bind()ing to member *data*

From: jkharris01 (john.harris_at_[hidden])
Date: 2002-02-20 18:15:57

Should bind() be extended to include binding to member data?  If it 
already allows convenient binding to member functions, wouldn't 
member data be a natural extension?

Perhaps the operator() of the class resulting from such a bind would 
return a reference to the member data.

I've run this by the author of boost::function, Douglas Gregor 
(privately), and he thinks it would be a useful addition.

The specific problem that got me thinking about this is that I 
couldn't find an easy way to make a predicate compare only 
the ".second" member of a pair, as in a map lookup by value:

    typedef std::map<int,float> map_t;

    // find the map entry whose value is 3.14
    void findpi(const map_t& themap)
    {
        std::find_if(themap.begin(), themap.end(),
                     member_equal(&map_t::value_type::second, 3.14));
    }

My solution to this, by the way, goes like this:

template<typename R, typename T>
class MemData_t
{
public:
        MemData_t(R T::* pmd_)
                : pmd(pmd_)
        {
        }
        const R& operator()(const T& inst_)
        {
                return inst_.*pmd;
        }
private:
        R T::* pmd; // ptr to member data
};

template <typename T, typename R> boost::function<bool,T>
member_equal(R T::* pmd_, R stator_)
{
        return boost::bind<bool>(
            std::equal_to<R>(), 
            boost::bind<const R&>(MemData_t<R,T>(pmd_),_1), 
            stator_);
}