(SOLVED) Help Sorting List of Pointers by Derived Class Member

[Code2Code]

Hi.

I am just getting into using STL and am having issues with sorting lists of pointers based on values of derived class members, when I took my C++ class in college we never went into lists and they are quite a bit trickier in my opinion, yet faster for several reasons which is why I wanted to learn how to use them correctly. Anyway, I could really use some help.

Here is an example of my base class and two derived:

class Base
{
private:
	int length;

public:
	virtual void printLength() = 0;

};
class Derived1 : public Base
{
private:
	int length;
        int length2


public:
	virtual void printLength();

};
class Derived2 : public Base
{
private:
	int length;
        int length2
        int length3


public:
	virtual void printLength();

};

I am trying to get them to sort by total length, so if derived1's length + length2 = 10 and derived2's length + length2 + length3 = 11 it would sort it accordingly.

I tried overloading the > operator for all classes, and making an operator() that took two Base*'s and then inside compared Base* < Base* and returned a bool but no luck with that even though I got that method working with a vector of pointers.

Here is another fragment to give you an idea of what I am talking about, this code compiles and runs but it doesn't actually sort it by value:

int main()
{
   list<Base*> List;
   list<Base*>::iterator it;

   ......fill list here with instances of both derived classes

   List.sort(value());

   return 0
}

bool value::operator() (Base* v1, Base* v2)
{
	if(v1 < v2)
		return false;
	else
		return true;

}

Any help would be appreciated, I'm lost. Also if you need me to post more in depth code let me know, I tried to just post fragments to avoid a huge post.

[viliml]

your "derived" classes dont inherit the "base" class. try fixing that first.

[Code2Code]

your "derived" classes dont inherit the "base" class. try fixing that first.

obviously just a typo. I think you can tell by the names of the classes this is an example, not the actual code, I typed it all out from scratch and didn't type in the inheritance. Sorry.

[JLBorges]

Provide a virtual function in the base class that returns the total length.

struct base
{
    explicit base( int aa = 0 ) : a(aa) {}
    virtual ~base() {}

    virtual int total_length() const { return a ; }

    virtual std::ostream& print( std::ostream& stm ) const
    { return stm << "base{" << a << '}' ; }
    // ...

    const int a ;
};

std::ostream& operator<< ( std::ostream& stm, const base& b ) { return b.print(stm) ; }

Override the function in the derived classes

struct derived : base
{
    explicit derived( int aa = 0, int bb = 0 ) : base(aa), b(bb) {}

    virtual int total_length() const override { return a + b ; }

    virtual std::ostream& print( std::ostream& stm ) const override
    { return stm << "derived{" << a << ',' << b << '}' ; }

    const int b ;
};

Use the virtual function in the binary predicate for std::sort()

int main( int argc, char* argv[] )
{
    // prefer using std::vector as the default sequence container
    std::vector< base* > seq = { new derived(1,2), new base(3), new derived(0,0),
         new base(17), new derived(10,15), new derived(5,5), new base(7) } ;
    for( const base* p : seq ) std::cout << *p << ' ' ;
    std::cout << '\n' ;

    std::sort( seq.begin(), seq.end(), [] ( const base* a, const base* b )
                                { return a->total_length() < b->total_length() ; } ) ;
    for( const base* p : seq ) std::cout << *p << ' ' ;
    std::cout << '\n' ;

    // ideaLLY, use std::unique_ptr<base> instead
    for( base*& p : seq ) { delete p ; p = nullptr ; }
}

[Code2Code]

Thanks JLBorges! I had to change a few things around to get it working with list but you got my mind in the right direction and I got it working. I was focused on trying access all the members directly and totally overlooked the option of making a virtual function (*face palm).