错误:没有''成员函数在类中声明''

Question

我正在尝试创建一个包含虚函数的类,我想在两个子类中继承它.

我知道有些人已经问过这个(例如这里和那里),但我无法理解答案.

所以我做了一个我正在尝试的简化示例代码:

//Mother .h file 

#ifndef _MOTHER_H_
#define _MOTHER_H_
#include <iostream>

class Mother
{
 protected :
   std::string _name;
 public:
   Mother(std::string name);
   ~Mother();
   virtual std::string getName() = 0; 
};

#endif

//Mother .cpp file

#include "Mother.h"

Mother::Mother(std::string name)
{
  this->_name = name; 
}

Mother::~Mother()
{
}


//Child.h file

#ifndef _CHILD_H_
#define _CHILD_H_
#include "Mother.h"

class Child : public Mother
{
 private : 
   std::string _name;
 public:
   Child(std::string name);
   ~Child();
};

#endif


//Child .cpp file

#include "Mother.h"
#include "Child.h"

Child::Child(std::string name) : Mother(name)
{
  this->_name = name;
}

Child::~Child()
{
}

std::string Mother::getName()
{
  return this->_name;
}

这是我的main.cpp文件:

//Main.cpp file
#include "Child.h"

int main()
{
   Child l("lol");

   std::cout << l.getName() << std::endl;

   Mother& f = l;

   std::cout << f.getName() << std::endl;

   return 0;
 }

这是compilator所说的:( 用g ++编译.cpp -W -Wall -Wextra -Werror)

main.cpp: In function ‘int main()’:
main.cpp:5:9: error: cannot declare variable ‘l’ to be of abstract type‘Child’
In file included from main.cpp:1:0:
Child.h:8:7: note:   because the following virtual functions are pure within ‘Child’:
In file included from Child.h:6:0,
             from main.cpp:1:
Mother.h:14:23: note:   virtual std::string Mother::getName()

我究竟做错了什么 ？

(对不起,如果我犯了一些英语错误,我不是母语人士).

Answer 1

在Mother你的宣言中你有:

virtual std::string getName() = 0; 

这不仅仅是一个virtual,而是一个纯粹的 virtual.a virtual和pure 之间的区别在于,即使您在基类中提供了实现,virtual纯类还必须在派生类中实现重写.例如:

class Foo
{
public:
  virtual void DoIt() = 0 {};  // pure virtual.  Must be overridden in the derived class even though there is an implementation here
};

class Bar : public Foo
{
public:
  void DoIt(); // override of base
};

void Bar::DoIt()
{
  // implementation of override
}

您无法使用未实现的纯virtual方法实例化类.如果您尝试,您将收到编译器错误:

int main()
{
  Foo f;  // ERROR
  Bar b;  // OK
}

这正是你想要做的.你声明getName()是纯粹 virtual的 Mother,但你没有覆盖它Child.然后你试图实例化一个Child

int main()
{
   Child l("lol");

这导致编译器错误.

要修复它,请getName()在Child类中提供覆盖.