Re: [boost] RE: Submission: typelist

From: Hamish Mackenzie (boost_at_[hidden])
Date: 2001-11-27 19:49:26

On Wed, 2001-11-28 at 00:21, David Abrahams wrote:
> Firstly, runtime and compile-time behavior are different domains, and so may
> well live in different namespaces.
> 
> Secondly, Aleksey and Emily were just discussing a hybrid:
> 
> template<typename T>
> struct identity : public unary_function<T, T> {
>    const T& operator()(const T& x) const throw()
>    {
>       return x;
>    }
>    typedef T type;
> };

Cool.

On a similar note does boost have a lightweight runtime typeid system?

Here is what I use. It can only tell you if two types are exactly
matched (no is_a information) but the overhead should be just 4 to 8
bytes per type depending on alignment. And perhaps more importantly it
works on any type.

From memory, as I don't have it in front of me right now...

class lightweight_type_id
{
public:
  lightweight_type_id() : ptr_( 0 ) {}

  template< class Type >
  static lightweight_type_id get() { return &id< Type >::dumby_; }  

  bool operator ==( const lightweight_type_id & id ) const
  {
    ptr_ == id.ptr_;
  }
private:
  template< class Type >
  class id
  {
  public:
    static const char dumby_ = 0;
  }
  
  lightweight_type_id( const char *p ) : ptr_( p ) {}
  const char *ptr_;
};

Is there a way to reduce the static storage overhead?

Hamish