错误：在C ++中从“计数器”转换为非标量类型“ CountDn”

Question

#include <iostream>
using namespace std; 
class Counter
{
protected:
unsigned int count;
public:
    Counter ():count ()
  { }
    Counter (int c):count (c)
  { }
  unsigned int get_count () const
  { return count;}
  Counter operator ++ ()
  {return Counter (++count);}
  };

  class CountDn:public Counter
  {
  public:CountDn ():Counter (){ }
  CountDn (int c):Counter (c){ }
  CountDn operator -- ()
  {return CountDn (--count);}
  };

  int main ()
  {
    CountDn c1;
    CountDn c2 (100);
    cout << "\nc1=" << c1.get_count ();
    cout << "\nc2=" << c2.get_count ();
    ++c1;
    ++c1;
    ++c1;
    cout << "\nc1=" << c1.get_count ();
    --c2;
    --c2;
    cout << "\n c2=" << c2.get_count ();
    CountDn c3 = ++c2;
    cout << "\n c3=" << c3.get_count ();
    cout << endl;
    return 0;
}

使用重载的“ ++”运算符时出现错误，而使用“-”则可以正常工作。

错误消息是

37:14: error: conversion from ‘Counter’ to non-scalar type ‘CountDn’ requested
 CountDn c3 = ++c2;

为什么会出现此错误？ 此错误是什么意思，我该如何解决？

Answer 1

使用重载的“ ++”运算符时出现错误，而使用“-”则可以正常工作。

您可以在CountDn类中实现-而不是++的运算符重载实现

您必须在CountDn类中缺少++的运算符重载

CountDn operator ++ ()
{return CountDn (++count);}

修改后

#include <iostream>
using namespace std;
class Counter
{
protected:
    unsigned int count;
public:
    Counter ():count ()
    { }
    Counter (int c):count (c)
    { }
    unsigned int get_count () const
    { return count;}
    Counter operator ++ ()
    {return Counter (++count);}

};

class CountDn:public Counter
{
public:CountDn ():Counter (){ }
    CountDn (int c):Counter (c){ }
    CountDn operator -- ()
    {return CountDn (--count);}
    CountDn operator ++ ()
    {return CountDn (++count);}
};

int main ()
{
    CountDn c1;
    CountDn c2 (100);
    cout << "\nc1=" << c1.get_count ();
    cout << "\nc2=" << c2.get_count ();
    ++c1;
    ++c1;
    ++c1;
    cout << "\nc1=" << c1.get_count ();
    --c2;
    --c2;
    cout << "\n c2=" << c2.get_count ();
    CountDn c3 = ++c2;
    cout << "\n c3=" << c3.get_count ();
    cout << endl;
    return 0;
}

产量

c1=0
c2=100
c1=3
 c2=98
 c3=99
Program ended with exit code: 0

Answer 2

的结果

Counter operator ++ ()
    {return Counter (++count);}

属于Counter类型，并且没有从Counter到CountDn转换。 以下行可以编译：

Counter c3 = ++c2;