whether shared ownership (i.e., std::shared_ptr) would be more appropriate. By returning a std::unique_ptr, factories provide callers with the most efficient smart pointer, but they don’t hinder callers from replacing it with its more flexible sibling.

std::shared_ptr to std::unique_ptr is not allowed. Once you’ve turned lifetime management of a resource over to a std::shared_ptr, there’s no changing your mind. Even if the reference count is one, you can’t reclaim ownership of the resource in order to, say, have a std::unique_ptr manage it.

Reference: Effective Modern C++. 42 SPECIFIC WAYS TO IMPROVE YOUR USE OF C++11 AND C++14. Scott Meyers.

简而言之，您可以轻松高效地将 std::unique_ptr 转换为 std::shared_ptr，但无法将 std::shared_ptr 转换为 std::unique_ptr。

例如：

std::unique_ptr<std::string> unique = std::make_unique<std::string>("test");
std::shared_ptr<std::string> shared = std::move(unique);

或：

std::shared_ptr<std::string> shared = std::make_unique<std::string>("test");

Answer 2

给定 unique_ptr u_ptr，创建 shared_ptr s_ptr:

std::shared_ptr<whatever> s_ptr(u_ptr.release());

走另一条路是不切实际的。

Answer 3

我更愿意这样做。

std::unique_ptr<int> up_ = std::make_unique<int>();
std::shared_ptr<int> sp_ = std::move(up_);