Date: Tue, 19 Jan 1999 14:18:34 +0000 (BST)
From: M P Tidball <M DOT P DOT Tidball AT cs DOT cf DOT ac DOT uk>
To: DJ Delorie <djgpp AT delorie DOT com>
Subject: Virtual functions not deferring to Derived class implementation
Message-ID: <Pine.SOL.3.95.990119141213.7439B-100000@thrall.cs.cf.ac.uk>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
Reply-To: djgpp AT delorie DOT com

I have put this on the newsgroup a couple of times but have had no joy.

This test program tries to enter 2 instances of Thing's derived classes,
Thing_a and Thing_b, into a vector of Things.

When it comes to reading the Things from the vector, the Thing
implementations of the functions are run instead of the Thing_a and
Thing_b ones. This is clearly not what is meant to happen.

Can you explain this phenomenon for me - I think there is something funny
going on in the compiler.

Cheers,

Matthew
 

#include<iostream>
#include<string>
#include<vector>

class Thing
{
	public:
	   	virtual int number() { return -50; }
        	virtual string str() { return "Base class str"; }
		virtual void print() { cout << "Base class print"; }
};

class Thing_a : public Thing
{
	public:
		Thing_a( int num ) :
			_number( num ) {};
        	int number() { return _number; }
        	string str() { return "Thing_a str"; }
		void print();

	private:
		int _number;
};

void
Thing_a::print()
{
	cout << _number << endl;
}

class Thing_b : public Thing
{
	public:
		Thing_b( const string str );
        int number() { return -100; }
        string str() { return *_string; }
	void print();
	private:
		string *_string;
};

Thing_b::Thing_b( const string str )
{
	_string = new string( str );
}

void
Thing_b::print()
{
	cout << *_string << endl;
}

main()
{
	vector < Thing > thingVec;
	Thing_a a( 50 );
	string str = "bollox";
	Thing_b b( str );

	thingVec.push_back( a );
	thingVec.push_back( b );

	cout << "The number of things in the vec is: " << thingVec.size()
<< endl;

   	vector< Thing >::iterator iter;

	for (iter = thingVec.begin(); iter != thingVec.end(); iter++)
   		iter->print();

	int number = thingVec[ 0 ].number();
	string str1 = thingVec[ 1 ].str();
	cout << number << " " << str1 << endl;
   	cout << endl;

	return( 0 );
}

Sanctus fortis, Sanctus Deus,
De profundis oro te,
Miserere, Judex Meus,
Parce mihi, Domine

http://www.cs.cf.ac.uk/User/M.P.Tidball/index.html