Re: [OT] Re: Teaching new tricks to an old dog (C++ -->Ada)

["Peter Koch Larsen" <pklspam@mailme.dk>]

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"Dr. Adrian Wrigley" <amtw@linuxchip.demon.co.uk.uk.uk> skrev i en 
meddelelse 
news:pan.2005.03.14.14.41.47.813082@linuxchip.demon.co.uk.uk.uk...
> On Sat, 12 Mar 2005 09:42:26 +0100, Georg Bauhaus wrote:
>
>> Falk Tannh�user wrote:
>>> Dr. Adrian Wrigley wrote:
>>>
>>>> But what of features not present in either?
>>>
>>> [...]
>>>
>>>> associative arrays (from Perl)
>>>
>>> Wouldn't that be std::map in C++?
>>
>> and in Ada 2005,
>>
>> Ada.Containers.Hashed_Maps  and  Ada.Containers.Hashed_Maps
>
> I have probably missed a trick in the C++, but I couldn't get
> std::map code to compile (except in the trivial cases):
>
> #include <map>
>
> struct compoundindex {
>  int a, b, c;
> };
>
> int main()
> {
>    std::map<compoundindex, float> hash;
>    compoundindex fred = { 1, 2, 4 };
>
>    hash[fred] = 0.123;

First off, std::map is not hash-based. It is a sorted container. Now in 
order to use this, you need a comparing function. It can be the "standard" 
ordering function for your key or it can be a user specified function.

> }
>
> cpptest.cpp: In function `int main()':
> cpptest.cpp:14: error: `main()::compoundindex' uses local type
> `main()::compoundindex'
> (8 more lines of errors here!)

You must have snipped the most interesting errormessage.
>
> (what is the simplest C++ code to get this intent?)

What you need here is a comparator:

bool operator<(compoundindex const& lhs,compoundindex const& rhs)
{
....
}

And the map should work.
> --
> The Ada associative arrays from the new draft standard
> are specified as something like:
>
> generic
>   type Key_Type is private;
>   type Element_Type is private;
>   with function Hash (Key : Key_Type)
>      return Hash_Type is <>;
>   with function Is_Equal_Key (Left, Right : Key_Type)
>      return Boolean is "=";
>   with function "=" (Left, Right : Element_Type)
>      return Boolean is <>;
> package Ada.Containers.Hashed_Maps is...
>
>
> Which clearly won't work unless you can find the three
> function generic parameters.  I don't see how this can be
> used easily in a generic context.
>
> I don't think I am being *that* unreasonable in asking for arrays
> indexed by arbitrary types, without jumping through hoops :)

But I do! How can you index without at least knowing if two values are 
different or the same?
The C++ standard will have a hash-based container later. Those interested in 
having one can download one from e.g. boost (I believe this container also 
to be implemented by boost and prbably several others).

> -- 
> Adrian
>
/Peter