Question

在下面的代码中，使用==比较两个对象显示的不同输出比比较两个字符指针。请帮我解决它的工作原理。

 #include <iostream>

 class A
 {
 public:
   A() : m_i(0) { }

 protected:
   int m_i;
 };

 class B
 {
 public:
    B() : m_d(0.0) { }

 protected:
    double m_d;
 };

 class C
      : public A
       , public B
 {
  public:
      C() : m_c('a') { }

  private:
      char m_c;
  };

  int main()
  {
    C c;
    A *pa = &c;
    B *pb = &c;

    const int x = (pa == &c) ? 1 : 2;
    const int y = (pb == &c) ? 3 : 4;
    const int z = (reinterpret_cast<char*>(pa) ==   reinterpret_cast<char*>(pb)) ? 5 : 6;

    std::cout << x << y << z << std::endl;

    return 0;
  }

输出： 136

为什么y的值为3，其中z的值为6.这里到底发生了什么任何意见都表示赞赏。

Answer 1

const int y = (pb == &c) ? 3 : 4;

相当于使用

B *pb2 = &c;
const int y = (pb == bp2) ? 3 : 4;

将&c与B*进行比较后，它会自动转换为B*，从而抵消c的地址以匹配B部分C。这就是表达式(pb == &c)评估为true的原因。

但是，pa和pb是不同的地址。因此，当您使用

比较它们的值时

(reinterpret_cast<char*>(pa) ==   reinterpret_cast<char*>(pb))

该表达式的值为false。

Answer 2

You are using multiple inheritance. In this scenario, when you create an instance of Derived class typically what happens is :

Enough memory is reserved to hold data for all the base classes and the Derived class.
Order in which the data of base classes in laid out in memory is the inheritance order you specify for Derived class. For your case:

class C: public A, public B

the memory layout of object looks like: A data members, B data members followed by C data members.

In your case pa and pb are actually pointing to different addresses but y is assigned a value 3 because of implicit conversion during comparison. But by doing reinterpret cast to a char* you are taking away the information that allows the compiler to do implicit casting. Instead a plain pointer comparison is done and the result is false.

If you do C* pc = &c;

and check the value of pc in debugger, you will notice that it is the same as pa which highlights the above point.

Answer 3

你传递了对象的地址＆＃39; c＆＃39;指针＆＃39; pa＆＃39; A级和A级指针＆＃39; pb＆＃39; B级。

As＆＃39; pa＆＃39; ＆安培;＆安培; ＆＃39; PB＆＃39;保持相同的内存参考对象＆＃39; c＆＃39; C类，因此表达变量x＆amp;你是真的。

对于z变量的表达，你要比较＆＃39; pa＆＃39;的地址。用＆＃39; pb＆＃39;强制它们解释为强制转换指针，但这两个指针可能已分配在内存中的两个不同位置，因此它们不匹配，你得到z = 6（即假）

继承中的对象比较

3 个答案: