Question

我无法理解Foo移动构造函数在以下示例中尝试调用~ptr的原因：

#include <utility>

template <typename T, typename Policy>
class ptr {
    T * m_t;
public:
    ptr() noexcept : m_t(0) {}
    explicit ptr(T *t) noexcept : m_t(t) {}
    ptr(const ptr &other) noexcept : m_t(Policy::clone(other.m_t)) {}
    ptr(ptr &&other) noexcept : m_t(other.m_t) { other.m_t = 0; }
    ~ptr() noexcept { Policy::delete_(m_t); }
    ptr &operator=(const ptr &other) noexcept
    { ptr copy(other); swap(copy); return *this; }
    ptr &operator=(ptr &&other) noexcept
    { std::swap(m_t,other.m_t); return *this; }

    void swap(ptr &other) noexcept { std::swap(m_t, other.m_t); }

    const T * get() const noexcept { return m_t; }
    T * get() noexcept { return m_t; }
};

class FooPolicy;
class FooPrivate;
class Foo {
    // some form of pImpl:
    typedef ptr<FooPrivate,FooPolicy> DataPtr;
    DataPtr d;
public:
    // copy semantics: out-of-line
    Foo();
    Foo(const Foo &other);
    Foo &operator=(const Foo &other);
    ~Foo();

    // move semantics: inlined
    Foo(Foo &&other) noexcept
      : d(std::move(other.d)) {} // l.35 ERR: using FooDeleter in ~ptr required from here
    Foo &operator=(Foo &&other) noexcept
    { d.swap(other.d); return *this; }
};

GCC 4.7：

foo.h: In instantiation of ‘ptr<T, Policy>::~ptr() [with T = FooPrivate; Policy = FooPolicy]’:
foo.h:34:44:   required from here
foo.h:11:14: error: incomplete type ‘FooPolicy’ used in nested name specifier

Clang 3.1-pre：

foo.h:11:14: error: incomplete type 'FooPolicy' named in nested name specifier
    ~ptr() { Policy::delete_(m_t); }
             ^~~~~~~~
foo.h:34:5: note: in instantiation of member function 'ptr<FooPrivate, FooPolicy>::~ptr' requested here
    Foo(Foo &&other) : d(std::move(other.d)) {}
    ^
foo.h:23:7: note: forward declaration of 'FooPolicy'
class FooPolicy;
      ^
foo.h:11:20: error: incomplete definition of type 'FooPolicy'
    ~ptr() { Policy::delete_(m_t); }
             ~~~~~~^~
2 errors generated.

发生了什么事？我正在编写移动构造函数以避免运行复制ctors和dtors。请注意，这是一个试图隐藏其实现的头文件（pimpl idiom），因此不能选择使用FooDeleter完整类型。

编辑：在Bo的回答之后，我尽可能地添加了noexcept（在上面编辑过）。但错误仍然是一样的。

Answer 1

您创建一个包含Foo成员的新ptr<something>对象。如果Foo构造函数失败，编译器必须为部分构造的Foo的任何完全构造的成员调用析构函数。

但它无法实例化~ptr<incomplete_type>()，因此失败。

你有一个类似的私有析构函数的情况。这也阻止您创建该类型的对象（除非从朋友或成员函数完成）。

析构函数隐藏在此代码中的哪个位置？

1 个答案: