cannot instantiate an abstract class

Question

4.00/5 (2 votes)

See more:

C++

Hi All!
The below code works fine VS6.0, but In VS2005,2008 it is giving compilation Error C2559: "cannot instantiate an abstract class"
Due to Derived class calling base class(abstract class) constructor.

Please lets give the solution.

Thanks
Arjun

C++

class Base
{
public:
    int a;
    Base()
    {
        cout<<"Base::Base()"<<endl;
    }
    virtual void f1()= 0;

    ~Base()
    {
        cout<<"Base::~Base()"<<endl;
    }

};

class Derived : public Base
{
public:
    Derived()
    {
        cout<<"Derived::Derived()"<<endl;
    }
    void f1()
    {
        cout<<"Derived::f1()"<<endl;
    }

    void f2()
    {
        Base::Base();  //Cannot instantiate an abstract class

    }
    ~Derived()
    {
        cout<<"Derived::~Derived()"<<endl;
    }
};

Edit by Aescleal: Added code tags so lesser mortals like me can read it...

Posted 14-May-12 23:46pm

tgm.arjun09

Updated 14-May-12 23:50pm

Aescleal

v2

Add a Solution

2 solutions

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Aescleal · Accepted Answer · 2012-05-14T23:59:00

The error message says it all really, it's what any sane compiler should give you for the code you've written. You're trying to create an instance of Base which is abstract.

In case you don't know, you're trying to create the instance of base when you write Base::Base(). It's an unnamed temporary that'll be constructed and destructed immediately but there's still a request to make an object in there.

The only way you're going to make base something you can create instances of is to remove the pure virtual ( = 0 ) from f1 and give it an implementation. Nothing else will do...

And if it "worked" in VC++6.0 that's yet another reason to bury that compiler under several feet of rock and treat it like it's intensely radioactive.

Cheers,

Ash

PS: With reference to the comment below...

Repeat after me - you are not calling the constructor, you're trying to create an instance of an object. It's a bit confusing as in C++:

C++

Base::f1();

means "explicitly call f1() as found in the class Base on whatever the current object is" but:

C++

Base::Base();

means "reserve a lump of memory big enough for an object of class Base, call it's default constructor on that memory, call it's destructor then drop the memory back in the bit bucket."

Anyway, just remember you don't call constructors (even when using placement new, although it comes close) the compiler does for you and ONLY when initialising a new object from raw memory.

enhzflep · Accepted Answer · 2012-05-15T00:20:00

I'm afraid I'm not able to think of an instance where one would _want_ to call a base class's constructor in a derived class's member function. Forgive me if I overlook a situation that this is required.

That would mean that if one were to declare an instance of Derived, following this with a call to instanceDerived->f2(), that the base constructor would be called twice. My feeling is that you should have another function (say, init) that was called from the Base class's constructor. You then have the option of invoking this init method from within your f2() member function.

Here, this works just fine in VS2010 (your code had same errors that you reported):

C++

// deleteMe.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
#include <iostream>

using std::cout;
using std::endl;

class Base
{
public:
    int a;
    Base()
    {
        cout<<"Base::Base()"<<endl;
		// alloc mem here
		//
		//
		someNumber = new int;

		// init alloced mem to some value
		init();
    }
    virtual void f1()= 0;
 
    ~Base()
    {
        cout<<"Base::~Base()"<<endl;
		delete someNumber;
    }

	void init()
	{
		*someNumber = rand()%128;
	}
protected:
	int *someNumber;
};
 
class Derived : public Base
{
public:
    Derived()
    {
        cout<<"Derived::Derived()"<<endl;
    }
    void f1()
    {
        cout<<"Derived::f1()"<<endl;
    }
 
    void f2()
    {
		cout << "f2()" << endl;
		//---  Base::Base();  //Cannot instantiate an abstract class ----
		// don't want to call Base::Base here, since that will re-alloc (i.e allocate for a 2nd or more time)
		
		// BUT, we would like to reset the allocated mem to the default values
		init();
    }

	int getSomeNumber()
	{
		return *someNumber;
	}

    ~Derived()
    {
        cout<<"Derived::~Derived()"<<endl;
    }
};



int _tmain(int argc, _TCHAR* argv[])
{
	{
		Derived a;
		int tmp;

		a.f2();
		tmp = a.getSomeNumber();
		cout << "tmp = " << tmp << endl;

		a.f2();
		tmp = a.getSomeNumber();
		cout << "tmp = " << tmp << endl;

		a.f2();
		tmp = a.getSomeNumber();
		cout << "tmp = " << tmp << endl;
	}
	getchar();
	return 0;
}

Output:

Base::Base()
Derived::Derived()
f2()
tmp = 35
f2()
tmp = 62
f2()
tmp = 4
Derived::~Derived()
Base::~Base()

cannot instantiate an abstract class

2 solutions

Solution 1

Solution 2

Add your solution here

Preview 0