眼见的事实尚有假，背后的言语未必真。---谚语

当使用另一种方式去实现相同的任务时，过往的经验可以帮助你更快速的分析和实现。但有时候经验也会产生负面影响。

下面的一段例子表示当一个对象以值的形式包含另一个对象时，会自动调用另一个对象的析构函数。

#include <iostream>
#include <cstring>
using namespace std;
class Tyre
{
	char* brand;
public:
	Tyre(Tyre &tyre)
	{
		cout<<"first construct."<<endl;
		this->brand = new char[strlen(tyre.brand)+1];
		strcpy(this->brand,tyre.brand);
	}
	Tyre(char* _brand)
	{
		cout<<"Tyre("<<_brand<<")"<<endl;
		this->brand = new char[strlen(_brand)+1];
		strcpy(this->brand,_brand);
	}
	~Tyre()
	{
		cout<<"Tyre destruct:"<<brand<<endl;
		delete brand;
	}
	void print()
	{
		cout<<"This is "<<brand<<" Tyre."<<endl;
	}	
};


class Car
{
	char* name;
	Tyre tyre2;
public:
	Car(char* name,Tyre tyre):tyre2(tyre)
	{
		this->name = new char[strlen(name)+1];
		strcpy(this->name,name);
		tyre2.print();
	}
	~Car()
	{
		cout<<"Car destructor:only delete name"<<endl;
		delete name;
	}
	void print()
	{
		cout<<"Car :";
		this->tyre2.print();
	}
};

int main()
{
	
	Tyre tyre1("g");
	Car theCar("lexus",tyre1);
	theCar.print();
	
	return 0;
}

编译：gcc -o leak main2.cpp -lstdc++
运行：

D:\workspace\C++\memory_leak\leak2>leak
Tyre(g)
first construct.
first construct.
This is g Tyre.
Tyre destruct:g
Car :This is g Tyre.
Car destructor:only delete name
Tyre destruct:g
Tyre destruct:g

以引用的方式也一样自动调用其析构：

//
#include <iostream>
#include <cstring>
using namespace std;
class Tyre
{
	char* brand;
public:
	Tyre(char* _brand);
	~Tyre();
	void print();	
};

Tyre::Tyre(char* _brand)
{
	cout<<"Tyre("<<_brand<<")"<<endl;
	this->brand = new char[strlen(_brand)+1];
	strcpy(this->brand,_brand);
}

Tyre::~Tyre()
{
	cout<<"Tyre destruct:"<<brand<<endl;
	delete brand;
}

void Tyre::print()
{
	cout<<"This is "<<brand<<" Tyre."<<endl;
}

class Car
{
	char* name;
	Tyre& tyre2;
public:
	Car(char* name,Tyre& tyre);
	~Car();
	void print();
};

Car::Car(char* name,Tyre& tyre):tyre2(tyre)
{	
	this->name = new char[strlen(name)+1];
	strcpy(this->name,name);
	tyre2.print();
}

Car::~Car()
{
	cout<<"Car destructor:only delete name"<<endl;
	delete name;
}

void Car::print()
{
	cout<<"Car :";
	this->tyre2.print();
}

int main()
{
	
	Tyre tyre1("g");
	Car theCar("lexus",tyre1);
	theCar.print();
	
	return 0;
}

编译运行结果：

D:\workspace\C++\memory_leak\leak2>leak
Tyre(g)
This is g Tyre.
Car :This is g Tyre.
Car destructor:only delete name
Tyre destruct:g

而当包含对象的指针时，就不会像之前那样自动调用析构了，必须要像以前的规则，new和delete要成对出现：

//
#include <iostream>
#include <cstring>
using namespace std;
class Tyre
{
	char* brand;
public:
	Tyre(char* _brand);
	~Tyre();
	void print();	
};

Tyre::Tyre(char* _brand)
{
	cout<<"Tyre("<<_brand<<")"<<endl;
	this->brand = new char[strlen(_brand)+1];
	strcpy(this->brand,_brand);
}

Tyre::~Tyre()
{
	cout<<"Tyre destruct:"<<brand<<endl;
	delete brand;
}

void Tyre::print()
{
	cout<<"This is "<<brand<<" Tyre."<<endl;
}

class Car
{
	char* name;
	Tyre* tyre;
public:
	Car(char* name,char* _tyre);
	~Car();
	void print();
};

Car::Car(char* name,char* _tyre)
{
	this->name = new char[strlen(name)+1];
	strcpy(this->name,name);
	tyre = new Tyre(_tyre);
}

Car::~Car()
{
	cout<<"Car destructor:only delete name"<<endl;
	delete name;
//	delete tyre;//Do not forget.
}

void Car::print()
{
	cout<<"Car :";
	tyre->print();
}

int main()
{			
	Car car1("benz","goodyear");
	car1.print();
	Car car2("bmw","michelin");
	car2.print();

	Car *car3 = new Car("audi","continental");
	car3->print();
	delete car3;
	
	return 0;
}

运行结果如下：

D:\workspace\C++\memory_leak\leak2>leak
Tyre(goodyear)
Car :This is goodyear Tyre.
Tyre(michelin)
Car :This is michelin Tyre.
Tyre(continental)
Car :This is continental Tyre.
Car destructor:only delete name
Car destructor:only delete name
Car destructor:only delete name

有三个Tyre没有释放内存，Car::~Car中解开注释，编译运行如下：

D:\workspace\C++\memory_leak\leak2>leak
Tyre(goodyear)
Car :This is goodyear Tyre.
Tyre(michelin)
Car :This is michelin Tyre.
Tyre(continental)
Car :This is continental Tyre.
Car destructor:only delete name
Tyre destruct:continental
Car destructor:only delete name
Tyre destruct:michelin
Car destructor:only delete name
Tyre destruct:goodyear

生活中不能缺少经验，但不要迷信经验。