c++ - 这里发生了什么？我失去了常量吗？

Question

我这里有一个 Singleton 类，旨在由想要使用它的类继承。但我对编译器允许的某些问题有疑问，但我不明白为什么:

template<class DerivedT>
class Singleton {
  static DerivedT*& internalPointer() {
    static DerivedT* object(nullptr);
    return object;
  }

  void reassignIf(Singleton* instance) {
    auto& object(internalPointer());
    if (object == static_cast<DerivedT*>(instance)) {
      object = static_cast<DerivedT*>(this);
    }
  }

  Singleton(Singleton const& other); // = delete

  Singleton& operator=(Singleton const& other); // = delete

public:
  static DerivedT* instance() { return internalPointer(); }

protected:
  Singleton() { reassignIf(nullptr); }

  Singleton(Singleton&& other) { reassignIf(&other); }

  ~Singleton() {
    auto& object(internalPointer());
    if (object == static_cast<DerivedT*>(this)) {
      object = nullptr;
    }
  }

  Singleton& operator=(Singleton&& other) { reassignIf(&other); return *this; }
};

class SingletonMock: Singleton<SingletonMock> {
public:
  using Singleton<SingletonMock>::instance;

  SingletonMock() : Singleton<SingletonMock>() { }
};

int main() {
  SingletonMock x;
  SingletonMock const y;  // why is this line allowed?
}

为什么允许底线？ Singleton 基类有一个名为 internalPointer 的静态成员函数，它有一个静态 DerivedT*。这应该成为 SingletonMock*，但它允许将 const SingletonMock 分配给该变量。

编辑:

我想我从评论中理解了它现在起作用的原因。但是，特别是它允许我这样做:

SingletonMock const y;
SingletonMock* yPtr(SingletonMock::instance());

我怎样才能阻止这样的事情被允许？

Answer 1

C++ 不允许该行。规范说 (8.5p9):

If no initializer is specified for an object, and the object is of (possibly cv-qualified) non-POD class type (orarray thereof), the object shall be default-initialized; if the object is of const-qualified type, the underlyingclass type shall have a user-declared default constructor.

(强调我的)。 C++0x (FDIS) 措辞(8.5p6 和 8.5p11):

If no initializer is specified for an object, the object is default-initialized;

If a program calls for the default initialization of an object of a const-qualified type T, T shall be a class type with a user-provided default constructor.