c++ - 具有已删除析构函数的类型的动态对象是如何构造的？

Question

假设我有这样的东西:

struct foo {
    ~foo() = delete;
}

假设我稍后动态分配了一个类型为 foo 的对象:

foo *f = new foo;

这很好；我假设合成的默认构造函数用于构造由 f 表示的对象，但是:

foo f2;

给我一​​个错误:

Attempt to use a deleted function

那么如果foo的默认构造函数被隐式删除，那么f表示的对象是如何构造的呢？

此外，假设 foo 有私有(private)成员 size_t n。对于动态分配的 foo，例如 f 表示的 n 的值是多少？

Answer 1

当你写作时

foo f;

编译器需要能够在 f 最初创建时构造它。由于该变量具有自动存储持续时间(花哨的 C++ 术语“在堆栈上”)，编译器也负责生成代码来清理它。这需要访问析构函数，但由于您已删除它，因此会出现错误。

当你写作时

foo* f = new foo;

您正在堆栈上创建一个指向 foo 对象的指针，编译器可以在不访问 foo 析构函数的情况下销毁该指针本身。另一方面，用 new foo 创建的对象具有动态存储持续时间，这意味着您 promise 手动销毁它。因此，编译器不需要访问析构函数，因此创建步骤没问题。那就是说，如果你再写

delete f;

您应该得到一个错误，因为该操作确实需要析构函数。

编辑:从你的后续行动来看，我的感觉是你想知道为什么即使删除了析构函数，仍然会生成默认构造函数。我有一份 C++14 规范草案，在 §12.1.4 中，它说了以下内容:

A default constructor for a class X is a constructor of class X that can be called without an argument. If there is no user-declared constructor for class X, a constructor having no parameters is implicitly declared as defaulted (8.4). An implicitly-declared default constructor is an inline public member of its class. A defaulted default constructor for class X is defined as deleted if:

— X is a union-like class that has a variant member with a non-trivial default constructor,

— any non-static data member with no brace-or-equal-initializer is of reference type,

— any non-variant non-static data member of const-qualified type (or array thereof) with no brace-or- equal-initializer does not have a user-provided default constructor,

— X is a union and all of its variant members are of const-qualified type (or array thereof),

— X is a non-union class and all members of any anonymous union member are of const-qualified type (or array thereof),

— any direct or virtual base class, or non-static data member with no brace-or-equal-initializer, has class type M (or array thereof) and either M has no default constructor or overload resolution (13.3) as applied to M’s default constructor results in an ambiguity or in a function that is deleted or inaccessible from the defaulted default constructor, or

— any direct or virtual base class or non-static data member has a type with a destructor that is deleted or inaccessible from the defaulted default constructor.

换句话说，删除析构函数对默认构造函数的自动生成没有影响。