[Boost-users] Re: boost::unary_traits

From: Matthias Kaeppler (nospam_at_[hidden])
Date: 2005-03-04 14:46:24

Ian McCulloch wrote:
>>template< typename Operation >
>>class indirecter_unary
>>    : public std::unary_function< typename
>>boost::unary_traits<Operation>::argument_type,
> 
> 
> Is this correct?  You are advertising that your function has an
> argument_type that is the same as Operation::argument_type.  Don't you want
> your argument_type to be a pointer? 

Hm yes, that is true. So let me correct this to:

     class indirecter_unary
        : public std::unary_function< typename 
boost::unary_traits<Operation>::argument_type*,
                                      typename boost::unary_traits<Operation>::result_type >

 > In any event, since you need to
 > redefine argument_type and result_type yourself, there is no point
 > inheriting from std::unary_function.
 >

Why? I thought those types are boost::unary_traits specific types? I 
thought I have to inherit from std::unary_function if I want to create 
what is called an "Adaptable Unary Function"?

> 
>>typename boost::unary_traits<Operation>::result_type >
>>{
>>    typedef typename boost::unary_traits<Operation>::argument_type
>>    arg_type; 
> 
> 
> I think, the argument_type you are looking for is
> 
>    typedef typename boost::remove_reference<arg_type>::type* argument_type;

Thanks, that sounds reasonable.

> 
>>    typedef typename boost::unary_traits<Operation>::param_type 
>>param_type;
> 
> 
> IIUC, param_type is supposed to be a type used to pass this function object
> as a parameter.  ie, it should be something like indirecter_unary const&,
> NOT the param_type of Operation.

Yes, I know. That's how I used it: To pass objects of type Operation to 
the constructor of indirecter_unary.

> 
>>    typedef typename boost::unary_traits<Operation>::result_type
>>result_type;
>>    typedef typename boost::unary_traits<Operation>::function_type
>>function_type;
>>    function_type op_;
>>public:
>>    explicit indirecter_unary( param_type op ): op_(op) {}
>>    result_type operator() (arg_type *arg) const {
> 
>                               ^^^^^^^^^^
> should be:
>     result_type operator()(argument_type arg) const {
> 
> 

Should it? operator() is supposed to take pointers! Where would the 
indirection be if I would pass non-pointer types?

-- 
Matthias Kaeppler