Why won't std::list sort?

Hi all,

I can't compile program in which I want to sort std::list:

Code:

---

void CGeomUtils::sortByRelativePolarAngle(const myPoint & relativeTo, std::list<myPoint> & aList)
{
        CGeomUtils::relativePoint = relativeTo;

        std::sort(aList.begin(), aList.end(), CGeomUtils::sortByRelativePolarAngle_callback);
}

---

Code:

---

bool CGeomUtils::sortByRelativePolarAngle_callback(myPoint a, myPoint b)
{
        double pa1 = getRelativePolarAngle(a, CGeomUtils::relativePoint);
        double pa2 = getRelativePolarAngle(b, CGeomUtils::relativePoint);

        if (IS_EQUIV(pa1, pa2))
                return true;

        if (pa1 < pa2)
                return true;

        return false;
}

---

It won't compile and I'm getting errors like this:

e:\program files\microsoft visual studio\vc98\include\algorithm(592) : error C2784: '_D __cdecl std::operator -(const class std::reverse_iterator<_RI,_Ty,_Rt,_Pt,_D> &,const class std::reverse_iterator<_RI,_Ty,_Rt,_Pt,_D> &)' : could not deduce temp
late argument for 'const class std::reverse_iterator<_RI,_Ty,_Rt,_Pt,_D> &' from 'class std::list<struct _myPoint,class std::allocator<struct _myPoint> >::iterator'

Can someone see where is error?

Thank you.

You should use std::list::sort, and not std::sort on std::list.

Reason being that std::sort needs Random Access Iterators - something std::list does not offer. Hence for sorting's sake, std::list offers it's own member sort method.

1) std::sort requires random access iterators. std::list has
bi-directional iterators.

2) you should use the sort() member function for std::list

3) If you are using vc++6 , the predicated version incorrectly
requires the predicate to be in a special form (I don't recall
the exact form (something about dreived from std::greater).

If possible, define operator < for MyPoint, and use the
non-predicated version of the std::list::sort member function.

Thank you both guys!

Unfortunately defining operator< for myPoint wouldn't be that nice since it is just representing a point in the coordinate system (x, y) and it depends on whether we want to sort the points according to y or x-coordinate. Yes, it would be possible setting some flag, but...I switched to std::vector after all.

Thank you for the info, it's very useful yet not very frequently noticed (especially the note about VC++ 6.0 compiler)

Quote:

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Originally Posted by s_k

Unfortunately defining operator< for myPoint wouldn't be that nice since it is just representing a point in the coordinate system (x, y) and it depends on whether we want to sort the points according to y or x-coordinate. Yes, it would be possible setting some flag,

---

You don't need a flag or overload operator >, all you need are two separate sorting criteria. Then you call the 3 argument version of std::list::sort() that specifies which criteria to use for the sort.

Overloading operator < is just one way to do this. The other way is to specify the sorting criteria using a function object or a function pointer.

Regards,

Paul McKenzie