类B用公有继承的方式继承了类A的私有成员变量a,b,c,d，

#include <iostream>
using namespace std;
struct point {
float x, y;
};
class Quadrangle
{
public:
void init() // sets coordinates for A, B, C, D
{
cout << "a:";
cin >> a;
cout << "Input A coordinates (x, y): ";
cin >> A.x >> A.y;
cout << "Input B coordinates (x, y): ";
cin >> B.x >> B.y;
cout << "Input C coordinates (x, y): ";
cin >> C.x >> C.y;
cout << "Input D coordinates (x, y): ";
cin >> D.x >> D.y;
};
void show()
{
cout << a << endl;
cout << "A:(" << A.x << "," << A.y << ")\n";
cout << "B:(" << B.x << "," << B.y << ")\n";
cout << "C:(" << C.x << "," << C.y << ")\n";
cout << "D:(" << D.x << "," << D.y << ")\n";
}
void square()
{
// square calculation
};
int a;
private:
point A, B, C, D;
//int a;
};
class Rectangle : public Quadrangle
{
public:
void square()
{
// square calculation
};
/*
void show()
{
cout << a << endl;
cout << "A:(" << A.x << "," << A.y << ")\n";
cout << "B:(" << B.x << "," << B.y << ")\n";
cout <

class Rectangle : public Quadrangle
{
public:
void square()
{
// square calculation
};
/*
void show()
{
cout << a << endl;
cout << "A:(" << A.x << "," << A.y << ")\n";
cout << "B:(" << B.x << "," << B.y << ")\n";
cout << "C:(" << C.x << "," << C.y << ")\n";
cout << "D:(" << D.x << "," << D.y << ")\n";
}
*/
private:
point A, B, C, D;
int a;
};
int main()
{
Rectangle tmp;
tmp.init();
tmp.show();
}

在子类重写和不重写show方法得到的结果不同

我通过sizeof打印验证确实子类占的内存是父类的2倍，因为我在子类都声明了同名的变量

被嵌套的作用域中的名称会被嵌套其中的作用域中的同名名称覆盖。
对于non-virtual成员，其表达的是不变性，请不要在派生类中定义同名成员。

变量在类里有效

因为你子类的show打印的是子类的ABCD成员，然而你init是父类的函数，初始化的是父类的ABCD成员。子类再写个show把父类的覆盖了，除非你使用b.A::show()来调用父类的show，不然调的只会是子类的show。然后你show打印的是子类没初始化的成员。如果你子类不写show，那么没有覆盖父类的show，之后调用的也是父类的show，父类的成员你是在init里初始化过了的，所以不是无效值。