C++之virtual虚函数的几种用法

1. Virtual function

Base class(virtual ), derived class(override)

1）没有使用 virtual 的结果

#include <iostream> 
#include <string>

class base 
{
public:
	std::string GetName() { return "base class"; }
};

class derived :public base
{
public:
	std::string GetName() { return "derived class"; }
private:
	std::string m_Name;
};


int main()
{
	base* Base = new base();
    std::cout <<Base->GetName() << std::endl;
	std::cout << "--------------------------\n";
	derived* Derived = new derived();
	std::cout <<Derived->GetName() << std::endl;
	std::cout << "--------------------------\n";
	base* Poly = Derived;
	std::cout << Poly->GetName() << std::endl;

	std::cin.get();
	return 0;
}

结果：

2）加入virtual 和 override

#include <iostream> 
#include <string>

class base 
{
public:
	virtual std::string GetName() { return "base class"; }  //虚函数
};

class derived :public base
{
public:
	std::string GetName() override { return "derived class"; }
private:
	std::string m_Name;
};


int main()
{
	base* Base = new base();
    std::cout <<Base->GetName() << std::endl;
	std::cout << "--------------------------\n";
	derived* Derived = new derived();
	std::cout <<Derived->GetName() << std::endl;
	std::cout << "--------------------------\n";
	base* Poly = Derived;
	std::cout << Poly->GetName() << std::endl;

	std::cin.get();
	return 0;
}

结果：

3. Pure virtual function

It is Claimed in base class without implementation, and enforce the subclass (derived class) to implement

#include <iostream> 
#include <string>

class base 
{
public:
	virtual std::string GetName() = 0; //虚函数
};

class derived :public base
{
public:
	std::string GetName() override { return "derived class"; }  // 必须定义，否则报错
private:
	std::string m_Name;
};


int main()
{
	//base* Base = new base(); //不允许生成base实体
	//std::cout << "--------------------------\n";
	derived* Derived = new derived();
	std::cout <<Derived->GetName() << std::endl;
	std::cout << "--------------------------\n";
	base* Poly = Derived;
	std::cout << Poly->GetName() << std::endl;

	std::cin.get();
	return 0;
}

2. Virtual destructor

1）没有使用virtual destructor代码及其结果：

#include <iostream> 

class base 
{
public:
	base() { std::cout << "base constructor\n"; }
	~base() { std::cout << "base destructor\n"; }
};

class derived :public base
{
public:
	derived() { m_array = new int[5];  std::cout << "derived constructor\n"; }
	~derived() { delete[] m_array;  std::cout << "derived destructor\n"; }
private:
	int* m_array;
};


int main()
{
	base* Base = new base();
	delete Base;
	std::cout << "--------------------------\n";
	derived* Derived = new derived();
	delete Derived;
	std::cout << "--------------------------\n";
	base* Poly = new derived();   //多态
	delete Poly;


	std::cin.get();
	return 0;
}

结果：

此时的 m_array 没有释放掉，存在内存泄漏。

2）让 base class 中的 destructor为虚函数即可

#include <iostream> 
#include <functional>

class base 
{
public:
	base() { std::cout << "base constructor\n"; }
	virtual ~base() { std::cout << "base destructor\n"; }  //虚析构函数
};

class derived :public base
{
public:
	derived() { m_array = new int[5];  std::cout << "derived constructor\n"; }
	~derived() { delete[] m_array;  std::cout << "derived destructor\n"; }
private:
	int* m_array;
};


int main()
{
	base* Base = new base();
	delete Base;
	std::cout << "--------------------------\n";
	derived* Derived = new derived();
	delete Derived;
	std::cout << "--------------------------\n";
	base* Poly = new derived();   //多态
	delete Poly;


	std::cin.get();
	return 0;
}