member binary predicate problems!

[bibalasvegas]

Hi,

I want to use max_element with a predicate. I have 2 vectors stored in a class and I want to use min_element on one vector using a predicate which refers to the second. i.e. the first vector (seeds) holds indices referring to values in the second (nodes).

unsigned int Map::GetSeed()
{
	unsigned int seed = 0;

	if(seeds.size() > 1)
	{
		std::vector<unsigned int>::iterator iter;
		iter = std::min_element(seeds.begin(),seeds.end(), XLesser);
		seed = *iter;
		seeds.erase(iter);
		return seed;
	}
	if(seeds.size() == 1)
	{
		seed = seeds[0];
		seeds.erase(seeds.begin());
		return seed;
	}
	
	return seed;
}

where Xlesser is defined by

bool Map::XLesser(unsigned int index1, unsigned int index2)
{
	return nodes[index1].x < nodes[index2].x;
}

My problem is that predicates are meant to have 2 arguments but XLesser will have 3 (the 3rd being a pointer to the object). I've read that this can be overcome by declaring XLesser as static but that doesn't work here because XLesser accesses the private vector nodes and I get error C2597: illegal reference to non-static member 'Map::nodes'! How do I get around this!?

For some explanation nodes holds vertices and seeds holds indices referring to certain vertices in nodes.

[Lindley]

If you have Boost, you can use boost::bind() to get around this.

If not, I'll explain what bind() is actually doing in this case.

See, the predicate doesn't need to be a function. A functor will do just as well. So if you write a trivial functor which takes in a pointer to a Map in its constructor, and has operator() defined to take two ints, it can call m->XLesser() within its operator().

[bibalasvegas]

Thanks very much for the reply. I am using boost but would prefer to implement things myself the first time to make sure I understand what is going on! After reading about functors a bit here's what I've done:

I have a class defined as:

class Functor
{
	Map* map_pt;
public:
	Functor(Map* _map_pt) : map_pt(_map_pt)
	{
	}
	bool operator()(unsigned int index1, unsigned int index2)
	{
		return map_pt->XLesser(index1, index2);
	}

};

and now my GetSeed function looks like this:

unsigned int Map::GetSeed()
{
	static Functor functor(this);
	unsigned int seed = 0;

	if(seeds.size() > 1)
	{
		std::vector<unsigned int>::iterator iter;
		iter = std::min_element(seeds.begin(),seeds.end(), functor);
		seed = *iter;
		seeds.erase(iter);
		return seed;
	}
	if(seeds.size() == 1)
	{
		seed = seeds[0];
		seeds.erase(seeds.begin());
		return seed;
	}
	
	return seed;
}

Is what you meant and how it should be done?

[jwbarton]

You probably don't want to make the functor a static variable. It would then only be initialized the first time that Map::GetSeed is called. This would be bad if you have more than one Map object.

Instead, you can directly create a Functor object in the call to min_element:

iter = std::min_element(seeds.begin(),seeds.end(), Functor(this));

[bibalasvegas]

Yes, good advice!

Thanks again.