Class member functions instantiated by traits [policies, actually]

Question

I am reluctant to say I can't figure this out, but I can't figure this out. I've googled and searched Stack Overflow, and come up empty.

The abstract, and possibly overly vague form of the question is, how can I use the traits-pattern to instantiate member functions? [Update: I used the wrong term here. It should be "policies" rather than "traits." Traits describe existing classes. Policies prescribe synthetic classes.] The question came up while modernizing a set of multivariate function optimizers that I wrote more than 10 years ago.

The optimizers all operate by selecting a straight-line path through the parameter space away from the current best point (the "update"), then finding a better point on that line (the "line search"), then testing for the "done" condition, and if not done, iterating.

There are different methods for doing the update, the line-search, and conceivably for the done test, and other things. Mix and match. Different update formulae require different state-variable data. For example, the LMQN update requires a vector, and the BFGS update requires a matrix. If evaluating gradients is cheap, the line-search should do so. If not, it should use function evaluations only. Some methods require more accurate line-searches than others. Those are just some examples.

The original version instantiates several of the combinations by means of virtual functions. Some traits are selected by setting mode bits that are tested at runtime. Yuck. It would be trivial to define the traits with #define's and the member functions with #ifdef's and macros. But that's so twenty years ago. It bugs me that I cannot figure out a whiz-bang modern way.

If there were only one trait that varied, I could use the curiously recurring template pattern. But I see no way to extend that to arbitrary combinations of traits.

I tried doing it using boost::enable_if, etc.. The specialized state information was easy. I managed to get the functions done, but only by resorting to non-friend external functions that have the this-pointer as a parameter. I never even figured out how to make the functions friends, much less member functions. The compiler (VC++ 2008) always complained that things didn't match. I would yell, "SFINAE, you moron!" but the moron is probably me.

Perhaps tag-dispatch is the key. I haven't gotten very deeply into that.

Surely it's possible, right? If so, what is best practice?

UPDATE: Here's another try at explaining it. I want the user to be able to fill out an order (manifest) for a custom optimizer, something like ordering off of a Chinese menu - one from column A, one from column B, etc.. Waiter, from column A (updaters), I'll have the BFGS update with Cholesky-decompositon sauce. From column B (line-searchers), I'll have the cubic interpolation line-search with an eta of 0.4 and a rho of 1e-4, please. Etc...

UPDATE: Okay, okay. Here's the playing-around that I've done. I offer it reluctantly, because I suspect it's a completely wrong-headed approach. It runs okay under vc++ 2008.

#include <boost/utility.hpp>
#include <boost/type_traits/integral_constant.hpp>

namespace dj {

struct CBFGS {
    void bar() {printf("CBFGS::bar %d\n", data);}
    CBFGS(): data(1234){}
    int data;
};

template<class T>
struct is_CBFGS: boost::false_type{};

template<>
struct is_CBFGS<CBFGS>: boost::true_type{};

struct LMQN {LMQN(): data(54.321){}
    void bar() {printf("LMQN::bar %lf\n", data);}
    double data;
};

template<class T>
struct is_LMQN: boost::false_type{};

template<>
struct is_LMQN<LMQN> : boost::true_type{};

// "Order form"
struct default_optimizer_traits {
    typedef CBFGS update_type; // Selection from column A - updaters
};

template<class traits> class Optimizer;

template<class traits>
void foo(typename boost::enable_if<is_LMQN<typename traits::update_type>, 
         Optimizer<traits> >::type& self) 
{
    printf(" LMQN %lf\n", self.data);
}

template<class traits>
void foo(typename boost::enable_if<is_CBFGS<typename traits::update_type>,  
         Optimizer<traits> >::type& self) 
{
    printf("CBFGS %d\n", self.data);
}

template<class traits = default_optimizer_traits>
class Optimizer{
    friend typename traits::update_type;
    //friend void dj::foo<traits>(typename Optimizer<traits> & self); // How?
public:
    //void foo(void); // How???
    void foo() {
        dj::foo<traits>(*this);
    }
    void bar() {
        data.bar();
    }
//protected: // How?
    typedef typename traits::update_type update_type;
    update_type data;
};

} // namespace dj



int main() {
    dj::Optimizer<> opt;
    opt.foo();
    opt.bar();
    std::getchar();
    return 0;
}

Answer 1

I think template specialization is a step in the right direction. This doesn't work with functions so I switched to classes. I changed it so it modifies the data. I'm not so sold on the protected members and making friends. Protected members without inheritance is a smell. Make it public or provide accessors and make it private.

template <typename>
struct foo;

template <>
struct foo<LMQN>
{
    template <typename OptimizerType>
    void func(OptimizerType& that)
    {
        printf(" LMQN %lf\n", that.data.data);
        that.data.data = 3.14;
    }
};

template <>
struct foo<CBFGS>
{
    template <typename OptimizerType>
    void func(OptimizerType& that)
    {
        printf(" CBFGS %lf\n", that.data.data);
    }
};

template<class traits = default_optimizer_traits>
class Optimizer{
public:
    typedef typename traits::update_type update_type;
    void foo() {
        dj::foo<typename traits::update_type>().func(*this);
    }
    void bar() {
        data.bar();
    }
    update_type data;
};

Answer 2

Your use of enable_if is somewhat strange. I've seen it used it only 2 ways:

• in place of the return type
• as a supplementary parameter (defaulted)

Using it for a real parameter might cause the havoc.

Anyway, it's definitely possible to use it for member functions:

template<class traits = default_optimizer_traits>
class Optimizer{
  typedef typename traits::update_type update_type;
public:

  typename boost::enable_if< is_LQMN<update_type> >::type
  foo()
  {
    // printf is unsafe, prefer the C++ way ;)
    std::cout << "LQMN: " << data << std::endl;
  }

  typename boost::enable_if< is_CBFGS<update_type> >::type
  foo()
  {
    std::cout << "CBFGS: " << data << std::endl;
  }


private:
  update_type data;
};

Note that by default enable_if returns void, which is eminently suitable as a return type in most cases. The "parameter" syntax is normally reserved for the constructor cases, because you don't have a return type at your disposal then, but in general prefer to use the return type so that it does not meddle with overload resolution.

EDIT:

The previous solution does not work, as noted in the comments. I could not find any alternative using enable_if, only the "simple" overload way:

namespace detail
{
  void foo_impl(const LMQN& data)
  {
    std::cout << "LMQN: " << data.data << std::endl;
  }

  void foo_impl(const CBFGS& data)
  {
    std::cout << "CBFGS: " << data.data << std::endl;
  }
} // namespace detail

template<class traits = default_optimizer_traits>
class Optimizer{
  typedef typename traits::update_type update_type;

public:
  void foo() { detail::foo_impl(data); }

private:
  update_type data;
};

It's not enable_if but it does the job without exposing Optimizer internals to everyone. KISS ?

Answer 3

A simple solution might be to just use tag-based forwarding, e.g. something like this:

template<class traits>
void foo(Optimizer<traits>& self, const LMQN&) {
    printf(" LMQN %lf\n", self.data.data);
}

template<class traits>
void foo(Optimizer<traits>& self, const CBFGS&) {
    printf("CBFGS %d\n", self.data.data);
}

template<class traits = default_optimizer_traits>
class Optimizer {
    friend class traits::update_type;
    friend void dj::foo<traits>(Optimizer<traits>& self, 
                            const typename traits::update_type&);
public:
    void foo() {
        dj::foo<traits>(*this, typename traits::update_type());
    }
    void bar() {
        data.bar();
    }
protected:
    typedef typename traits::update_type update_type;
    update_type data;
};

Or if you want to conveniently group several functions together for different traits, maybe something like this:

template<class traits, class updater=typename traits::update_type> 
struct OptimizerImpl;

template<class traits>
struct OptimizerImpl<traits, LMQN> {
    static void foo(Optimizer<traits>& self) {
        printf(" LMQN %lf\n", self.data.data);
    }
};

template<class traits> 
struct OptimizerImpl<traits, CBFGS> {
    static void foo(Optimizer<traits>& self) {
        printf("CBFGS %d\n", self.data.data);
    }
};

template<class traits = default_optimizer_traits>
class Optimizer{
    friend class traits::update_type;
    friend struct OptimizerImpl<traits>;
public:
    void foo() {
        OptimizerImpl<traits>::foo(*this);
    }
    // ...
};

Answer 4

It would be trivial to define the traits with #define's and the member functions with #ifdef's and macros. But that's so twenty years ago.

Although it may be worth learning new methods, macros are often the simplest way to do things and shouldn't be discarded as a tool just because they're "old". If you look at the MPL in boost and the book on TMP you'll find much use of the preprocessor.

Answer 5

Here's what I (the OP) came up with. Can you make it cooler?

The main Optimizer template class inherits policy-implementation classes. It gives those classes access to the Optimizer's protected members that they require. Another Optimizer template class splits the manifest into its constituent parts and instantiates the main Optimizer template.

#include <iostream>
#include <cstdio>

using std::cout;
using std::endl;

namespace dj {

// An updater.
struct CBFGS {
    CBFGS(int &protect_)
        : protect(protect_)
    {}

    void update () {
        cout << "CBFGS " << protect << endl;
    }   

    // Peek at optimizer's protected data
    int &protect;

};

// Another updater
struct LMQN {
    LMQN(int &protect_)
        : protect(protect_)
    {}

    void update () {
        cout << "LMQN " << protect << endl;
    }

    // Peek at optimizer's protected data
    int &protect;

};

// A line-searcher
struct cubic_line_search {
    cubic_line_search (int &protect2_)
        : protect2(protect2_)
    {}

    void line_search() {
        cout << "cubic_line_search  " << protect2 << endl;
    }   

    // Peek at optimizer's protected data
    int &protect2;

};

struct default_search_policies {
    typedef CBFGS update_type;
    typedef cubic_line_search line_search_type;
};

template<class Update, class LineSearch>
class Opt_base: Update, LineSearch
{
public:
    Opt_base()
        : protect(987654321) 
        , protect2(123456789)
        , Update(protect)
        , LineSearch(protect2)
    {}
    void minimize() {
        update();
        line_search();
    }

protected:
    int protect;
    int protect2;
};

template<class Search_Policies=default_search_policies>
class Optimizer: 
    public Opt_base<typename Search_Policies::update_type
                  , typename Search_Policies::line_search_type
                    >
{};

} // namespace dj



int main() {
    dj::Optimizer<> opt; // Use default search policies
    opt.minimize();

    struct my_search_policies {
        typedef dj::LMQN update_type;
        typedef dj::cubic_line_search line_search_type;
    };

    dj::Optimizer<my_search_policies> opt2;
    opt2.minimize();

    std::getchar();
    return 0;
}