error: no ‘’ member function declared in class ''

Question

I'm trying to create a class containing a virtual function, which i would like to inherit in two child classes.

I know some people already asked this (here and there for example), but i couldn't understand the answers.

So i made a simplified example code of what i'm trying :

//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;
}

Here is my main.cpp file :

//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;
 }

Here's what the compilator says : (compiling with g++ *.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()

What am i doing wrong ?

(sorry if i made some english mistakes, i am not a native speaker).

You have not overridden the pure virtual function getName() in your child class. — metal, Jan 21 '14 at 13:39
You need to implement getName() function in your child class. — Abhishek Bansal, Jan 21 '14 at 13:39
Children don't really inherit from mothers in the OOP world. — mike3996, Jan 21 '14 at 13:43
@Joker_vD This is assignment that i have to do, so getName has to be pure... — trolologuy, Jan 21 '14 at 13:47
@AbhishekBansal but that's what i did? no? or shouldn't i inherit from mother, and just rewrite it in the Child? — trolologuy, Jan 21 '14 at 13:49
The names of your include guards use reserved identifiers, https://stackoverflow.com/questions/228783/what-are-the-rules-about-using-an-underscore-in-a-c-identifier/228797#228797 — Benjamin Bannier, Jan 21 '14 at 13:56

score 5 · Accepted Answer · edited Jul 06 '14 at 20:22

5

In Mother's declaration you have:

virtual std::string getName() = 0;

This is not just a virtual, but a pure virtual. The distinction between a virtual and a pure virtual is that the pure variety must have an override implemented in a derived class, even if you have provided an implementation in the base class. For example:

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
}

You can't instantiate a class with un-implemented pure virtual methods. If you try, you will get a compiler error:

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

And that is exactly what you tried to do. You declared getName() to be pure virtual in Mother, but you did not override it in Child. Then you tried to instantiate a Child

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

Which resulted in the compiler error.

To fix it, provide an override of getName() in the Child class.

edited Jul 06 '14 at 20:22

Huckle

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answered Jan 21 '14 at 13:50

John Dibling

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`std::string Child::getName() { return Mother::getName(); }`? – Joker_vD Jan 21 '14 at 13:55
Thanks a lot ! Now i understood my mistake ! :) I still don't understand the logic behind it, but now i now how to use it. – trolologuy Jan 21 '14 at 13:55
@Joker_vD: that won't compile. There's no implementation in 'Mother' – John Dibling Jan 21 '14 at 13:58
@JohnDibling There is, scroll down the code in the question all the way down. – Joker_vD Jan 21 '14 at 14:00
@Joker_vD: Ah, yes, I had missed that. – John Dibling Jan 21 '14 at 15:17

score 3 · Answer 2 · answered Jan 21 '14 at 13:42

3

You class child should override getName() method as it is pure virtual defined in class mother

Seems typo to me.. as std::string Mother::getName() is defined in child.cpp..

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

answered Jan 21 '14 at 13:42

Digital_Reality

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Well... this worked... but what's the point of redefining the Function in the Child if i already defined it in the Mother class? – trolologuy Jan 21 '14 at 13:51
1

Infact that is wrong.. there should be no function name getName in class mother... as it is pure virtual (body =0). In this example, there must be only one getName ..and that must be in Child! – Digital_Reality Jan 21 '14 at 13:52
@Digital_Reality Well, having definition for pure virtual function *in the class where it was declared* is not against the standard. It would allow you to implement `Child::getName` as `return Mother::getName()`. – Joker_vD Jan 21 '14 at 13:54
@Joker_vD yes but I am not really sure, if defining a pure virtual function does makes any sense!! – Digital_Reality Jan 21 '14 at 13:58

score 0 · Answer 3 · answered Jan 21 '14 at 13:51

The pure virtual function in your base class Mother does not make sense from an OOP perspective, it is a common trait in all children so the same function could be used. There is no need to override it.

struct Person
{
  Person(std::string name) : _name(name) {}
  std::string _name;
  std::string getName() {return _name; }
};

struct Mother : Human
{
   Mother(std::string name) : Person(name) {}
};

struct Child : Human
{
   Child(std::string name) : Person(name) {}
};

error: no ‘’ member function declared in class ''

3 Answers3