undefined reference to `vtable

c++ 编译错误

undefined reference to vtable
undefined reference to Abstract_base::~Abstract_base()

c:/mingw/bin/../lib/gcc/mingw32/9.2.0/../../../../mingw32/bin/ld.exe: C:\Users\ssas0e\AppData\Local\Temp\ccwtXlSL.o:abstract_class.cpp:(.text$_ZN13Abstract_baseC2Ev[__ZN13Abstract_baseC2Ev]+0xa): undefined reference to `vtable for Abstract_base'
c:/mingw/bin/../lib/gcc/mingw32/9.2.0/../../../../mingw32/bin/ld.exe: C:\Users\ssas0e\AppData\Local\Temp\ccwtXlSL.o:abstract_class.cpp:(.text$_ZN17derived_from_baseC1Ev[__ZN17derived_from_baseC1Ev]+0x4d): undefined reference to `Abstract_base::~Abstract_base()'
c:/mingw/bin/../lib/gcc/mingw32/9.2.0/../../../../mingw32/bin/ld.exe: C:\Users\ssas0e\AppData\Local\Temp\ccwtXlSL.o:abstract_class.cpp:(.text$_ZN17derived_from_baseD1Ev[__ZN17derived_from_baseD1Ev]+0x3e): undefined reference to `Abstract_base::~Abstract_base()'
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错误代码示例子

#include 

using namespace std;

class Abstract_base
{
    public:
    virtual ~Abstract_base();
    virtual void interface() const{};
    
};

class derived_from_base : public Abstract_base
{
    public:
    derived_from_base()
    {
        cout <<"derived ctor" << endl;

    }

    ~derived_from_base() override
    {
        cout << "~ derived dtor" << endl;
    }

    void interface() const override{}

};

int main()
{
    derived_from_base b;
    return 1;
}
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分析

vtable是虚函数表，报错说的是 undefined reference to
就是说，指向这个表的reference是未定义的。
为什么？ 这个表不存在？ 还是reference不存在？

子对象构造的过程

当derived_from_base b 开始构造时候，先构造父类的，然后构造自己的，是一种自上而下的过程。我们看到编译的错误也是先父类出的问题，然后是子类出的问题。

父类哪里出的问题？
我们看下父类构造virtual fucntion过程。

• 每一个class 产生出一堆指向virtual functions的指针,放在表格中。这个表格称为virtual table vtbl
• 每一个class object被安插一个指针，指向相关的virtual table。通常这个指针被称为vptr. vptr的设定和重置都是由每一个class的constructor 和destructor和copy assignment运算符自动完成的。每个class所关联的type_infor_object（用以支持runtime type identification）也经由virtual table被指出来，通常放在表格的第一个slot.

国外一个针对此问题更加详细的讨论
总结下

• look at your class definition. Find the first non-inline virtual function that is not pure virtual (not “= 0”) and whose definition you provide (not “= default”).

  If there is no such function, try modifying your class so there is one. (Error possibly resolved.)
  See also the answer by Philip Thomas for a caveat.

• Find the definition for that function. If it is missing, add it! (Error possibly resolved.)

  If the function definition is outside the class definition, then make sure the function definition uses a qualified name, as in ClassName::function_name.

• Check your link command. If it does not mention the object file with that function’s definition, fix that! (Error possibly resolved.)

• Repeat steps 2 and 3 for each virtual function, then for each non-virtual function, until the error is resolved. If you’re still stuck, repeat for each static data member.

回到最初问题

上述代码的错误，是未定义function，只是声明。所以function pointer应该没产生成功，更别说后面的virtual table了。