Re: [boost] Formal Review Request: TypeErasure

Subject: Re: [boost] Formal Review Request: TypeErasure
From: lcaminiti (lorcaminiti_at_[hidden])
Date: 2012-06-19 07:54:16

Steven Watanabe-4 wrote
> 
> AMDG
> 
> On 06/18/2012 12:25 PM, lcaminiti wrote:
>> 
>> lcaminiti wrote
>>>
>>> I will use the lib in some use cases that I ran into the past and let
>>> you
>>> know if I find a nice example not too complex (I don't remember what the
>>> use cases were but they were about type erasure).
>>>
>> 
>> Is this a sensible use of this library or is there a much better way to
>> do
>> the following?
>> 
> 
> This is a textbook use case for the library.
> Use TypeErasure states the intent of the code clearly.
> obj_ can hold any type that is CopyConstructible,
> and Ostreamable. (you don't actually need typeid_ here).
> 

Oh yes, because I'm not doing type casting. Also I meant for obj_ to be
private. A member like the following could also be added:

class display
{
    ...

    public: template< typename U >
    void set ( U const& obj ) { obj_ = obj; }
};

This might be a reasonable example for the motivation section maybe with a
comparison of why this example cannot be worked with Boost.Any alone.

>> #include &lt;boost/type_erasure/any.hpp&gt;
>> #include &lt;boost/type_erasure/builtin.hpp&gt;
>> #include &lt;boost/type_erasure/operators.hpp&gt;
>> #include &lt;boost/mpl/vector.hpp&gt;
>> #include <iostream>
>> 
>> struct display
>> {
>>     public: template< typename T >
>>     explicit display ( T const& obj ) : obj_(obj) {}
>> 
>>     public: void print ( void ) { std::cout << obj_ << std::endl; }
>> 
>>     public: boost::type_erasure::any<
>>         boost::mpl::vector<
>>               boost::type_erasure::copy_constructible<>
>>             , boost::type_erasure::typeid_<>
>>             , boost::type_erasure::ostreamable<>
>>         >
>>     > obj_;
>> };
>> 
>> int main ( void )
>> {
>>     display i(-1), d('x');
>>     i.print();
>>     d.print();
>>     return 0;
>> }
>> 
>> I had to do something like this in the past where:
>> 1. I didn't want display to be a template but I wanted it to handle
>> generic
>> types as passed to the constructor.
>> 2. I knew a-priori the operations that display needed from T (e.g.,
>> operator<<).
>> 3. I had to store the object of the generic type T so the operation on T
>> could be performed later (e.g., by print).
>> I needed to do this as an implementation detail so the original problem I
>> was trying to solve has little relevance in general. But I wanted to ask
>> if
>> using this library is a reasonable approach to do the above of it there
>> are
>> better ways.
> 

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