C++——我们为什么要在类定义之外定义纯虚析构函数？

Question

class Object {
public:
  ...
  virtual ~Object() = 0;
  ...
};

Object::~Object() {} // Should we always define the pure virtual destructor outside?

问题:我们是否应该始终在类定义之外定义纯虚析构函数？

换句话说，这就是我们不应该定义任何内联虚函数的原因？

谢谢

Answer 1

您可以内联定义虚函数。您不能内联定义纯虚拟函数。

根本不允许以下语法变体:

virtual ~Foo() = 0 { }
virtual ~Foo() { } = 0;

但这是完全有效的:

virtual ~Foo() { }

如果您打算实例化它或子类，则必须定义一个纯虚拟析构函数，引用 12.4/7:

A destructor can be declared virtual (10.3) or pure virtual (10.4); if any objects of that class or any derived class are created in the program, the destructor shall be defined. If a class has a base class with a virtual destructor, its destructor (whether user- or implicitly- declared) is virtual.