Re: [boost] [Ann] Variadic Templates [Updated]

From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2006-08-05 21:06:26

[moderator: this might be a bit off-topic, sorry]

Douglas Gregor wrote:
> On Aug 5, 2006, at 3:04 PM, Ion Gaztañaga wrote:
> 
>> -> First-class parameter packs
> 
> Will not be implemented. There are serious technical problems with  
> implementing first-class parameter packs that we had not foreseen. In  
> particular, to do a decent job of making sure parameter packs are  
> used properly when a template is defined, you need to know *exactly*  
> which things are parameter packs and which things aren't. With first- 
> class parameter packs, you don't always have this information because.

Umm. Native tuple without needing recursive instantiation trick was a 
very good idea to speed up compilation. Maybe the next time!

>> -> Initializer lists are parameter packs
> 
> Still thinking about this one. It looks like it might be a good idea.

In your vector example, the parameter list is introduced with a 
recursive function:

template<typename T>
class vector {
public:
    template<typename... Values>
    vector(const Values&... values) {
       reserve(sizeof(values...));
       push back all(values...);
    }
private:
    void push_back_all() { }
    template<typename... Values>
    void push_back_all(const T& value, const Values&... values) {
       push_back(value);
       push_back_all(values...);
    }
};

It would nice (just an idea) to have a way to execute an expression for 
every parameter, just like it was a function unrolling, implemented with 
with macros:

    template<typename... Values>
    void push back all(const T& value, const Values&... values) {
       std::variadic_unroll(push_back(value));
    }

Just an idea for the future. I don't know if implementing this is too 
difficult. That would help with compilation time.

>> Lots of good stuff implemented

*Very* nice work. Looks impressive.

Ion