Re: [boost] Formal Review Request: TypeErasure

Subject: Re: [boost] Formal Review Request: TypeErasure
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2012-06-19 00:12:15

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).

> 
> #include <boost/type_erasure/any.hpp>
> #include <boost/type_erasure/builtin.hpp>
> #include <boost/type_erasure/operators.hpp>
> #include <boost/mpl/vector.hpp>
> #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.
> 

In Christ,
Steven Watanabe