Re: [Boost-users] Extending boost::function for mathematics?

Subject: Re: [Boost-users] Extending boost::function for mathematics?
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2009-01-28 22:11:38

AMDG

jesseperla wrote:
> Stephen:  Thanks as always, a few follow-ups:
>   

Err, I usually spell my name with a 'v'

>> Question 3: How would we adapt a structure implementing the operator()
>> and gradient() to be stored here?  Just like boost::function accepting
>> any function object?
>> struct square
>> {
>>    double operator()(double x) {return x * x;}
>>    double gradient(double x){return 2 * x;}
>> };
>> math_function<double (double)> myfunc(square);
>> template<class F>
>>    math_function(const F& f) : f_(f), g_(bind(F::gradient, f, _1...?)
>> //Here I don't see how to bind to the arbitrary signature, extracting
>> the inputs from the Signature which (may) be more than 1
>>     

You'll have to either specialize math_function for different arities or
push the construction of the function that holds the gradient into a 
separate
class which is specialized for each arity or use a custom function object
that contains all the overloads of operator() needed instead of Boost.Bind.

>> However, it is fairly easy to give math_function members that return these traits at runtime.
>>     
> Is there a canonical example of a way to do this the best way?
>   

It's pretty straightforward:

// default implementation
template<class T>
bool is_convex(const T& t) {
    return(false);
}

// overload for math_function
template<class S>
bool is_convex(const math_function<S>& m) {
    return(m.is_convex());
}

// overloads for other objects should be found by ADL.

template<class S>
class math_function {
    // ...
    math_function(F f) : is_convex_(is_convex(f)) {}
    bool is_convex_;
};

This is the simplest solution.  Of course, if necessary, you can use more
type erasure to delay the calculation of is_convex until its needed.

In Christ,
Steven Watanabe