From: Reece Dunn (msclrhd_at_[hidden])
Date: 2004-11-03 15:47:25
Vladimir Prus wrote:
> On Thursday 28 October 2004 12:26, Reece Dunn wrote:
>> Hi All,
>> I have been working on the library and have got a local
>> my Output Formatters library based on the review comments. There are
>> a few issues that need to be resolved and I would like to expand stream
>> state support.
>> I have created a preliminary evolution document available at:
>> that outlines what the new functionality looks like. This redesign
>> addresses several of the issues brought up during the review.
> Hi Reece, here are my comments on the design.
>> Use () if you want to get the value of a decoration:
>> sep() == "|"; // instrinsinc value
> Why would I want to do this?
In case you need to get the value directly and not lookup based on the
>> sep( std::cout ) == "|"; // stream default if sep() == std::string()
> So, this means: 'value in sep if any, otherwise value in stream'?
Yes. There is currently a bug in the implementation if you use "" as a
decorator value (i.e. no decoration) where it will choose the default
value. I am planning to use "\1 def" - a value that is not likely to be
>> If you want to see if a decorator is on an input stream, you can use
>> io::match( std::cin, sep );
> What are the use cases for this?
It is primarily used to implement the read function of a format object.
template< class TraitsT, typename F, typename S >
std::basic_istream< CharT, TraitsT > &
in( std::basic_istream< CharT, TraitsT > & is, F & f, S & s ) const
match( is, open ) && first.read( is, f ) &&
match( is, separator ) && second.read( is, s ) &&
match( is, close )
is.setstate( std::ios_base::failbit );
return( is );
>> boost::io::wrapper_decorators< CharT, RetType = void >
> While I see why it's called "wrapper", the name still sounds a bit
strange for me. Maybe, "open_close_decorators"?
That is better. I'll update the name.
>> Holds an (open, close) decorator set.
>> boost::io::wrapper_decorators< char > wrap( "< ", " >" );
>> boost::io::wrapper_decorators< char > wrap2( wrap );
>> wrap.decorate( "[[ ", " ]]" );
>> wrap2.decorate( wrap );
> Why do I need the 'decorate' method as opposed to
> wrap = wrapper_decorators<char>("<", ">")
> wrap.assign("[[", "]]")
> The idea that I can decorate a decorator does not seems good for me.
Why do I need to change decorators in this way?
At the moment, the default values on the stream applies to all types.
That is, the default value is associated with the decorator id argument.
I can see why decorate would not be a good name, but assign is not good
io::object( vec ).decorate( " / " ); // decorate io::object
io::object( vec ).assign( " / " ); // assign what?
What I am trying to aim for is:
io::object( vec ).separator( " | " )
.openclose( "< ", " >" );
but I need to work on implementability (i.e. maintaining the current
functionality). Maybe 'set' would be better for setting the decoration
based on all 3 values or another sequence_decoration object.
> To change them, I need to access decorators stored on the stream. How
do I do that?
Use io::decorate< CharT > or the c/wdecorate typedefs like this:
std::cout << io::cdecorate( "[[ ", " ]]" )
<< io::object( vec ) << '\n';
>> RATIONALE: RetType -- a class may derive from this class and you may
>> chain function calls. For example:
>> derived_class dc;
>> dc.decorate( "/ ", " /" ).derived_fn();
>> RetType is needed to allow this type of behaviour.
> Do you have the need for it at the moment? If not, I suggest you drop
this template argument. I had something similar in program_options and
was messy, did not work everywhere and added no value.
It is needed: the format objects can be passed as arguments to other
format objects. Without the above, this code will not work:
io::object( vec, fmt::container().decorate( " : " ));
[error]: cannot find function 'write' in 'sequence_decorators'.
>> : sequence decorators
>> boost::io::sequence_decorators< CharT, RetType = void >
>> Holds an (open, close, separator) decorator set. Derives from
>> io::sequence_decorators< char > seq1;
>> io::sequence_decorators< char > seq2( " : " );
>> io::sequence_decorators< char > seq3( "( ", " )" );
>> io::sequence_decorators< char > seq4( "[ ", " ]", " | " );
>> io::sequence_decorators< char > seq5( wrap );
> Do I ever need to use this constructor?
Do you mean seq5( wrap )? I don't know. It would depend on what you are
using the library for. I have tried to keep the same arguments for the
constructors and decorator() function.
>> ===== TYPE TRAITS AND INTROSPECTION
>>  type traits
>> In order to tell the framework about a type, you need to register it
>> BOOST_IO_CLASSIFY_TYPE( nargs, tempate_type, type_class )
>> nargs is the number of template arguments the class has
>> template_type is the name of the class (e.g. std::list)
>> template_class is the type to which the class belongs
>> (e.g. boost::io::seq_container_type)
>> BOOST_IO_CLASSIFY_TYPE( 1, std::complex, io::nary2value_type );
>>  boost::io::is_XXX< T >
>> These MPL objects assert whether T is of type XXX, e.g.:
>>  n-ary objects
>> An n-ary object (currently, 2-ary, 4-ary and 8-ary objects are
>> is a statically-sized sequence of n elements. There is support here for
>> describing how one of these types is constructed and for
>> In order to get the nth element of an n-ary object ob, you can use
>> std::cout << io::getval< n >( ob ) << '\n';
> Isn't 'getval' an implementation detail of the library?
In that it is used to implement the fmt::pair_t and fmt::nary_t classes.
>> BOOST_IO_NARY_PARAM2 is used to register a 2-ary type as containing two
>> different types for elements 1 and 2, e.g. std::pair.
>> is used in all other cases.
>> BOOST_IO_NARY_PARAM2( std::pair, seperable_pair )
>> BOOST_IO_NARY_PARAM1( std::complex, inseperable_pair< T > )
> And what about 4-ary objects? Still BOOST_IO_NARY_PARAM1? What if
there are 4 different template parameters?
BOOST_IO_NARY_PARAM1( boost::math::quaternion, nary4_type< T > )
BOOST_IO_NARY_PARAM1( boost::math::octonion, nary8_type< T > )
The current implementation of n-ary types only supports different
template parameters on 2-ary types. What would be better is something like:
naryN_type< T1, ..., Tn = Tn-1 >
The fmt::nary_t format object would need to be extended to support
FormatObject1..N format objects to render the individual elements.
>> This is implemented like this:
>> template< int n, typename T1, typename T2 >
>> typename io::detail::selector< n, T1, T2 >::ref_type
>> io::refval( std::pair< T1, T2 > & p )
>> return( detail::selector< n, T1, T2 >::ref( p.first, p.second ));
>> where the ref function of detail::selector takes 2, 4 or 8 arguments and
>> returns a reference to the nth argument.
>> An inseperable pair (io::inseperable_pair< T1, T2 = T1 >), 4-ary
>> (io::nary4_type< T >) or 8-ary (io::nary8_type< T >) object cannot
>> a reference to individual elements. They need to be set at the same time
>> using io::assignval:
>> io::assignval( ob2ary, a, b );
>> io::assignval( ob4ary, a, b, c, d );
>> io::assignval( ob8ary, a, b, c, d, e, f, g, h );
> I think you're again discussing implementation, not interface. I
don't think this function should be initially present in the library
I see why you would classify it as implementation. It really belongs in
an "extenders guide" rather than a "users guide".
For example, if you want to add support for NSPoint (a 2D position
structure from the MacOS Cocoa API), you need to know this information,
template< int n >
float io::getval( const NSPoint & pt )
return detail::selector< n, float >( pt.x, pt.y );
Likewise, if you are writing a format object that allows a different
seperator for each division in the output, e.g.:
open e1 s1 e2 s2 e3 s3 e4 close
where eX are the elements in the n-ary type
and sX are the seperators
you need to know how to use getval, refval and assignval in order to
implement the format object in a generic way.
>>  automatic type deduction
>> The fmt::deduce function:
>> template< typename CharT, typename T >
>> [undefined] fmt::deduce( const T & ob );
>> will return an instance to the nested format object construct that
>> the class T.
> Ok, we've got to interesting part. I don't understand what's "nested
format object", what's "construct", and what does it mean for
"construct" to map to some class. And you haven't defined "format
What I was trying to say with the above sentence is something like:
The type associated with ob may be a simple type or a more complex type
like a container or pair type. If the type is a container or other
sequence-like type, the elements in this type may be sequence-like types.
A format object is a class that is responsible for reading from and
writing to a stream. There are format objects for different types (like
containers, arrays and pairs). These take template arguments that
specify the format object used to render the elements in the sequence
associated with the format object.
The type of the object ob passed to fmt::deduce is mapped to the format
object that can render ob. For example, if
T = std::pair< std::list< char >, std::complex< float > >
then fmt::deduce will return a format object of the type
fmt::pair( fmt::container(), fmt::pair())
>> The deduction mechanism involves registering the format object with
a type class.
> Maybe, you should call that "type category" to avoid confusion.
>> In order to save writing code when several types are implemented by
a format object, you can map these types into a formatter
>> class type:
>> SEQ_TYPE_TO_FORMATTER( nary2value_type, pair_type );
>> SEQ_TYPE_TO_FORMATTER( nary2base_type, pair_type );
>> SEQ_TYPE_TO_FORMATTER( nary2int_type, pair_type );
> Hmm... what's nary2int_type, nary2base_type and nary2value_type? I
though there's only one type category for n-ary objects, now I see two more.
The nary2xxx_type categories belong to the type deduction system. They
are used to distinguish between the different typedefs for the element
types. Ideally, this would be better as a type trait:
element_type< T >::type
element_type< std::complex >::type == std::complex::value_type
element_type< boost::interval >::type == interval::base_type
element_type< boost::rational >::type == rational::int_type
This would make the deduction system simpler in that you would only need
nary2_type. The problem is how this works with things like std::pair.
One solution would be to return an mpl type list in this instance.
>> As a user, you don't need this, but if you are adding a format
>> the type deduction framework, you will need this to extract the nested
>> format object.
> Could you rephrase the above?
Yes. I shall update the evolution.html file with the revised wording.
>> A format object is deduced by defining fmt::detail::deduce_type for the
>> formatter class type
> What's "formatter class type"? Is that "type category"?
More like a collapsed type category. For example, the fmt::container_t
format object supports seq_container_type (sequential containers),
assoc_container_type (associative containers) and set_container_type
(set containers). Thus, the format class type for fmt::container_t is
seq_container_type. SEQ_TYPE_TO_FORMATTER is used to perform this collapse.
>> that the format object handles. It has the form:
> Say I've used SEQ_TYPE_TO_FORMATTER( nary2value_type, pair_type )
already. Do I need to define this?
You only need to register it once. If you have another decleration like
this it will most likely result in a compilation error.
>> struct fmt::detail::deduce_type< type >
>> template< typename CharT, typename T >
> What's 'T' here? The type being output?
>>  format object generators
>> The format objects provided by the library supply generator functions to
>> simplify the creation of format objects.
>> A generator gen will have the form gen< CharT >() to specify the
>> type used by the format object to store the decoration values. It
>> the form gen() that is equivalent to gen< char >().
> Such overloads (non-template + template) used to cause problems for
me. IIRC, on borland.
I am not sure on this because wrapper_decorators is producing compile
errors on borland (5.6 and 5.5). I want to sort these out as I revise
>> In addition, the format object may provide additional forms to allow
format object nesting.
> I did not understood what's generator and when it should be used.
A generator is a function that will return a format object. This is
similar to make_pair or make_tuple. For example, instead of doing:
fmt::pair_t< char, fmt::container_t< char > >()
you can use the generator functions like this:
>>  format objects
>> <2> pair format object
>> fmt::pair_t< CharT, First, Second > will render 2-ary object types using
>> First to render the first element and Second to render the second. The
>> generator function is fmt::pair() and has the additional form:
>> [unspecified] fmt::pair( const First & f, const Second & s = basic());
> "unspecified"? Shouldn't that be fmt::pair_t<charT, First, Second>?
It is actually
fmt::pair_t< typename First::char_type, First, Second >
I use "unspecified" because the types can be long and complex, for
example, io::object returns:
CharT, T, typename fmt::detail::get_deducer
and I don't want the return type to distract from the functions
> And BTW, I'm strongly against the "_t" suffix. Does it add any
information or you just _t because 'pair' is already used? What about
calling the type "pair_formatter"?
I will change the name in the update I am performing.
> And, why do I ever need to create the formatter explicitly? Can I
call any methods of it?
You can save a format style, for example:
fmt::pair_t< char > parens;
parens.decorate( "( ", " )" );
std::cout << io::object( p1, parens ) // ( a, b )
<< io::object( p1 ) // [ a, b ]
<< io::object( p2, parens ); // ( a, b )
You can also access properties and functions associated with the format
object. For example:
fmt::pair_t< char, fmt::pair_t< char > > po;
po.first.decorate( " : " );
po.decorate( "<: ", " :>" );
std::cout << io::object( p, po ); // <: [ a : b ], c :>
>> <7> state format object
>> fmt::state_t< CharT, StateObject, bool pre, FO > will render the state
>> defined by StateObject before the object being read or written if
> I think the name "state format" is a bit unclear. I'd suggest some
more explicit pre_format_hook/post_format_hook, or something like that.
BTW, some thinks (and I'd agree) that using 'bool' as parameter to
function/template is not good. If I look at
> state<Foobar, true>()
> I have no idea what 'true' means.
I shall find a better name. I am considering using an enumeration to
define pre, post and pre_post values. For example:
pre_post_hook< char, pre >() // [hook] elem
pre_post_hook< char, post >() // elem [hook]
pre_post_hook< char, pre_post >() // [hook] elem [hook]
It may be better to drop this functionality from the initial release of
>> <8> object manipulator
>> io::object_t< CharT, T, FO > is a special type of format object in
>> provides I/O manipulator functionality
> What's "I/O manipulator functionality"? Do you mean stream
> Finally: you mentioned you have revised version of the code. Looks
like it's not in sandbox. Is it possible to take a look?
I want to work on it further before committing it to the sandbox. The
current version is now available at:
NOTE: This is a work in progress.
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