類字節大小，字節對齊相關

1、空類爲1字節，虛函數指針爲4字節（字節按double對齊時爲8字節），子類在父類的字節大小上面累加

class B
{
	double a; //8字節
	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;
}

類B按8字節對齊，類D按4字節對齊，所以sizeof(B): 24  sizeof(D):32

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

在B類前面加上#pragma pack(1)時，則按1字節對齊， <span style="font-family: Arial, Helvetica, sans-serif;">所以sizeof(B): 16  sizeof(D):24