Backtrack algorithm

I coded this algorithm in order to have a generic backtracking algorithm. I expect the user to pass a POD for ELEMENT, hopefully as small as possible. Any idea how I could improve this?

#ifndef BACKTRACK_H
#define BACKTRACK_H

#include <array>
#include <vector>
#include <assert.h>

template< class ELEMENT, unsigned MAX_CAPACITY >
class backtrack
{
public:
    backtrack()
        : m_grid()
        , m_depth( 0 )
        , m_finish( false )
    {
    }

    virtual ~backtrack() {}

    template< typename ForwardIterator >
    backtrack( ForwardIterator begin, ForwardIterator end )
        : m_grid()
        , m_depth( 0 )
        , m_finish( false )
    {
        std::copy( begin, end, m_grid.begin() );
    }

    void step()
    {
        if ( is_a_valid_solution( m_grid ) )
            m_finish = process_solution( m_grid );

        if ( m_depth == MAX_CAPACITY )
            return;

        const auto & candidates = generate_candidates( m_grid );
        for( const auto & candidate : candidates )
        {
            push( candidate );
            step();
            pull();
        }
    }

private:
    virtual
    bool is_a_valid_solution( const std::array< ELEMENT, MAX_CAPACITY > & solution )
    {
        ( void )solution;
        return false;
    }

    virtual
    bool process_solution( const std::array< ELEMENT, MAX_CAPACITY > & solution )
    {
        ( void )solution;
        return false;
    }

    virtual
    std::vector<ELEMENT> generate_candidates( const
            std::array< ELEMENT, MAX_CAPACITY > & partial_solution )
    {
        ( void )partial_solution;
        return std::vector<ELEMENT>();
    }

    void push( ELEMENT candidate )
    {
        assert( m_depth < MAX_CAPACITY );
        m_grid[ m_depth++ ] = candidate;
    }

    void pull()
    {
        assert( m_depth <= MAX_CAPACITY );
        assert( m_depth > 0 );
        m_grid[ --m_depth ] = ELEMENT();
    }

private:
    std::array< ELEMENT, MAX_CAPACITY > m_grid;
    unsigned m_depth;
    bool m_finish;
};