类字节大小，字节对齐相关

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