错误：在 C++ 中从“Counter”转换为非标量类型“CountDn”答案

【问题标题】：error: conversion from 'Counter' to non-scalar type 'CountDn' in c++错误：在 C++ 中从“Counter”转换为非标量类型“CountDn”
【发布时间】：2017-08-16 02:40:47
【问题描述】：

#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;
}

当我使用重载的“++”运算符时出现错误，而当我使用“--”时效果很好。

错误信息是

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

为什么会出现此错误？这个错误是什么意思，我该如何解决？

【问题讨论】：

标签： c++ inheritance

【解决方案1】：

当我使用重载的 '++' 运算符时出现错误，而当我使用 '--' 时效果很好。

您在 CountDn 类中实现了运算符重载 -- 但没有 ++

CountDn 类中缺少 ++ 的运算符重载你必须添加

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;
}

输出

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

【讨论】：

【解决方案2】：

结果

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

属于Counter 类型，并且没有从Counter 到CountDn 的转换。以下行编译正常：

Counter c3 = ++c2;

【讨论】：