C++ 虚函数

虚函数是应在派生类中重新定义的成员函数。 当使用指针或对基类的引用来引用派生的类对象时，可以为该对象调用虚函数并执行该函数的派生类版本。

如果声明的类不提供 PrintBalance 函数的重写实现，则使用基类 Account 中的默认实现。

可通过使用范围解析运算符 (::) 显式限定函数名称来禁用虚函数调用机制。

#include <iostream>
using namespace std;

class Base {
public:
	virtual void NameOf();   // Virtual function.
	void InvokingClass();   // Nonvirtual function.
};

// Implement the two functions.
void Base::NameOf() {
	cout << "Base::NameOf\n";
}

void Base::InvokingClass() {
	cout << "Invoked by Base\n";
}

class Derived : public Base {
public:
	void NameOf();   // Virtual function.
	void InvokingClass();   // Nonvirtual function.
};

// Implement the two functions.
void Derived::NameOf() {
	cout << "Derived::NameOf\n";
}

void Derived::InvokingClass() {
	cout << "Invoked by Derived\n";
}

int main() {
	// Declare an object of type Derived.
	Derived aDerived;

	// Declare two pointers, one of type Derived * and the other
	//  of type Base *, and initialize them to point to aDerived.
	Derived* pDerived = &aDerived;
	Base* pBase = &aDerived;

	// Call the functions.
	pBase->NameOf();           // Call virtual function.
	pBase->InvokingClass();    // Call nonvirtual function.
	pBase->Base::NameOf();

	pDerived->NameOf();        // Call virtual function.
	pDerived->InvokingClass(); // Call nonvirtual function.
}

Derived::NameOf
Invoked by Base
Base::NameOf
Derived::NameOf
Invoked by Derived