using comparing function - c++

Question

I have this function
std::vector <std::vector <int> > puddles;
std::set <int> is_checked;
size_t M, N;
bool v_compare(int a, int b){
return puddles[(a-a%M)/M][a%M] < puddles[(b-b%M)/M][b%M];
}
and I want to find the set element which corresponds to the minimal value in puddles (I use i*M+j as a key in the set). So I am trying to use my function as a predicate for min_element
close(*std::min_element(is_checked.begin(), is_checked.end(), v_compare));
but I got this error
:113:82: error: no matching function for call to 'min_element(std::set<int>::iterator, std::set<int>::iterator, <unresolved overloaded function type>)'
:113:82: note: candidates are:
/usr/include/c++/4.6/bits/stl_algo.h:6173:5: note: template<class _FIter> _FIter std::min_element(_FIter, _FIter)
/usr/include/c++/4.6/bits/stl_algo.h:6201:5: note: _FIter std::min_element(_FIter, _FIter, _Compare) [with _FIter = std::_Rb_tree_const_iterator<int>, _Compare = bool (TFlood::*)(int, int)]
/usr/include/c++/4.6/bits/stl_algo.h:6201:5: note: no known conversion for argument 3 from '<unresolved overloaded function type>' to 'bool (TFlood::*)(int, int)'

Answer 1

There is a difference between a pointer to function and a pointer to member function. A class member which is not static gets you a pointer to member function. But std::min_element only accepts a pointer to function or other callable type, and a pointer to member function doesn't qualify since it can't be called without a pointer or reference to a class object. And as you found, if you try making the function a static member, then it can't get at the member M. (Maybe puddles too, if that's also a member.)
It looks like you're using g++ 4.6, which supports lambdas. So the easiest solution is:
close(*std::min_element(is_checked.begin(), is_checked.end(),
[this](int a, int b) { return puddles[a/M][a%M] < puddles[b/M][b%M]; }));
(I got rid of the subtractions, since division of integer types always rounds toward zero, so (a-(a%M))/M is always the same result as a/M.) Make sure you use the -std=c++0x compiler flag. The lambda magic takes care of "capturing" the this pointer for later use by the comparison object.