std::greater not working with sort

[janac]

Why isn't std::greater working here?
I have two versions of the code below. The only difference is in the first I used my function cmp() as the third parameter of sort(). It outputs 3 2 1
In the second version, I used greater<MyClass> and I get this error:
'std::greater<MyClass>': illegal use of this type as an expression

I looked up std::greater:
"Function object class for greater-than inequality comparison
Binary function object class whose call returns whether the its first argument compares greater than the second (as returned by operator >)."
"The type shall support the operation (operator>)."
This was the only restriction I saw on the type.

Then I looked up std::sort. Regarding the third parameter:
"Binary function that accepts two elements in the range as arguments, and returns a value convertible to bool."
"This can either be a function pointer or a function object."

Can someone explain why greater<MyClass> doesn't work:

// This works with cmp()
#include<vector>
#include<iostream>
#include<algorithm> 
#include<functional> 

using namespace std;

class MyClass
{
	int memb;
public:
	MyClass(int x) : memb(x){}
	int get_memb() const { return memb; }
	bool operator > (const MyClass& b) { return memb > b.memb; }
};

bool cmp(const MyClass& a, const MyClass& b)
{
	return a.get_memb() > b.get_memb();
}

ostream& operator << (ostream& os, const MyClass& inst)
{
	os << inst.get_memb();
	return os;
}

void display(MyClass m){cout << m << " ";}

int main()
{
	int values[] = { 1, 2, 3 };
	vector<MyClass> my_vect(values,values + 3);
	sort(my_vect.begin(), my_vect.end(), cmp);
	for_each(my_vect.begin(), my_vect.end(), display); 

	return 0;
}

// does not work with greater<MyClass>
#include<vector>
#include<iostream>
#include<algorithm> 
#include<functional> 

using namespace std;

class MyClass
{
	int memb;
public:
	MyClass(int x) : memb(x){}
	int get_memb() const { return memb; }
	bool operator > (const MyClass& b) { return memb > b.memb; }
};

bool cmp(const MyClass& a, const MyClass& b)
{
	return a.get_memb() > b.get_memb();
}

ostream& operator << (ostream& os, const MyClass& inst)
{
	os << inst.get_memb();
	return os;
}

void display(MyClass m){cout << m << " ";}

int main()
{
	int values[] = { 1, 2, 3 };
	vector<MyClass> my_vect(values,values + 3);
	sort(my_vect.begin(), my_vect.end(), greater<MyClass>);
	for_each(my_vect.begin(), my_vect.end(), display); 

	return 0;
}

[jlb]

You have a const correctness issue with your operator > overload. You need to declare the overload with the const keyword:

bool operator > (const MyClass& b) const { return memb > b.memb; }

And your sort call is missing a pair of ():

sort(my_vect.begin(), my_vect.end(), greater<MyClass>)();

[janac]

Great, thanks!

[seeplus]

L35

sort(my_vect.begin(), my_vect.end(), greater<MyClass>());