Whats wrong with the following code? It's not compiling

Question

#include <iostream> 
#include <vector>
using namespace std;

class Base
{
public:
      void Display( void )
      {
            cout<<"Base display"<<endl;
      }

      int Display( int a )
      {
            cout<<"Base int display"<<endl;
            return 0;
      }

};

class Derived : public Base
{
public:

      void Display( void )
      {
            cout<<"Derived display"<<endl;
      }
};


void main()
{
   Derived obj;
   obj.Display();  
   obj.Display( 10 );
}

$test1.cpp: In function ‘int main()’:  
test1.cpp:35: error: no matching function for call to ‘Derived::Display(int)’  
test1.cpp:24: note: candidates are: void Derived::Display()

On commenting out obj.Display(10), it works.

What compiler are you using ? It looks like `gcc` but is this the most recent code ? Why you get `$test1.cpp: In function ‘int main()’:` even though you have defined main as `void main()` ? — , Mar 17 '11 at 08:09
tried with gcc and vc++. Copied code from the vc++ editor, pasted gcc output. — Ganesh Kundapur, Mar 17 '11 at 08:14

score 3 · Answer 1 · edited May 23 '17 at 11:44

3

You need to use the using declaration. A member function named f in a class X hides all other members named f in the base classes of X.

Why?

Read this explanation by AndreyT

You can bring in those hidden names by using a using declaration:

using Base::Display is what you need to include in the derived class.

Furthermore void main() is non-standard. Use int main()

edited May 23 '17 at 11:44

Community

1
1

answered Mar 17 '11 at 08:06

Prasoon Saurav

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He has defined main as `void main()` but how come he gets `$test1.cpp: In function ‘int main()’:` ? – Mar 17 '11 at 08:07
5

@Muggen: It's illegal to declare `main` as having a return type of anything other than `int`. This particular implementation seems to just ignore this error and treat the code as if it declared `main` correctly. – CB Bailey Mar 17 '11 at 08:11

score 2 · Accepted Answer · answered Mar 17 '11 at 08:04

2

You need to place -

using Base::Display ; // in Derived class

If a method name is matched in the Derived class, compiler will not look in to the Base class. So, to avoid this behavior, place using Base::Display;. Then the compiler will look into the Base class if there is any method that can actually take int as argument for Display.

class Derived : public Base
{
    public:
    using Base::Display ;
    void Display( void )
    {
        cout<<"Derived display"<<endl;
    }
};

answered Mar 17 '11 at 08:04

Mahesh

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is this something compiler-specific? – Nathan Fellman Mar 17 '11 at 08:06
1

It works with the using. Could you please explain me whats happening without the using? – Ganesh Kundapur Mar 17 '11 at 08:08
Shouldn't int Display(int) get derived anyway as it's declared as public. – Shamim Hafiz - MSFT Mar 17 '11 at 08:10
@Gunner : Its gets derived but is hidden in the derived class. – Prasoon Saurav Mar 17 '11 at 08:15
@Gunner - It gets derived. But the problem is both the base and derived class has a method bearing same name. So, it is hidden. – Mahesh Mar 17 '11 at 08:17
@Prasoon: Why is it kept hidden if it got derived. What is the notion behind that? – Shamim Hafiz - MSFT Mar 17 '11 at 08:18
1

@Gunner : For historical reasons. Read my answer. – Prasoon Saurav Mar 17 '11 at 08:18
@Mahesh: The name maybe same, but signature is different. It should be possible to invoke it without any ambiguity of redefinition. – Shamim Hafiz - MSFT Mar 17 '11 at 08:19
@Gunner - In `void Display( void );` - **Display** is the method name, right ? – Mahesh Mar 17 '11 at 08:22
@Mahesh,Prasoon Saurav: I read the historical reason and now it's clear to me. Thanks all for the clarification. – Shamim Hafiz - MSFT Mar 17 '11 at 08:26

score 2 · Answer 3 · answered Mar 17 '11 at 08:09

2

You are masking the original function definition by creating a function in the derived class with the same name: http://www.parashift.com/c++-faq-lite/strange-inheritance.html#faq-23.8

answered Mar 17 '11 at 08:09

jonsca

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Whats wrong with the following code? It's not compiling

3 Answers3