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

From: Reece Dunn (msclrhd_at_[hidden])
Date: 2004-09-20 14:20:06

Vladimir Prus wrote:
>John Torjo wrote:
>
> > The FORMAL Review of "Output Formatter" library begins today,
> > Sept 12, 2004.
>
>The primary case is debugging. I want to either output small vectors to 
>some
>log/dump file (in which case the output will be small one-line), or output
>some huge structures (vector<Function>, where Function has a lot of data).
>In the latter case, the output should be multiline, with nice indentation,
>or I won't understand anything.

You can explicitally state where/how new lines and indentation are going to 
be within my library by adding them to the open/close/separator formatting 
(see output-3D.cpp for an example). As for more intelligent indentation, it 
is hard to know in advance if an object you are writing to the stream should 
be wrapped or not. This is where feeding it through Jonathan's library (via 
an indentation filter) would be useful.

>A particularly interesting question is how the proposed library overlaps
>with serialization. When outputting vector<Function> I'd prefer the content
>of 'Function' to be outputted too, preferably by describing the members
>with the 'serialize' method.

The default behaviour for an unrecognised type is to output it directly to 
the stream so it will use the << operator associated with that type. If you 
want to use a particular method, such as 'serialize', then you will need to 
write your own format object that calls that function.

> > 1. What is your evaluation of the design?
>
>There are some things I don't like. First, as many mentioned, the naming is
>not optimal. For example:
>
>    boost::io::formatob(v).format(" | ");

The naming scheme for the various elements has undergone an overhaul. See my 
other posts for comments. From here on in, I will use the new naming, with:
   namespace io = boost::io;
   namespace fmt = boost::io::format;

>or maybe even
>
>   ("[" + io(v)/"," + "]")

Cool! I shall look into trying to implement something like this.

>It's desirable that the library support some multiline output style out of
>the box, so that I could write:
>
>   os << io::multiline(v) << ...

Do you mean something like:

   std::cout << io::object( v ).decorate( "[\n   ", "\n]", ",\n   " );

resulting in:

[
   1,
   2,
   3
]

> > 2. What is your evaluation of the implementation?
>
>There should be the <boost/outfmt.hpp> header, including everything else of
>the library.

ok

>Some of the lines are longer than 80 characters (e.g. template header of
>formatob_t has 111 characters).

ok

>Methods defined in the body of the class are implicitly inline, there's no
>need to put the "inline" in front of them, as it's just one more word to
>read.

ok

>I don't think defining methods in the class is a good idea -- this makes 
>the
>class interface less obvious.

I agree. I prefer to keep interface and implementation seperate, but MS VC 
(prior to 7.1) will choke on nested templates declared out of line, e.g.:

   template< typename T >
   struct foo
   {
      template< typename U >
      void bar();
      template< typename U >
      void ok(){}
   };

   template< typename T > template< typename U > // VC<=7.0 ==> error
   void foo< T >::bar(){}

I also try to be consistent where possible, so don't tend to mix the two.

>Why is the formatob_t necessary? It seems to work by delegating everything
>to the underlying formatter. Can't the 'formatob' just return the
>appropriate formatter?

It's necessary because the format objects don't define << and it is 
necessary to keep a refererence to the object passed to it for formatting so 
that it can be accessed when inside the implementation of the << operator.

>I might be missing something, but the mechanism for getting the type of
>formatter from a type to be output seems too complex. First, the
>type_deducer.hpp file is used, and 'select' computes a 'category'. Then
>format_deducer.hpp takes the category, and again uses 'select' to obtain
>the real type of formatter. Why the type_deducer.hpp is needed?

I have been working on improving this. There are 3 parts to the type 
deduction:
[1] identifying the type and mapping it to its generic type (e.g. container, 
array, etc.) This is done by type_traits.hpp.
[2] working out the formatter type that is needed to render this to the 
stream. In the review implementation this is done by format_deducer.hpp, 
using the information gathered by type_deducer.hpp to compute the nested 
structure (e.g. std::vector< std::pair< char, std::list< int > > >).
[3] creating an instance of the format object. This is also done by 
format_deducer.hpp in the review version.

In the version I am working on at the moment, I have a deduce_type< int > 
template that is used to simulate partial specialization (for compilers that 
don't support it). The int parameter is the format object category and is 
taken from type_traits.hpp (or your own value if you add a custom type and 
format object).

The decude_type template contains
   template< typename CharT, typename T > struct type_from{ ... };
that has a format_object typedef and the method:
   static format_object deduce( const T & );
for constructing an instance of that format object. For example:

   template<> struct deduce_type< io::seq_container_type >
   {
      template< typename CharT, typename T >
      struct type_from
      {
         typedef typename T::value_type value_type;
         typedef typename get_deducer< CharT, value_type >::type 
value_deducer;

         typedef container_t
         < CharT, typename value_deducer::format_object > format_object;

         static format_object deduce( const T & )
         {
            return( format_object( value_deducer::deduce( value_type())));
         }
      };
   };

> > 3. What is your evaluation of the documentation?
>
>The documentation leaves much to be desired. I'll walk though some of the
>aspects (indented text is quote from the docs)

I will attempt to address the issues you have raised.

>      "providing an extensible framework that sits"
>
>How it's extensible? I see only one section about extending and it's just
>one paragraph long

I really need to add more documentation regarding this. The idea is that you 
can:

*  add your own decorators (delimiters in the review docs) to support more 
complex types (e.g. trees and graphs).

*  add your own format objects to control the way the data is rendered, for 
example adding a format object to call the 'serialize' method on a class.

*  create state objects used by fmt::state to perform custom formatting 
(e.g. in the john-torjo.cpp example, a state object is used to render the 
position:
   { [0] Vladimir, [1] Terje, [2] John, [3] Dave, [4] Rene }
)

*  register a type with the type deduction system so it can be implicitly 
deduced, e.g.:
   BOOST_IO_CLASSIFY_TYPE( 2, boost::array, io::seq_container_type );
   // ...
   boost::array< int, 5 > a;
   std::cout << io::object( a ); // output: [ 1, 2, 3, 4, 5 ]

>  boost::io::formatobex< DelimeterType >( const T & ob );
>
>  This will format ob according to it's underlying type.
>
>How is this different from using the 'formatob' function. What's 
>"underlying
>type".

Underlying type is: typeid(ob).name(). That is std::vector< int > will be 
formatted differently to std::pair< char, std::list< std::complex< float > 
 >.

>I'd also suggest
>for formatobex use the "basic" prefix to indicate it's templated on the
>character type. E.g. stlio and basic_stlio, or something like that.

That is a good idea. Especially considering the revisions I am in the 
process of making.

>    If you need to specify a range or sub-range, boost::io::formatob will 
>not
>    recongnise it unless it is a container.
>
>    boost::io::range( ForwardIterator first, ForwardIterator last );
>
>This is more of a language problem. Range, by definition, is a pair (but 
>not
>std::pair) of iterators. It's never a container.

What I meant is that:

   std::cout << io::object( vec ); // [1] ok - know how to process 
containers
   std::cout << io::object( a, a + 5 ); // [2] oops! don't know how to 
handle ranges
   std::cout << io::object( io::range( a, a + 5 )); // [3] ok - range is now 
explicit

The reason I don't allow variant 2 is because that would mess up argument 
resolution.

>For a reference, the code examples are not necessary. For tutorial,
>you don't need to list every overload, just mention that there are others.

noted.

>    boost::io::openclose_formatter is a class that allows the user to 
>change
>    and access the format used for open and close delimeters
>
>Here we're definitely in the reference docs already, while I did not get an
>overall picture yet. Then, what's "change and access the format". If the
>format can be changed, it is stored somewhere. Where?

It is stored inside the [openclose_]formatter object (wrapper_decorators and 
sequence_decorators in the new version). You can set the delimiters 
(decorators in the new version) using the various 'format' functions 
('decorate') and get their values via open(), close() and separator() (used 
when implementing your own format objects).

>The code block before
>this comment defines two classes openclose_formatter_t and
>openclose_formatter. Is this a typo, or you really have two classes?

This is not a typo. The _t variants take a template parameter that contains 
the return value of the 'format' ('decorate') functions. This is so that:
   std::cout << io::object( vec, fmt::container().decorate( " | " ));
works properly.

>Do you mean that every FormatObject class should have this signature? Then,
>the phrase about "FormatObject::write(os, fo.ob)" is not necessary.

ok.

Regards,
Reece

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