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list sort problem

I am using Embarcadero C++ builder XE10.1 Berlin update 2.I am having trouble sorting a TList .
To illustrate the problem, I have coded a simple example below which generates 30 random integers and puts them into a class which is added into a list.
The original list content is displayed in Memo1. the list is sorted using a compare function and the sorted list is displayed in Memo2.
The resultant display shows that the sort operation fails. I have inspected the comarisons by setting a breakpoint in the compare function and can see no failure there.
What am I doing wrong ?

//---------------------------------------------------------------------------

#include <vcl.h>
#pragma hdrstop
using namespace std;
#include <list>
#include "Unit1.h"
//---------------------------------------------------------------------------
#pragma package(smart_init)
#pragma resource "*.dfm"
TForm1 *Form1;

class MyClass
{
	public:
	int n;
	MyClass(int a){n=a;};
	~MyClass(){};
};

int SortOnInt(void *Item1, void *Item2)
{
	MyClass *a = reinterpret_cast<MyClass *>(Item1);
	MyClass *b = reinterpret_cast<MyClass *>(Item2);
	int rtn = 0;

	if(a!=NULL && b!=NULL)
	{
		if(a->n  > b->n)
			rtn = 1;
		else
		if(a->n  < b->n)
			rtn = -1;
	}
	return rtn;
}
//---------------------------------------------------------------------------
__fastcall TForm1::TForm1(TComponent* Owner)
	: TForm(Owner)
{
	list<MyClass *> myclasslist;
	list<MyClass *>::iterator k;
	int i,j;

	Memo1->Clear();
	Memo2->Clear();
	for(i=0;i<30;i++)
	{
		MyClass *mc;
		j = rand()%100;
		mc = new MyClass(j);
		Memo1->Lines->Add(mc->n);
		myclasslist.push_back(mc); // random number in the range 0 to 99
	}
	// now sort he list
	myclasslist.sort(SortOnInt);

	for(k=myclasslist.begin();k!=myclasslist.end();k++)
	{
		Memo2->Lines->Add((*k)->n);
	}
}
//---------------------------------------------------------------------------

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ASKER CERTIFIED SOLUTION

evilrix

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sarabande

when using std::list::sort your class simply should provide a less operator (as suggested by evilrix).

class MyClass
{
	public:
	int n;
	MyClass(int a){n=a;};
	~MyClass(){};
        bool operator< (MyClass & other) const
        {
              return n < other.n;
        }

};

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with that the sorting will be done by

// now sort the list after fill
myclasslist.sort();

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Sara

Roger Alcindor

ASKER

Thanks for your comments.
In the project application, I will be sorting the list on various other members of the class depending upon circumstances so in this case, the use of a single compare function in myclass would require my adding a member to myclass that defines which member the sort should be made on.

In an old Borland C++ project of mine (BCB 5) ,the STL List Sort used A Quicksort function which returns 1,0 or -1 depending on the result of the comparison as greater than, equal or less than respectively. I had assumed (incorrectly) that this applied to the Embarcadero C++ compilers especially as the compiler didn't issue any warnings or errors.