Re: [boost] Re: Formal review of "Output Formatters"library beginstoday

From: John Torjo (john.lists_at_[hidden])
Date: 2004-09-13 07:08:43

> This is true.
> 
>> That said, in this case there shouldn't be many penalties.
>> Again, you can use
>> ios_base::pword and ios_base::iword for this.
>> It's an advanced technique, but I think it can prove very neat and 
>> flexible.
> 
> 
> okay.
> 
>>> It will also change the way all sequence constructs are rendered. 
>>> (n-ary types have a different rendreing so won't be affected). As I 
>>> understand, this is the desired effect. But then:
>>
>>
>> You can make it possible to associate a fmt object with a delimeter 
>> object.
>>
>> Example: pairfmt() with openclose_formatter.
>> basicfmt() with formatter. Etc.
> 
> 
> That is basically what I do already:
> wrappedfmt() is associated with openclose_formatter;
> pairfmt() is associated with formatter (using default_nary_traits)
> containerfmt() is associated with formatter (using default_sequence_traits)
> etc.

Yes, and that is great!
But you hold them in the formatob object.

What I'm saying is that (overridable) defaults for each of the above can 
also be kept in the stream object itself.

That's why ios_base::iword and ios_base::pword are good for: for keeping 
delimeter objects *inside the stream object itself*.

Thus, when you write something, and the user hasn't provided a specific 
delimeter, you read that delimeter from the underlying stream.
(again, it's quite an advanced technique ;))

Same goes for reading.

> 
> 
> so you can theoretically use any string type to store the default 
> delimeter values. How do we do the equivalent with ios_base::pword/iword?

That's your job - to come up with a technique for this ;)
Seriously, I'm sure it's possible (see above).

Basically, the library's underlying concept doesn't have to change. This 
is just an extra goodie you can provide.

> 
> openclose_formatter/formatter set their delimeter values to the values 
> provided by the default traits object passed to them. You can then 
> override these defaults by calling format(). How do you tell that the 
> default has been overrided?

It's certainly possible. Internally, in the delim object, you can keep 
the data like this:

template<class value_type> struct val_keeper {
     value_type val;
     bool has_been_set;
     //getter and setter - I assume you get the point
};

> 
>> Then, you can store (default) delimeter objects within the stream.
>> Once you want to output/input something, it requires a specific delim 
>> object. You'll somehow use ios_base::pword/iword to get a reference to 
>> that object.
> 
> 
> The main questions are:
> [1] how do you store different DelimeterTypes in this mechanism?

Which brings us to ;) :
I think one type of delimeter is enough (I assume you wholeheartidly 
disagree with this).
This type should be the std::basic_string<underlying_char_type>().

This would simplify things a lot.

But, in case you want multiple DelimeterTypes, it will still be possible 
to implement this (but only harder)

> [2] how do you tell if the default is needed (see above)?

see the val_keeper above.

Best,
John

-- 
John Torjo
-- john_at_[hidden]
Contributing editor, C/C++ Users Journal
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