Re: [Boost-users] [partly: Boost.Function] typesafe (templated) wrapper around class with void* API

Subject: Re: [Boost-users] [partly: Boost.Function] typesafe (templated) wrapper around class with void* API
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2009-01-29 11:34:08

AMDG

Christoph Duelli wrote:
> template<typename T>
> class B : public A
> {
> public:
>    void do_something(T* t) { A::do_something(t); }
> protected:
>    // Force user to provide a correctly typed callback.
>    virtual bool help_me(const T*) =0;
> private:
>    // call the user's safe callback
>    bool help_me(const void* t)
>    {
>       return help_me(static_cast<const T*>(t));
>    }
> };
>
> More specific questions:
> i) I assume the penalty for my callback "help_me" is one
> more indirection. Can't see that optimized away.
>   

Use CRTP

template<class Derived, class T>
class B {
private:
    bool help_me(const void* t) {
        return static_cast<Derived*>(this)->help_me(static_cast<const 
T*>(t));
    }
};

> ii) Assuming A's interface gets passed a real callback
> function (a Boost.Function) instead:
>
> class A
> {
> public:
>    typedef boost::function<bool(const void*)> helper_func;
>    void do_something(void*, helper_func);
> };
>
> is it somehow possible to get a typesafe templated B such that
> template<typename T>
> class B : public A
> {
> public:
>    typedef boost::function<bool(const T*)> helper_func;
>    void do_something(T* t, helper_func hf)
>    {
>       // this makes my compiler unhappy (when instanced).
>       // I do understand that... can it be worked around somehow?
>       A::do_something(t, hf);
>    }
> };
>   

A::do_something(t, boost::lambda::bind(hf, 
boost::lambda::ll_static_cast<const T*>(boost::lambda::_1)));

In Christ,
Steven Watanabe