Re: [boost] Getting the contained object from boost::function

From: Douglas Gregor (gregod_at_[hidden])
Date: 2001-10-07 15:44:02

On Sunday 07 October 2001 11:05, you wrote:
> I have encountered a situation where I have a boost::function:
>
> boost::function<void> onClick;
>
> initialized to a function object with a known type F (which is similar to a
> std::vector<boost::function<void> > with a broadcasting operator().)
>
> At some later point I need to ask onClick "do you contain an instance of F?
> if yes, please give me a reference to it so I can add another handler."

With the current interface, this isn't possible, but I think it would be a 
worthwhile addition. I suggest the following interface (modeled after 
boost::any)

template<typename R, typename T1, ..., typename TN>
class function {
public:
  // type of target function object, or typeid(void) if empty
  const std::type_info& type() const; 
};

// 
template<typename To, typename R, typename T1, ..., typename TN>
To& function_cast(function<R, T1, ..., TN>& f); 

template<typename To, typename R, typename T1, ..., typename TN>
const To& function_cast(const function<R, T1, ..., TN>& f); 

> Ideas?

I would think that onClick shouldn't be a boost::function<void>, but a 
full-fledged signal. Unfortunately, we don't have one of those in Boost yet.

> BTW, the function documentation mentions 'operator bool' which has been
> replaced by 'operator safe_bool.' 

It's an intentional trade-off. On one hand, the function documentation 
out-and-out lies to the user about a conversion. On the other hand, the user 
doesn't need to go look up what safe_bool is in the (nonexistant) 
documentation.

safe_bool should go into utility.hpp, and then it would have documentation so 
this problem would be avoided. I promised to do that but haven't yet 
delivered :(.

> Also, operator! is not necessary.

It's a workaround. At least one compiler (Borland, perhaps?) doesn't find the 
appropriate conversion sequence without an explicit operator!.

        Doug