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tmpw2001-paper/iter-adaptor.tex
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@ -205,8 +205,17 @@ in which a variety of adaptor types are enumerated.
\section{The Boost Iterator Adaptor Library}
% Do we need a section body?
In this section, we present the Boost Iterator Adaptor Library.
% We need an intro here (do we?), but I don't think the text below
% fits in. Suggestions?
% For an iterator to be usable with the Standard algorithms (and other
% generic algorithms), it must fulfill the Standard requirements for an
% iterator type, which range from the few requirements of an
% \stlconcept{InputIterator} to the many requirements of a
% \stlconcept{RandomAccessIterator}. Implementing an iterator class that
% meets these requirements is a tedious and error-prone task despite the
% fact that most iterators are conceptually simple. The Boost Iterator
% Adaptor Library addresses these problems.
\subsection{Overall Design}
@ -222,7 +231,7 @@ the use of inheritance. We have followed the lead of the standard
library, which provides a \code{base()} function to access the
underlying iterator object of a \reverseiterator\ adaptor.}
\subsubsection{Core Elements of the Concept}
\subsection{Core Elements of the Concept}
The first step in designing such a generalized Model is to identify
the core elements of its interface. We have identified the following
@ -247,16 +256,16 @@ these: as traditional and also as \emph{named} template parameters
(described below), and uses a system of smart defaults which in most
cases reduces the number of these types that must be specified.
\subsubsection{From Building Models to Building Adaptors}
\subsection{From Building Models to Building Adaptors}
A generalized iterator is useful, but a generalized iterator
\emph{adaptor} would be even more useful. One could then build
specialized adaptors to generate new families of iterator instances
based on existing iterators. In the Boost Iterator Adaptor Library,
the role of adaptor is also played by the \iteratoradaptor\ class
template. The behaviors of \iteratoradaptor{} instances are supplied
through a Policies class~\cite{alexandrescu01:_modern_cpp_design}
which allows allows users to specialize adaptation. They go beyond
the \iteratoradaptor\ class template plays the roles of both Adaptor
and Model. The behaviors of \iteratoradaptor{} instances are supplied
through a policies class~\cite{alexandrescu01:_modern_cpp_design}
which allows allows users to specialize adaptation. Users go beyond
generating new iterator types to easily generating new iterator
adaptor families.
@ -277,8 +286,16 @@ show how this adaptor is implemented below.
Iterator Adaptor except that when dereferenced it returns by-reference
instead of by-value.
\item Filter Iterator Adaptor, which view of an iterator range in
which some elements of the range are skipped over.
\item Filter Iterator Adaptor, which provides a view of an iterator
range in which some elements of the underlying range are skipped.
\item Counting Iterator Adaptor, which adapts any incrementable type
(e.g. integers, iterators) so that incrementing/decrementing the
adapted iterator and dereferencing it produces successive values of
the \code{Base} type.
\item Function Output Iterator Adaptor, which makes it easier to
create custom output iterators.
\end{itemize}
@ -288,222 +305,6 @@ permutation to a \stlconcept{RandomAccessIterator}, and a strided
adaptor, which adapts a \stlconcept{RandomAccessIterator} by
multiplying its unit of motion by a constant factor.
% In addition, large number of iterator adaptors are now in use:
% iterators that that conforms to the Conceptual
% requirements of its iterator category
% For an iterator to be usable with the Standard algorithms (and other
% generic algorithms in third-party libraries), it must fulfill the
% Standard requirements for an iterator type, which range from the few
% requirements of an \stlconcept{InputIterator} to the many requirements
% of a \stlconcept{RandomAccessIterator}. Implementing an iterator class
% that meets these requirements is a tedious and error-prone task
% despite the fact that most iterators are conceptually simple.
\subsection{Redundant Operators}
Perhaps the most obvious reason that implementing an iterator can
be tedious is that there are lots of redundant operators. That is,
there are many operators that can be trivially defined in terms of
other operators. For example, the postfix \code{operator++(int)} is often best
implemented in terms of prefix \code{operator++()} as shown below.
{\footnotesize
\begin{verbatim}
class iter {
// ...
iter& operator++() { /* ... */ return *this; }
iter operator++(int) { iter tmp(*this); ++*this; return tmp; }
};
\end{verbatim}
}
For a full \stlconcept{RandomAccessIterator}, there are a total of 17
operators. 7 of the operators are fundamental while the other 10 are
redundant.
% 7 core operations
% 10 derived operations
% \code{operator->}
% \code{operator[]}
% \code{operator++(int)},
% \code{operator--(int)},
% \code{operator-=},
% \code{operator+},
% \code{operator!=},
% \code{operator>},
% \code{operator<=},
% \code{operator>=}
\subsection{Delegation of Operators and Type Definitions}
An iterator adaptor used to adapt an underlying iterator type often
changes the meaning of one or two operators while leaving the rest of
the operators defined in the same way as in the base iterator. This
is typically implemented with delegating functions.\footnote{Although
one might normally consider parametrized inheritance for cases where
many functions must be forwarded, that is not possible for a
generalized iterator adaptor because the underlying type may be a
pointer} The following example shows an excerpt from an
\code{indirect\_iterator} adaptor, which takes an iterator over
pointers or smart-pointers and creates an iterator over the things
pointed to. The \code{operator*} and \code{operator->} are changed to
dereference twice but all the other operators stay the same. Writing
all of the delegating functions for the \code{indirect\_iterator} is a
tedious job.
{\footnotesize
\begin{verbatim}
template <class Base> class indirect_iterator {
public:
// Adapt the meaning of dereference
reference operator*() const {
return **iter; // dereference twice
}
// a ``redundant'' operator
pointer operator->() const { return &this->operator*(); } % dwa -- changed for clarity. I don't think the review comment was correct.
// Delegating the implementation to the underlying iterator.
indirect_iterator& operator++() { ++iter; return *this; }
indirect_iterator& operator--() { --iter; return *this; }
// delegate for all the other operators...
private:
Base iter;
};
\end{verbatim}
}
% I think it would be better to use reverse_iterator as an example
% here, because it delegates more types than indirect_iterator does.
% But reverse_iterator doesn't delegate many operators... it changes
% the meaning of all of them. I wonder if there's a good example
% for both operators and typedefs.
A standard-conforming iterator must either come with a specialization
of \code{std::iterator\_\-traits<>} or it must define five nested
types: \valuetype{}, \code{reference}, \code{pointer},
\differencetype{}, and \iteratorcategory{}. In the
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 Implementation Complexities}
In addition to the tedious aspects of iterator implementation, there
are some complexities that trip up even the most experienced of
programmers. Ideally, an iterator implementer should not have to worry
about these details.
\subsubsection{Constant/Mutable Iterator Interactions}
\label{sec:constant-mutable-iterations}
Iterators over containers and other sequences of stored objects
usually come in pairs: a constant iterator type and a mutable iterator
type. It is desirable to allow the constant and mutable iterators to
interoperate through comparison and
subtraction. For example, suppose
that you are implementing a container type \code{C}. Then you ought to
define the following four versions of \code{operator==}, along with
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);
bool operator==(const C::const_iterator& x, const C::iterator& y);
bool operator==(const C::iterator& x, const C::const_iterator& y);
bool operator==(const C::const_iterator& x, const C::const_iterator& y);
\end{verbatim}
}
Implementers often forget to define the operators for constant/mutable
iterator interaction. In addition, iterator adaptors applied to these
kinds of iterators should propagate the ability to interact. For
example, a reverse iterator adaptor applied to \code{C::iterator} and
\code{C::const\_iterator} should result in mutable and constant reverse iterator types that
have the same ability to interact as the \code{Base} iterators do.
\subsubsection{Constant/Mutable Iterator Code Duplication}
% Is this section really worthwhile? It was confusing to read until I
% finally figured out what you were driving at. Who makes this conceptual mistake?
% It may not be worthwhile... though I've seen other grad students make
% this mistake, and the TMPW2000 VTL paper contains this mistake.
The implementations of the constant and mutable versions of an
iterator typically differ only in their \code{reference} and
\code{pointer} types. Therefore it is desirable to merge the two
iterator classes into a single class template with the
\code{reference} and \code{pointer} types as template parameters.
Some care must be taken when merging the constant and mutable
iterators. One common mistake is that the programmer will confuse the
ideas of a \code{const} iterator object and a \emph{constant
iterator}. Such a misunderstanding can, for example, lead to an
iterator class template that has two versions of \code{operator*}, one
that is a \code{const} member function and one that is not.
{\footnotesize
\begin{verbatim}
// this is a mistake
reference operator*();
const_reference operator*() const;
\end{verbatim}
}
There should be only one \code{operator*} that returns the
\code{reference} type and the member function should be \code{const} since
dereferencing an iterator does not change the state of the iterator
object itself (unlike \code{operator++}).
{\footnotesize
\begin{verbatim}
// this is right
reference operator*() const;
\end{verbatim}
}
\subsubsection{Input Iterators and \code{operator->}}
\label{sec:operator-arrow}
When creating an iterator adaptor that produces an
\stlconcept{InputIterator} some extra care must be taken in the
implementation of \code{operator->}. Remember that an input iterator
need not iterate over stored objects: it can manufacture new objects
when it is dereferenced as is the case for
\code{std::istream\_iterator}. If the iterator's \valuetype\ is
of class type, we need to support \code{operator->}. Since the result
of using \code{operator->} must produce a true pointer even when
dereferencing the iterator does not yield a true reference type, we
need a \code{const} lvalue to which a pointer can be formed. In
\S\ref{sec:operator-arrow} we show how this can be accomplished.
\subsubsection{The Return Type of \code{operator[]} for Adaptors}
\label{sec:operator-bracket}
The \Cpp\ Standard specifies that the return type of \code{operator[]}
of a random access iterator must be ``convertible to \code{T}''. This
is a rather lenient requirement since \code{operator*} is required to
return the exact type \code{T\&}, and one might think that
\code{operator[]} and \code{operator*} should be same in this respect.
The \Cpp\ Standards Committee is currently debating as to whether the
random access iterator requirements should be changed.
To complicate the matter, returning \code{T\&} from \code{operator[]}
causes a run-time error in a certain class of situations. We will
discuss this in detail in \S\ref{sec:op-bracket-impl}.
% Automatic implementation of redundant operators
% Default delegation to adapted iterator
% complexities:
% const-non const interaction
% const/mutable iterator distinction
% input iterator \code{operator->}
\section{The Boost \iteratoradaptor\ Class Template}
The \iteratoradaptor\ class template simplifies the creation
@ -511,11 +312,11 @@ of iterators by automating the implementation of redundant operators
and delegating functions and by taking care of the complex details of
iterator implementation.
The central design feature of \iteratoradaptor\ is the
The central design feature of \iteratoradaptor\ is
parameterization by an iterator policies class. The policies class is
the primary communication mechanism between the iterator implementer
and the \iteratoradaptor{}; it specifies how the new iterator
type is different from the \code{Base} type. Unlike the policy classes
type is behaves. Unlike the policy classes
in~\cite{alexandrescu01:_modern_cpp_design}, we group several policies
into a single class as this proved more convenient for iterator
implementation.
@ -782,6 +583,255 @@ negate the numbers over which it iterates.
\end{verbatim}
}
\section{The Policy Adaptor Design Pattern}
The Iterator Adaptor Library illustrates how a generalized Model
(\iteratoradaptor{}) of a Concept family (iterators) combined with
default policy delegation allows users to easily build new Models and
behavioral adaptors for existing Models. We can capture this strategy
in the Policy Adaptor design pattern:\footnote{This is not quite the
same as the Policy Class pattern which has been discussed previously
in the literature~\cite{alexandrescu01:_modern_cpp_design}. The
construction of an adaptor which can easily transforms existing Models
into new ones is the key difference}\begin{itemize}
% Jeremy, can we get a numbered list here?
\item First, identify the core elements of the Adaptor Concept's
public interface. In our case, the Adaptor Concept is Iterator.
\item then, encapsulate core elements of the Concept family in a
Policies Concept.
\item Write a default policies class which delegates behavior to the
public interface of the Adaptor Concept. This is the mechanism that
supplies default adaptation behavior.
\item build an Adaptor class template parameterized on Policies. The
Adaptor should be a generalized model of the Adaptor Concept,
providing the public interface, but delegating functionality to the
policies class.
\item Store a member of the Policies parameter in the Adaptor
template so that users can store additional data while taking
advantage of default behavior delegation.
\end{itemize}
While the This pattern has not yet been widely tested for
applicability, we believe it will be useful for modelling any Concept
which varies along several axes and contains significant
redundancy.
% In addition, large number of iterator adaptors are now in use:
% iterators that that conforms to the Conceptual
% requirements of its iterator category
\section{The Iterator Implementation Generator}
\subsection{Redundant Operators}
Perhaps the most obvious reason that implementing an iterator can
be tedious is that there are lots of redundant operators. That is,
there are many operators that can be trivially defined in terms of
other operators. For example, the postfix \code{operator++(int)} is often best
implemented in terms of prefix \code{operator++()} as shown below.
{\footnotesize
\begin{verbatim}
class iter {
// ...
iter& operator++() { /* ... */ return *this; }
iter operator++(int) { iter tmp(*this); ++*this; return tmp; }
};
\end{verbatim}
}
For a full \stlconcept{RandomAccessIterator}, there are a total of 17
operators. 7 of the operators are fundamental while the other 10 are
redundant.
% 7 core operations
% 10 derived operations
% \code{operator->}
% \code{operator[]}
% \code{operator++(int)},
% \code{operator--(int)},
% \code{operator-=},
% \code{operator+},
% \code{operator!=},
% \code{operator>},
% \code{operator<=},
% \code{operator>=}
\subsection{Delegation of Operators and Type Definitions}
An iterator adaptor used to adapt an underlying iterator type often
changes the meaning of one or two operators while leaving the rest of
the operators defined in the same way as in the base iterator. This
is typically implemented with delegating functions.\footnote{Although
one might normally consider parametrized inheritance for cases where
many functions must be forwarded, that is not possible for a
generalized iterator adaptor because the underlying type may be a
pointer} The following example shows an excerpt from an
\code{indirect\_iterator} adaptor, which takes an iterator over
pointers or smart-pointers and creates an iterator over the things
pointed to. The \code{operator*} and \code{operator->} are changed to
dereference twice but all the other operators stay the same. Writing
all of the delegating functions for the \code{indirect\_iterator} is a
tedious job.
{\footnotesize
\begin{verbatim}
template <class Base> class indirect_iterator {
public:
// Adapt the meaning of dereference
reference operator*() const {
return **iter; // dereference twice
}
// a ``redundant'' operator
pointer operator->() const { return &this->operator*(); } % dwa -- changed for clarity. I don't think the review comment was correct.
// Delegating the implementation to the underlying iterator.
indirect_iterator& operator++() { ++iter; return *this; }
indirect_iterator& operator--() { --iter; return *this; }
// delegate for all the other operators...
private:
Base iter;
};
\end{verbatim}
}
% I think it would be better to use reverse_iterator as an example
% here, because it delegates more types than indirect_iterator does.
% But reverse_iterator doesn't delegate many operators... it changes
% the meaning of all of them. I wonder if there's a good example
% for both operators and typedefs.
A standard-conforming iterator must either come with a specialization
of \code{std::iterator\_\-traits<>} or it must define five nested
types: \valuetype{}, \code{reference}, \code{pointer},
\differencetype{}, and \iteratorcategory{}. In the
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 Implementation Complexities}
In addition to the tedious aspects of iterator implementation, there
are some complexities that trip up even the most experienced of
programmers. Ideally, an iterator implementer should not have to worry
about these details.
\subsubsection{Constant/Mutable Iterator Interactions}
\label{sec:constant-mutable-iterations}
Iterators over containers and other sequences of stored objects
usually come in pairs: a constant iterator type and a mutable iterator
type. It is desirable to allow the constant and mutable iterators to
interoperate through comparison and
subtraction. For example, suppose
that you are implementing a container type \code{C}. Then you ought to
define the following four versions of \code{operator==}, along with
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);
bool operator==(const C::const_iterator& x, const C::iterator& y);
bool operator==(const C::iterator& x, const C::const_iterator& y);
bool operator==(const C::const_iterator& x, const C::const_iterator& y);
\end{verbatim}
}
Implementers often forget to define the operators for constant/mutable
iterator interaction. In addition, iterator adaptors applied to these
kinds of iterators should propagate the ability to interact. For
example, a reverse iterator adaptor applied to \code{C::iterator} and
\code{C::const\_iterator} should result in mutable and constant reverse iterator types that
have the same ability to interact as the \code{Base} iterators do.
\subsubsection{Constant/Mutable Iterator Code Duplication}
% Is this section really worthwhile? It was confusing to read until I
% finally figured out what you were driving at. Who makes this conceptual mistake?
% It may not be worthwhile... though I've seen other grad students make
% this mistake, and the TMPW2000 VTL paper contains this mistake.
The implementations of the constant and mutable versions of an
iterator typically differ only in their \code{reference} and
\code{pointer} types. Therefore it is desirable to merge the two
iterator classes into a single class template with the
\code{reference} and \code{pointer} types as template parameters.
Some care must be taken when merging the constant and mutable
iterators. One common mistake is that the programmer will confuse the
ideas of a \code{const} iterator object and a \emph{constant
iterator}. Such a misunderstanding can, for example, lead to an
iterator class template that has two versions of \code{operator*}, one
that is a \code{const} member function and one that is not.
{\footnotesize
\begin{verbatim}
// this is a mistake
reference operator*();
const_reference operator*() const;
\end{verbatim}
}
There should be only one \code{operator*} that returns the
\code{reference} type and the member function should be \code{const} since
dereferencing an iterator does not change the state of the iterator
object itself (unlike \code{operator++}).
{\footnotesize
\begin{verbatim}
// this is right
reference operator*() const;
\end{verbatim}
}
\subsubsection{Input Iterators and \code{operator->}}
\label{sec:operator-arrow}
When creating an iterator adaptor that produces an
\stlconcept{InputIterator} some extra care must be taken in the
implementation of \code{operator->}. Remember that an input iterator
need not iterate over stored objects: it can manufacture new objects
when it is dereferenced as is the case for
\code{std::istream\_iterator}. If the iterator's \valuetype\ is
of class type, we need to support \code{operator->}. Since the result
of using \code{operator->} must produce a true pointer even when
dereferencing the iterator does not yield a true reference type, we
need a \code{const} lvalue to which a pointer can be formed. In
\S\ref{sec:operator-arrow} we show how this can be accomplished.
\subsubsection{The Return Type of \code{operator[]} for Adaptors}
\label{sec:operator-bracket}
The \Cpp\ Standard specifies that the return type of \code{operator[]}
of a random access iterator must be ``convertible to \code{T}''. This
is a rather lenient requirement since \code{operator*} is required to
return the exact type \code{T\&}, and one might think that
\code{operator[]} and \code{operator*} should be same in this respect.
The \Cpp\ Standards Committee is currently debating as to whether the
random access iterator requirements should be changed.
To complicate the matter, returning \code{T\&} from \code{operator[]}
causes a run-time error in a certain class of situations. We will
discuss this in detail in \S\ref{sec:op-bracket-impl}.
% Automatic implementation of redundant operators
% Default delegation to adapted iterator
% complexities:
% const-non const interaction
% const/mutable iterator distinction
% input iterator \code{operator->}
\section{The Implementation of \iteratoradaptor{}}