shared_ptr从unique_ptr到数组 [英] shared_ptr from unique_ptr to an array

问题描述

Scott Meyers在他的书Effective Modern C ++中提到，使用 shared_ptr 到数组是不鼓励的，因为当转换为Base类指针时，它创建

Scott Meyers mentions in his book, Effective Modern C++, that using shared_ptr to arrays is discouraged because when converted to Base class pointers it creates "holes" in the type system.

但是，可以从<$ c $>创建 shared_ptr< T> c $ c> unique_ptr< T []> 。

However, it is possible to create shared_ptr<T> from unique_ptr<T[]> in the following way

std::shared_ptr<D> pDerived = std::shared_ptr<D>(std::make_unique<D[]>(3)); // Create an array of 3 D's

上述代码有潜在的危险吗？如果 pDerived 以后被复制到 pBase ？

Is the above code potentially dangerous? Are there pitfalls if pDerived is later copied into a pBase?

std::shared_ptr<B> pBase = pDerived; // B is the base class for D

推荐答案

上述代码有潜在的危险吗？

Is the above code potentially dangerous?

这取决于你做什么。

它不会泄露资源，因为使用 delete [] default_delete c $ c>将从 unique_ptr 中复制并存储在 shared_ptr 中（如从unique_ptr< T []> 初始化shared_ptr< T> ）。

It won't leak resources because the default_delete<D[]> that uses delete[] will be copied from the unique_ptr and stored in the shared_ptr (as described at Initialization of shared_ptr<T> from unique_ptr<T[]>).

如果 pDerived 稍后复制到 pBase ？

是的，如果你执行 pBase.get（）[1] 指向第二个元素的有效指针if sizeof（B）！= sizeof（D）

Yes, if you do something like pBase.get()[1] then that is not a valid pointer to the second element if sizeof(B) != sizeof(D)

std :: experimental :: shared_ptr 从 Library Fundamentals TS 正确地支持数组（由

std::experimental::shared_ptr from the Library Fundamentals TS supports arrays properly (as proposed by N3939).

使用TS中的版本， shared_ptr< D> 不允许从 unique_ptr ，但 shared_ptr< D []> 。不能将 shared_ptr< D []> 转换为 shared_ptr< B []>

With the version in the TS, shared_ptr<D> does not allow construction from a unique_ptr<D[]>, but shared_ptr<D[]> does. A shared_ptr<D[]> cannot be converted to shared_ptr<B[]>, to avoid the safety problem you refer to.

对于数组的支持可能会在未来的C ++标准中变成 std :: shared_ptr ，但是现在只在TS中。

The support for arrays might make it into std::shared_ptr in a future C++ standard, but is only in the TS for now.

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