Re: [Boost-users] Fwd: Describing "traits" of a class's members

Subject: Re: [Boost-users] Fwd: Describing "traits" of a class's members
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2013-02-27 20:13:23

AMDG

On 02/27/2013 04:08 PM, Chris Stankevitz wrote:
> 
> Can you modify the attached program so that the object
> TCPositionTraits holds all the traits for the TCPosition object.  Not
> just the traits for the "double" parameters, as is currently the case.
>  Notably missing from TCPositionTraits is a reference to
> TCPosition::mSteps.
> 

That's actually pretty easy:

change TCDoubleTraits, TCUnsignedTraits, etc. to

template<class T>
struct TCTraits;
template<>
struct TCTraits<double>;
template<>
struct TCTraits<unsigned>;

change

struct TSItem { ... };

to

template<class T>
struct TSItemImpl
{
  const TCTraits<T> *mTraits;
  boost::function<const T&(const TCPosition*)> mGet;
};

typedef boost::variant<
  TSItemImpl<double>,
  TSItemImpl<unsigned> > TSItem;

then use boost::apply_visitor for all
operations on TSItems.

> Difficulty:
> 
> This is a difficult question to answer due to the fact that
> TCPositionTraits has hard-coded in it the keyword "double".  I cannot
> figure out how to remove this.
> 
> Bonus:
> 
> Bonus points if your answer looks something like "yes it is possible
> if you start using a boost concept which you apparently are not aware
> of".
> 
> Note:
> 
> You'll need the C++11 brace-initialization capability to compile this.
>  I used "g++ -std=c++0x test.cpp" with gcc 4.6.3.
> 

In Christ,
Steven Watanabe