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added section describing named parameters, default parameters, and

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a conclusion


[SVN r10328]
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Jeremy Siek
2001-06-14 16:02:58 +00:00
parent 5b716a8b19
commit c59a5e8783
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tmpw2001-paper/iter-adaptor.tex
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@ -66,7 +66,6 @@ Florham Park, NJ 07932, USA \\
\maketitle
\begin{abstract}
The iterator abstraction is one of the most commonly used in
programming and a considerable amount of time is spent building new
@ -214,7 +213,7 @@ types: \code{value\_\-type}, \code{reference}, \code{pointer},
example above, the last two would be delegated to the \code{Base}
iterator type. For many iterator adaptors, all five must be delegated.
\subsection{Iterator Complexities}
\subsection{Iterator Implementation Complexities}
In addition to the tedious aspects of iterator implementation, there
are some complexities that trip up even the most experienced of
@ -235,7 +234,6 @@ corresponding versions of \code{operator!=}, and (for
\stlconcept{RandomAccessIterator}), operators \code{<}, \code{>},
\code{<=}, \code{>=}, and \code{-}.
{\footnotesize
\begin{verbatim}
bool operator==(const C::iterator& x, const C::iterator& y);
@ -463,39 +461,53 @@ actual iterator type. The best way to package the construction of the
transform iterator is to create a \emph{type generator}, which is a
class template whose sole purpose is to simplify the instantiation of
some other complicated class template. It fulfills the same need as a
templated typedef would if that were part of the {C++} language.
The following code shows the type generator for the transform
iterator.
templated typedef would if that were part of the {C++} language. The
first template parameter to the generator is the type of the function
object and the second is the base iterator type. The following code
shows the type generator for the transform iterator.
{\footnotesize
\begin{verbatim}
template <class AdaptableUnaryFunction, class Iterator>
template <class AdaptableUnaryFunction, class BaseIterator>
struct transform_iterator_generator
{
typedef typename AdaptableUnaryFunction::result_type value_type;
typedef typename AdaptableUnaryFunction::result_type val_t;
public:
typedef iterator_adaptor<Iterator,
typedef iterator_adaptor<BaseIterator,
transform_iterator_policies<AdaptableUnaryFunction>,
value_type, value_type, value_type*, std::input_iterator_tag> type;
iterator_category_is<std::input_iterator_tag>,
value_type_is<val_t>, reference_is<val_t> > type;
};
\end{verbatim}
}
We use \code{iterator\_adaptor} to define the transform iterator type
as a nested \code{typedef} inside the
\code{transform\_iterator\_generator} class. The first template
parameter to the generator is the type of the function object and the
second is the base iterator type. The remaining types specify the
traits of the iterator: \code{value\_type}, \code{reference},
\code{pointer}, and \code{iterator\_\-category}. Because the function
may return by-value, we must limit the \code{iterator\_category} to
\code{std::input\_\-iterator\_\-tag}, and the iterator's
\code{reference} type cannot be a true reference (the standard allows
this for input iterators). The last template parameter of the
\code{iterator\_adaptor} is for the \code{difference\_\-type}, which
in this case will default to the \code{difference\_\-type} of the base
iterator.
\code{transform\_\-iterator\_\-generator} class. The first parameter
to \code{iterator\_\-adaptor} is the base iterator type and the second
is the policies class. The remaining parameters specify the iterators
associated types and are given as \emph{named parameters}. We will
discuss this technique in \S\ref{sec:named-template-parameters}.
The \code{iterator\_category} is set to
\code{std::input\_\-iterator\_\-tag} because the function object may
return by-value. For the same reason the \code{reference} type (which
will be the return type of \code{operator*}) is set to \code{val\_t}
(and not \code{val\_t\&}). There are two parameters that are left out:
the \code{pointer} type defaults to \code{value\_type*} and the
\code{difference\_\-type} defaults to the \code{difference\_\-type} of
the base iterator.
It is tempting to create a \code{transform\_\-iterator} by deriving
from \code{iterator\_\-adaptor} (as an alternative to using the
generator). This approach does not work, for example, because the
return type of \code{operator++} of an iterator is required to be the
same iterator type, but in this case the return type would be
\code{iterator\_\-adaptor} and not \code{transform\_\-iterator}.
%It
%would be possible to arrange for using inheritance by applying the
%Barton and Nackman trick, but we felt that design
\subsection{Iterator Object Generator}
\label{sec:iter-object-generator}
@ -506,7 +518,6 @@ following constructor.
{\footnotesize
\begin{verbatim}
explicit
iterator_adaptor(const Base& it, const Policies& p = Policies())
\end{verbatim}
}
@ -556,7 +567,6 @@ printing the result to standard output.
#include <algorithm>
#include <iostream>
#include <boost/iterator_adaptors.hpp>
int main(int, char*[])
{
int x[] = { 1, 2, 3, 4, 5, 6, 7, 8 };
@ -592,14 +602,14 @@ discussed in the introduction were solved.
\begin{verbatim}
namespace boost {
template <class Base, class Policies,
class Value = default_argument,
class Reference = default_argument,
class Pointer = default_argument,
class Category = default_argument,
class Distance = default_argument>
class Param1 = default_argument,
class Param2 = default_argument,
class Param3 = default_argument,
class Param4 = default_argument,
class Param5 = default_argument>
struct iterator_adaptor {
// Deduce iterator associated types (value_type, etc.) from the
// template parameters, and handle any named template parameters.
// named template parameters, and resolve any defaults.
public:
// Core operators, delegate to policies class.
// Redundant operators, implemented in terms of the core operators.
@ -620,9 +630,106 @@ namespace boost {
\subsection{Deducing the Associates Types}
\subsubsection{Default Types}
Iterators have five associated types: \code{value\_\-type},
\code{reference}, \code{pointer}, \code{iterator\_\-category}, and
\code{difference\_\-type}. Each of these types must either be supplied
by the user, using the named parameter technique described below in
\S\ref{sec:named-template-parameters}, or a default must be computed
for the type.
\subsubsection{Defaults for the Associated Types}
The defaults for the \code{value\_type}, \code{iterator\_\-category},
and \code{difference\_\-type} are straightforward: they are the
respective types from the base iterator. So, for example, the default
for the \code{value\_type} is
\code{std::iterator\_\-traits\-<Base>::\-value\_type}.
Computing the defaults for \code{reference} and \code{pointer} is a
bit more complicated. If a \code{value\_type} is specified by the
user, then \code{value\_type\&} and \code{value\_type*} are used for
the \code{reference} and \code{pointer} type. Otherwise, if the
default was used for \code{value\_type} then the \code{reference} and
\code{pointer} types are taken from the base iterator using
\code{std::iterator\_traits}.
% Dave, is the above right? I'm not sure...
\subsubsection{Named Template Parameters}
\label{sec:named-template-parameters}
The \code{iterator\_adaptor} class has five template parameters that
specify the associated types of the iterator. Each of the parameters
has defaults as described above, so the user may specify zero or more
parameters. One difficulty with {C++} templates is that if a default
is used for a parameter then all the following parameters must also be
default. When there are a large number of parameters this becomes
inconvenient.
A solution to this problem is the idea of named parameters. Instead
of matching arguments to parameters based on order, the assignment of
arguments to parameters is made explicitly by name (so the ordering no
longer matters). The way this works for the \code{iterator\_\-adaptor}
class is that there is a wrapper class for each parameter, for
example:
{\footnotesize
\begin{verbatim}
template <class Value> struct value_type_is {
typedef value_type_tag tag;
typedef Value type;
};
\end{verbatim}
}
Instead of passing the argument for the \code{Value} type directly to
\code{iterator\_\-adaptor} the user passes
\code{value\_type\_is<Value>}. The \code{iterator\_\-adaptor} now has
five arguments for the associated types, each of which could be used
to specify any of the actual parameters. The
\code{iterator\_\-adaptor} must deduce which argument is for which
parameter based on the \code{tag} type inside the wrapper.
First we take all of the parameters and place them in a
lisp-style list, using \code{std::pair} for \code{cons}. Each
parameter wrapper has a key/value pair (the \code{tag} and \code{type}
respectively), so we can treat this as an associative list.
{\footnotesize
\begin{verbatim}
typedef pair<Param1, pair<Param2, pair<Param3, pair<Param4,
pair<Param5, list_end_type> > > > > NamedParamList;
\end{verbatim}
}
\noindent For each parameter we perform a look-up in the associative
list using a template meta-program utility class named
\code{find\_param}.
{\footnotesize
\begin{verbatim}
template <class AssociativeList, class Key>
struct find_param {
typedef ... type;
};
\end{verbatim}
}
\noindent So, for example, to retrieve the argument for the
\code{value\_type} we write the following:
{\footnotesize
\begin{verbatim}
typedef typename find_param<NamedParamList, value_type_tag>::type Value;
\end{verbatim}
}
The result of this look-up will either be the argument specified by the
user, or if there is none, the \code{default\_argument} type. If it is
the default, then a further step is taken to resolve what the default
should be for the parameter. The defaults for some of the parameters
depend on other parameters, so the order in which defaults are
resolved is tailored to respect these dependencies.
\subsection{Core Operators}
@ -732,7 +839,7 @@ appropriate type for the result of an iterator's \code{operator->}:
{
// Is it an input iterator, or something more?
static bool const is_input_iter
= is_convertible<Category*,std::input_iterator_tag*>::value
= is_convertible<Category*, std::input_iterator_tag*>::value
&& !is_convertible<Category*,std::forward_iterator_tag*>::value;
typedef typename type_if<is_input_iter,
@ -815,7 +922,6 @@ in a generic algorithm, you can not use \code{operator[]} for
assignment (on the left hand side) but instead must write \code{*(i +
n) = x}.
Alternatively, it would be nice to return by-reference for some
iterators and by-value for others. However, the current
\code{iterator\_\-traits} does not provide enough information make the
@ -826,11 +932,26 @@ iterator would either return by-value for both \code{operator*} and
in~\cite{siek01:_improved_iter_cat} would solves these problems, but
of course that will take some time to gain acceptance.
\section{Conclusion}
talk about how this approach generalizes to other things, containers,
etc.
Constructing iterators and iterator adaptors is a common task for
modern C++ programming. Despite the conceptual simplicity of most
iterators, implementing {C++} Standard conforming iterators requires
is non-trivial amount of code: some of which is challenging to get
right, and a lot of which is tedious to write. The
\code{iterator\_adaptor} class that we present in this paper solves
this problem by providing a mechanism by which the user provides a
minimal specification (by way of the policies class) for the iterator,
and then the \code{iterator\_\-adaptor} takes care of most of the
implementation details.
Taking a step back, the design approach was to create a canonical
implementation of a concept (iterator) and then delegate the core
implementation issues to a policies class. This approach can be
applied in situations where there is large family of components that
share the same interface. For example, we plan on applying this design
approach to containers and algebraic types.
\bibliographystyle{abbrv}
@ -841,6 +962,6 @@ etc.
% LocalWords: adaptors istream ostream iter MTL InputIterator adaptor const
% LocalWords: RandomAccessIterator dereference interoperate Implementers tmpw
% LocalWords: dereferencing adaptor's lvalues iterator's instantiation typedef
% LocalWords: AdaptableUnaryFunction templated dereferenced lvalue
% LocalWords: AdaptableUnaryFunction templated dereferenced lvalue val param
% LocalWords: parameterization implementers combinatorial InputIterators
% LocalWords: Convertibility