LNK2001: unresolved external symbol ... virtual ...

1

class A {
	virtual void fun2();
	virtual void fun3() {};
};

int main() {
	new A;
	return 0;
}

//LNK2001: unresolved external symbol "private: virtual void __cdecl A::fun2(void)" (?fun2@A@@EEAAXXZ)

2

class A {
	virtual void fun1()=0;
	virtual void fun2();
	virtual void fun3() {};
};

class B : A {
	void fun1() override {}
};

class C : B {
	void fun2() override {}
};

int main() {
	new B;
	return 0;
}

//LNK2001 : unresolved external symbol "private: virtual void __cdecl A::fun2(void)" (? fun2@A@@EEAAXXZ)

//B继承了A，实现了A中的纯虚函数fun1。A中的虚函数fun2没有实现。
//构造B对象前，先要构造A，A无法构造，导致无法构造B对象实例。

3

class A {
	virtual void fun1()=0;
	virtual void fun2();
	virtual void fun3() {};
};

class B : A {
	void fun1() override {}
	void fun2() override {};
};

class C : B {
	void fun2() override {}
};

int main() {
	new B;
	return 0;
}

//LNK2001: unresolved external symbol "private: virtual void __cdecl A::fun2(void)" (?fun2@A@@EEAAXXZ)

// B中实现了A中的虚函数fun2，还是无法构造成功。因为无法构造A对象。

4

class A {
	virtual void fun1()=0;
	virtual void fun2() {};
	virtual void fun3() {};
};

class B : A {
	void fun1() override {}
};

class C : B {
	void fun2() override {}
};

int main() {
	new B;
	return 0;
}

//OK

5

class A {
	virtual void fun1()=0;
	virtual void fun2() {};
	virtual void fun3() {};
};

class B : A {
	void fun1() override {}
};

class C : B {
};

int main() {
	new C;
	return 0;
}

//OK

6

class A {
	virtual void fun1()=0;
	virtual void fun2();
	virtual void fun3() {};
};

class B : A {
	void fun1() override {}
};

class C : B {
	void fun2() override {}
};

int main() {
	new C;
	return 0;
}

//LNK2001: unresolved external symbol "private: virtual void __cdecl A::fun2(void)" (?fun2@A@@EEAAXXZ)

//虽然C中实现了A中的虚函数fun2。
//构造C对象前，先要构造B对象，构造B对象前，要先构造A，无法构造A，导致无法构造C对象。

7

class A {
	virtual void fun1()=0;
	virtual void fun2();
	virtual void fun3() {};
};

class B : A {
	void fun1() override {}
	void fun2() override {};
};

class C : B {
	void fun2() override {}
};

int main() {
	new C;
	return 0;
}

//LNK2001: unresolved external symbol "private: virtual void __cdecl A::fun2(void)" (?fun2@A@@EEAAXXZ)