C++ -- why should we define the pure virtual destructor outside the class definition?

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

Objec开发者_运维问答t::~Object() {} // Should we always define the pure virtual destructor outside?

Question: Should we always define the pure virtual destructor outside the class definition?

In other words, it is the reason that we should not define any virtual function inline?

Thank you

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You can define virtual functions inline. You cannot define pure virtual functions inline.

The following syntax variants are simply not permitted:

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

But this is fully valid:

virtual ~Foo() { }

You must define a pure virtual destructor if you intend to instantiate it or subclasses, ref 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.

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A pure virtual function does not have an implementation by definition. It should be implemented by the sub-classes.

If you want to chain the destructor, just keep it virtual (non pure virtual).