Array of classes as a function parameter...pol ymorphism question.

[Jack]

I have a class called "Base". This class has a protected member
variable "m_base" which can be retrieved using the public member
function "GetBaseMember" . "m_base" is initialized to "1" and is never
changed.

I have another class which is a subclass of the "Base" class called
"Derived". This derived class has a member variable called
"m_derived" . "m_derived" is initialized to "2" and is never changed.

I pass an array of "Base" classes as a parameter to a function.The
individual items in this array may or may not be subclasses of the
"Base" class. Within this function, the member function
"GetBaseMem ber" is called, and the value of "m_base" is displayed to
the screen.

Intuitively, the output should always be "1", but unfortunately it's
not. The output alternates between "1" and "2". I don't understand why
this is. How else can I pass this "subclassab le" class array to a
function and retrieve the expected base class member variable? What
concept am I missing here?

#include <iostream>
using namespace std;

class Base
{
protected:
int m_base;

public:
Base() : m_base( 1 ){}
int GetBaseMember() { return m_base; }
};

class Derived : public Base
{
protected:
int m_derived;

public:
Derived() : Base(), m_derived( 2 ){}
};

void Foo( int cItems, Base b[] )
{
int i = 0;
for ( i = 0; i < cItems; i++ )
{
// I want the output to always be 1,
// but it alternates between 1 and 2
cout << "m_base = " << b[i].GetBaseMember( ) << endl;
}
}

int main()
{
const int NUM_ITEMS = 4;
Derived d[NUM_ITEMS];

Foo( NUM_ITEMS, d );

return 0;
}

[David W]

"Jack" <ja**@ss.comwro te in message news:j6******** *************** *********@4ax.c om...

On Thu, 5 Oct 2006 23:49:41 -0400, "Victor Bazarov"
<v.********@com Acast.netwrote:

The behaviour of your program is undefined. You pass an array of
Derived objects where an array of Base is expected. There is no
conversion between the two.

I was under the assumption that C++ would support polymorphism for
arrays of subclassed objects, but I guess I was wrong. IMHO, my
algorithm being described as "undefined" is a flaw in the language.

Rather, it would be a flaw in the language if it supported that, given the cost in space and/or
execution time needed and the very limited use it would get. For one thing, at run-time the program
would have to check the type stored to know how many bytes to advance to reach the array element
addressed - a complete waste when the objects are of the declared type.

You can always use an array of Base*, where each Base * may point to a Base or a Derived object, and
each would behave polymorphically .

DW

[Jack]

On Fri, 06 Oct 2006 18:45:34 +1300, Ian Collins <ia******@hotma il.com>
wrote:

>Jack wrote:

>On Thu, 5 Oct 2006 23:49:41 -0400, "Victor Bazarov"
<v.********@co mAcast.netwrote :

>>>The behaviour of your program is undefined. You pass an array of
Derived objects where an array of Base is expected. There is no
conversion between the two.

I was under the assumption that C++ would support polymorphism for
arrays of subclassed objects, but I guess I was wrong. IMHO, my
algorithm being described as "undefined" is a flaw in the language. No
conversion? Isn't it obvious? I guess not for the compiler.

No it isn't.

If you define and array of some base class of size N and attempt to
store derived objects of size N+M in said array, what would you expect
happen?

Use an array of pointers to base.

Very good explanation! It makes perfect sense now. An array of
pointers, of course! Thank you!

[Jack]

On Fri, 06 Oct 2006 00:04:30 -0400, Nate Barney <na********@gma il.com>
wrote:

>Jack wrote:

>I have a class called "Base". This class has a protected member
variable "m_base" which can be retrieved using the public member
function "GetBaseMember" . "m_base" is initialized to "1" and is never
changed.

[snip]

>class Base
{
protected:
int m_base;

public:
Base() : m_base( 1 ){}
int GetBaseMember() { return m_base; }
};

I know this is just an example, but if m_base never changes, why not
declare it const? Same goes for GetBaseMember.

Nate

Sure, why not. LOL!

class Base
{
protected:
const int m_base;

public:
Base() : m_base( 1 ){}
const int GetBaseMember() { return m_base; }
};

[Nate Barney]

Jack wrote:

On Fri, 06 Oct 2006 00:04:30 -0400, Nate Barney <na********@gma il.com>
wrote:

>Jack wrote:

>>class Base
{
protected:
int m_base;

public:
Base() : m_base( 1 ){}
int GetBaseMember() { return m_base; }
};

I know this is just an example, but if m_base never changes, why not
declare it const? Same goes for GetBaseMember.

Sure, why not. LOL!

class Base
{
protected:
const int m_base;

public:
Base() : m_base( 1 ){}
const int GetBaseMember() { return m_base; }
};

Actually, I meant:

int GetBaseMember() const { return m_base; }