std :: unique_ptr具有不完整的类型将不能编译

问题描述

我使用pimpl-idiom与 std :: unique_ptr ：

I'm using the pimpl-idiom with std::unique_ptr:

class window {
  window(const rectangle& rect);

private:
  class window_impl; // defined elsewhere
  std::unique_ptr<window_impl> impl_; // won't compile
};

However, I get a compile error regarding the use of an incomplete type, on line 304 in <memory>:

据我所知， std :: unique_ptr 应该能够使用不完整的类型。这是在libc ++中的错误还是我在这里做错了？

For as far as I know, std::unique_ptr should be able to be used with an incomplete type. Is this a bug in libc++ or am I doing something wrong here?

推荐答案

std :: unique_ptr 使用不完整的类型。

Here are some examples of std::unique_ptr with incomplete types. The problem lies in destruction.

如果你使用pimpl和 unique_ptr ，你需要声明一个析构函数： p>

If you use pimpl with unique_ptr, you need to declare a destructor:

class foo
{
    class impl;
    std::unique_ptr<impl> impl_;

public:
    foo(); // You may need a def. constructor to be defined elsewhere

    ~foo(); // Implement (with an empty body) where impl is complete
};

因为否则编译器会生成一个默认的，它需要一个完整的声明 foo :: impl 为此。

because otherwise the compiler generates a default one, and it needs a complete declaration of foo::impl for this.

如果你有模板构造函数，那么你被拧了，即使你不构造 impl _ 成员：

If you have template constructors, then you're screwed, even if you don't construct the impl_ member:

template <typename T>
foo::foo(T bar)
{
    // Here the compiler needs to know how to
    // destroy impl_ in case an exception is
    // thrown !
}

在命名空间范围，使用 unique_ptr 不会工作：

At namespace scope, using unique_ptr will not work either:

class impl;
std::unique_ptr<impl> impl_;

因为编译器必须知道如何销毁这个静态持续时间对象。解决方法是：

since the compiler must know here how to destroy this static duration object. A workaround is:

class impl;
struct ptr_impl : std::unique_ptr<impl>
{
    ~ptr_impl(); // Implement (empty body) elsewhere
} impl_;

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