c++ - 为什么允许我声明一个带有已删除析构函数的对象？

Question

考虑以下文本:

[C++11: 12.4/11]: Destructors are invoked implicitly

• for constructed objects with static storage duration (3.7.1) at program termination (3.6.3),
• for constructed objects with thread storage duration (3.7.2) at thread exit,
• for constructed objects with automatic storage duration (3.7.3) when the block in which an object is created exits (6.7),
• for constructed temporary objects when the lifetime of a temporary object ends (12.2),
• for constructed objects allocated by a new-expression (5.3.4), through use of a delete-expression (5.3.5),
• in several situations due to the handling of exceptions (15.3).

A program is ill-formed if an object of class type or array thereof is declared and the destructor for the class is not accessible at the point of the declaration. Destructors can also be invoked explicitly.

那为什么这个程序编译成功了呢？

#include <iostream>

struct A 
{
    A(){ };
    ~A() = delete;
};

A* a = new A;

int main() {}

// g++ -std=c++11 -O2 -Wall -pedantic -pthread main.cpp && ./a.out

GCC 只是宽容吗？

---

我倾向于这样说，因为它拒绝以下 the standard appears to have no particular rule specific to deleted destructors in an inheritance hierarchy (唯一松散相关的措辞与默认构造函数的生成有关):

#include <iostream>

struct A 
{
    A() {};
    ~A() = delete;
};

struct B : A {};

B *b = new B; // error: use of deleted function

int main() {}

Answer 1

第一部分不是格式错误的，因为标准文本不适用——那里没有声明类型 A 的对象。

对于第二部分，让我们回顾一下对象构造的工作原理。标准规定 (15.2/2) 如果构造的任何部分抛出，则到该点为止的所有完全构造的子对象都将按照构造的相反顺序销毁。

这意味着构造函数的底层代码，如果全部是手工编写的，将如下所示:

// Given:
struct C : A, B {
   D d;
   C() : A(), B(), d() { /* more code */ }
};

// This is the expanded constructor:
C() {
  A();
  try {
    B();
    try {
      d.D();
      try {
        /* more code */
      } catch(...) { d.~D(); throw; }
    } catch(...) { ~B(); throw; }
  } catch(...) { ~A(); throw; }
}

对于更简单的类，默认构造函数的扩展代码(new 表达式需要其定义)如下所示:

B::B() {
  A();
  try {
    // nothing to do here
  } catch(...) {
    ~A(); // error: ~A() is deleted.
    throw;
  }
}

在某些子对象的初始化完成后不可能抛出异常的情况下进行这项工作实在是太复杂了，无法指定。因此，这实际上并没有发生，因为 B 的默认构造函数首先被隐式定义为删除，这是由于 N3797 12.1/4 中的最后一个要点:

A defaulted default constructor for class X is defined as deleted if:

• [...]
• 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.

12.8/11 中的第四个项目符号存在用于复制/移动构造函数的等效语言。

12.6.2/10还有一个重要的段落:

In a non-delegating constructor, the destructor for each direct or virtual base class and for each non-static data member of class type is potentially invoked.