[Boost-users] Re: boost::unary_traits

From: Ian McCulloch (ianmcc_at_[hidden])
Date: 2005-03-04 15:10:07

Matthias Kaeppler wrote:

> 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 >

That will still cause problems with pointers to references.

> 
>  > 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"?

No, you only need to provide the typedef's for argument_type and
result_type.  std::unary_function is just a convenience to save typing. 
But in your case, it only adds to the typing.

> 
>> 
>>>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.

Ok, I misunderstood the first time I read it.

> 
>> 
>>>    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?

It does - from above:

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

argument_type is a pointer type.

There is still a subtle problem here with what the argument_type should be
when Operator takes is argument by value.  indirecter_unary should then
have an argument_type that is a pointer to const.  One way to achieve that
would be to use remove_reference<add_const<arg_type>::type>::type* as the
argument_type.

Cheers,
Ian