Bug 664

Recently I compiled my program with Eigen 3.2 stable and encouterd an error as follow:

error: no match for ‘operator<’ (operand types are ‘const std::_List_iterator<Eigen::Triplet<double> >’ and ‘const std::_List_iterator<Eigen::Triplet<double> >’)

This is caused by comparing "begin" end "end" in function set_from_triplets in SparseMatrix.h

"begin" and "end" are two iterator variable. This error is due the fact that iterator of std::list doesn't support < operation. This bug was introduced by the 9e8ce09 commit for "Fix setFromTripplet with empty inputs". We know that many containers support iterator, but not all their iterators support operator<. std::list is the case. To my best knowledge, all iterators support operator!=. 
I think operator!= is more general to check if a container is empty.

The example below can reprduce the error

///////////////////////////////////
#include <iostream>

#include <Eigen/Sparse>
#include <list>

using namespace std;

int main()
{
    Eigen::SparseMatrix<double> mat(3,3);
    std::list<Eigen::Triplet<double> > t;
    t.push_back(
    Eigen::Triplet<double>(0,0,1)
    );

    mat.setFromTriplets(t.begin(),t.end());
    return 0;
}

////////////////////////////////

I compiled my code under linux Mint 64bit with gcc 4.8.1.