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有两个小小的bug：C++ cast漏了类型，打印指针的时候字符串里变量名。打印加换行好看点。把父类指针cast到子类指针得非常小心。最好是不需要cast，而是依赖polymorphism。在c-tor和d-tor里调用virtual function也是搞笑。当然最有破坏力的是pB1->m_b = 1

wangqingshui(忘情水)

class A
{
public:
A() : m_a(0)
{
printf("A::A\r\n");
}
virtual ~A()
{
printf("A::~A m_a=%d\r\n", m_a);
foo();
}

virtual void foo()
{
printf("A::foo\r\n");
}

int m_a;
};

class B : public A
{
public:
B() : m_b(0)
{
printf("B::B\r\n");
}
virtual ~B()
{
printf("B::~B m_b=%d\r\n", m_b);
foo();
}

virtual void foo()
{
printf("B::foo\r\n");
throw 1;
}

int m_b;
};

int _tmain(int argc, _TCHAR* argv[])
{
printf("Test 1:\r\n");
try
{
B b;
}
catch(int)
{
}

printf("Test 2:\r\n");
A a;
B *pB1 = (B*)&a;
B *pB2 = static_cast<B*>(&a);
B *pB3 = dynamic_cast<B*>(&a);
B *pB4 = reinterpret_cast<B*>(&a);
printf("&a=0x%x, pB1=0x%x, pB2=0x%x, pB3=0x%x, pB4=0x%x\r\n", &a, pB1, pB2, pB3, pB4);

printf("Test 3:\r\n");
pB1->foo();
pB1->m_b = 1;
printf("m_a=%d, m_b=%d\r\n", pB1->m_a, pB1->m_b);

return 0;
}

(#6085350@0)
Last Updated: 2010-5-23
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工作学习 / 科技领域杂谈 / 典型C++题一道，请写出输出结果。 -liquid(豆泡松果之松果豆泡); 2010-5-22 {919} (#6084757@0)

class A
{
public:
	A() : m_a(0)
	{ 
		printf("A::A"); 
	}
	virtual ~A()
	{ 
		printf("A::~A m_a=%d", m_a); 
		foo();
	}

	virtual void foo()
	{
		printf("A::foo");
	}

	int m_a;
};

class B : public A
{
public:
	B() : m_b(0)
	{
		printf("B::B"); 
	}
	virtual ~B()
	{ 
		printf("B::~B m_b=%d", m_b); 
		foo();
	}

	virtual void foo()
	{
		printf("B::foo");
		throw 1;
	}

	int m_b;
};

int main(int argc, char* argv[])
{
	printf("Test 1:");
	try
	{
		B b;
	}
	catch(int)
	{
	}

	printf("Test 2:");
	A a;
	B *pB1 = (B*)&a;
	B *pB2 = static_cast(&a);
	B *pB3 = dynamic_cast(&a);
	B *pB4 = reinterpret_cast(&a);
	printf("&a=0x%x, pB1=0x%x, pB1=0x%x, pB1=0x%x, pB1=0x%x", &a, pB1, pB2, pB3, pB4);
	
	printf("Test 3:");
	pB1->foo();
	pB1->m_b = 1;
	printf("m_a=%d, m_b=%d", pB1->m_a, pB1->m_b);

	return 0;
}

有两个小小的bug：C++ cast漏了类型，打印指针的时候字符串里变量名。打印加换行好看点。把父类指针cast到子类指针得非常小心。最好是不需要cast，而是依赖polymorphism。在c-tor和d-tor里调用virtual function也是搞笑。当然最有破坏力的是pB1->m_b = 1 -wangqingshui(忘情水); 2010-5-23 {974} (#6085350@0)
class A
{
public:
A() : m_a(0)
{
printf("A::A\r\n");
}
virtual ~A()
{
printf("A::~A m_a=%d\r\n", m_a);
foo();
}

virtual void foo()
{
printf("A::foo\r\n");
}

int m_a;
};

class B : public A
{
public:
B() : m_b(0)
{
printf("B::B\r\n");
}
virtual ~B()
{
printf("B::~B m_b=%d\r\n", m_b);
foo();
}

virtual void foo()
{
printf("B::foo\r\n");
throw 1;
}

int m_b;
};

int _tmain(int argc, _TCHAR* argv[])
{
printf("Test 1:\r\n");
try
{
B b;
}
catch(int)
{
}

printf("Test 2:\r\n");
A a;
B *pB1 = (B*)&a;
B *pB2 = static_cast<B*>(&a);
B *pB3 = dynamic_cast<B*>(&a);
B *pB4 = reinterpret_cast<B*>(&a);
printf("&a=0x%x, pB1=0x%x, pB2=0x%x, pB3=0x%x, pB4=0x%x\r\n", &a, pB1, pB2, pB3, pB4);

printf("Test 3:\r\n");
pB1->foo();
pB1->m_b = 1;
printf("m_a=%d, m_b=%d\r\n", pB1->m_a, pB1->m_b);

return 0;
}
端个小板凳，带上笔记本，等高人评点。。。 -eagle_no1(瞎起哄); 2010-5-23 (#6085595@0)

我随便说说 -oscargee(oscargee); 2010-6-23 {422} (#6143170@0)
这是一道比较有趣的题目。主要考察方向大概有以下几点：
1，关于从析构函数里面抛出异常，这是允许的。但是不允许从构造函数抛。
2，关于DOWN CAST，这几种都是可以的。dynamic_cast会在你CALL了一个A中不存在的方法的时候抛出std::bad_cast异常。但是这里没有CALL,所以没有。
3，执行->foo的时候，因为A当中也有这个方法，是正常执行。
4，给mB给值的时候，超界了， sizeof(B)和sizeof(A)在这里是不同的。所以当你退出程序的时候，出错。STACK崩溃。

Q1:What really happens when you call virtual function and throw exception in dtors? Q2:What's the difference between those castings? Q3: What exactly will "pB1->m_b = 1;" do here? Thanks wangqingshui for formatting and correcting variable names. -liquid(豆泡松果之松果豆泡); 2010-5-24 (#6086597@0)

A1: calling virtual function in dtor I believe the function is considered non-virtual because the object of the derived class part has been destructed already. -wangqingshui(忘情水); 2010-5-25 (#6087476@0)

Yes, if virtual function was called directly from ctor/dtor, it is not considered as virtual; however if it is called indirectly (e.g. through another non-virtual function), it will be considered as a virtual function call and vtable will be used. -liquid(豆泡松果之松果豆泡); 2010-5-25 (#6087534@0)

A1: Throwing exception from dtor or from functions called in dtor may cause 2 parallel unhandled exception and abort the application. -wangqingshui(忘情水); 2010-5-25 (#6087478@0)

The try/catch will catch the C++ exception, and the exception didn't even stop stack unwinding. -liquid(豆泡松果之松果豆泡); 2010-5-25 (#6087544@0)

Q3: it is kind of similar to the situation that someone intrusively peed in his neighbour's yard, possibly burnt a precious flower, or maybe peed into empty area without any actual damage. -wangqingshui(忘情水); 2010-5-25 (#6087497@0)

Normally it corrupts stack depending how far it goes. With VC8, it runs without problem with release build and error of stack corruption in debug build. -liquid(豆泡松果之松果豆泡); 2010-5-25 (#6087548@0)

钻牛角尖的时间用在任何地方得到的回报都要比钻牛角尖高。 -wincity(toad 会英文的红卫兵); 2010-5-25 (#6087525@0)

拿一堆code，问人家结果，比钻牛角尖更浪费时间。 -wincity(toad 会英文的红卫兵); 2010-5-25 (#6087527@0)

@

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