类字节大小，字节对齐相关

1、空类为1字节，虚函数指针为4字节（字节按double对齐时为8字节），子类在父类的字节大小上面累加

class B
{
	double a; //8字节
	int b;

public:
	virtual ~B(){ cout << "B::~B()" << endl; }
};

class D : public B
{
 	int i;
 	int j;
public:
	virtual ~D() { cout << "D::~D()" << endl; }
};

int main(void)
{
	cout << "sizeB:" << sizeof(B) << " sizeD:" << sizeof(D) << endl;
	B *pb = new D[2];

	delete[] pb;

	return 0;
}

类B按8字节对齐，类D按4字节对齐，所以sizeof(B): 24  sizeof(D):32

<span style="font-family: Arial, Helvetica, sans-serif;">#pragma pack(1)</span>

在B类前面加上#pragma pack(1)时，则按1字节对齐， <span style="font-family: Arial, Helvetica, sans-serif;">所以sizeof(B): 16  sizeof(D):24