The multi_pass

| The multi_pass |

Backtracking in Spirit requires the use of the following types of iterator: forward, bidirectional, or random access. Because of backtracking, input iterators cannot be used. Therefore, the standard library classes istreambuf_iterator and istream_iterator, that fall under the category of input iterators, cannot be used. Another input iterator that is of interest is one that wraps a lexer, such as LEX.

Input Iterators In general, Spirit is a backtracking parser. This is not an absolute requirement though. In the future, we shall see more deterministic parsers that require no more than 1 character (token) of lookahead. Such parsers allow us to use input iterators such as the istream_iterator as is.

Unfortunately, with an input iterator, there is no way to save an iterator position, and thus input iterators will not work with backtracking in Spirit. One solution to this problem is to simply load all the data to be parsed into a container, such as a vector or deque, and then pass the begin and end of the container to Spirit. This method can be too memory intensive for certain applications, which is why the multi_pass iterator was created.

The multi_pass iterator will convert any input iterator into a forward iterator suitable for use with Spirit. multi_pass will buffer data when needed and will discard the buffer when only one copy of the iterator exists.

A grammar must be designed with care if the multi_pass iterator is used. Any rule that may need to backtrack, such as one that contains an alternative, will cause data to be buffered. The rules that are optimal to use are sequence and repetition. Sequences of the form `a >> b` will not buffer data at all. Any rule that repeats, such as kleene_star (`*a`) or positive such as (`+a`), will only buffer the data for the current repetition.

In typical grammars, ambiguity and therefore lookahead is often localized. In fact, many well designed languages are fully deterministic and require no lookahead at all. Peeking at the first character from the input will immediately determine the alternative branch to take. Yet, even with highly ambiguous grammars, alternatives are often of the form `*(a | b | c | d)`. The input iterator moves on and is never stuck at the beginning. Let's look at a Pascal snippet for example:

        program =
            programHeading >> block >> '.'
            ;

        block =
           *(    labelDeclarationPart
            |    constantDefinitionPart
            |    typeDefinitionPart
            |    variableDeclarationPart
            |    procedureAndFunctionDeclarationPart
            )
            >>   statementPart
            ;

Notice the alternatives inside the Kleene star in the rule block . The rule gobbles the input in a linear manner and throws away the past history with each iteration. As this is fully deterministic LL(1) grammar, each failed alternative only has to peek 1 character (token). The alternative that consumes more than 1 character (token) is definitely a winner. After which, the Kleene star moves on to the next.

Be mindful if you use the free parse functions. All of these make a copy of the iterator passed to them.

Now, after the lecture on the features to be careful with when using multi_pass, you may think that multi_pass is way too restrictive to use. That's not the case. If your grammar is deterministic, you can make use of flush_multi_pass in your grammar to ensure that data is not buffered when unnecessary.

Again, following up the example we started to use in the section on the scanner . Here's an example using the multi_pass: This time around we are extracting our input from the input stream using an istreambuf_iterator.

        #include <boost/spirit/core.hpp>
        #include <boost/spirit/iterator/multi_pass.hpp>

        using namespace boost::spirit;
        using namespace std;

        ifstream in("input_file.txt"); // we get our input from this file

        typedef char char_type;
        typedef multi_pass<istreambuf_iterator<char_type> > iterator_type;

        typedef skip_parser_iteration_policy<space_parser> iter_policy_type;
        typedef scanner_policies<iter_policy_type> scanner_policies_type;
        typedef scanner<iterator_type, scanner_policies_type> scanner_type;

        typedef rule<scanner_type> rule_type;

        iter_policy_type iter_policy(space_p);
        scanner_policies_type policies(iter_policy);
        iterator_type first(
            make_multi_pass(std::istreambuf_iterator<char_type>(in)));

        scanner_type scan(
            first, make_multi_pass(std::istreambuf_iterator<char_type>()),
            policies);

        rule_type   n_list = real_p >> *(',' >> real_p);
        match<>     m = n_list.parse(scan);

## flush_multi_pass

There is a predefined pseudo-parser called flush_multi_pass. When this parser is used with multi_pass, it will call multi_pass::clear_queue(). This will cause any buffered data to be erased. This also will invalidate all other copies of multi_pass and they should not be used. If they are, an boost::illegal_backtracking exception will be thrown.

## multi_pass Policies

multi_pass is a templated policy driven class. The description of multi_pass above is how it was originally implemented (before it used policies), and is the default configuration now. But, multi_pass is capable of much more. Because of the open-ended nature of policies, you can write your own policy to make multi_pass behave in a way that we never before imagined.

The multi_pass class has five template parameters:

- InputT - The type multi_pass uses to acquire it's input. This is typically an input iterator, or functor.
- InputPolicy - A class that defines how multi_pass acquires it's input. The InputPolicy is parameterized by InputT.
- OwnershipPolicy - This policy determines how multi_pass deals with it's shared components.
- CheckingPolicy - This policy determines how checking for invalid iterators is done.
- StoragePolicy - The buffering scheme used by multi_pass is determined and managed by the StoragePolicy.

## Predefined policies

All predefined multi_pass policies are in the namespace boost::spirit::multi_pass_policies.

### Predefined InputPolicy classes

#### input_iterator

This policy directs multi_pass to read from an input iterator of type InputT.

#### lex_input

This policy obtains it's input by calling yylex(), which would typically be provided by a scanner generated by LEX. If you use this policy your code must link against a LEX generated scanner.

#### functor_input

This input policy obtains it's data by calling a functor of type InputT. The functor must meet certain requirements. It must have a typedef called result_type which should be the type returned from operator(). Also, since an input policy needs a way to determine when the end of input has been reached, the functor must contain a static variable named eof which is comparable to a variable of result_type.

### Predefined OwnershipPolicy classes

#### ref_counted

This class uses a reference counting scheme. multi_pass will delete it's shared components when the count reaches zero.

#### first_owner

When this policy is used, the first multi_pass created will be the one that deletes the shared data. Each copy will not take ownership of the shared data. This works well for spirit, since no dynamic allocation of iterators is done. All copies are made on the stack, so the original iterator has the longest lifespan.

### Predefined CheckingPolicy classes

#### no_check

This policy does no checking at all.

#### buf_id_check

buf_id_check keeps around a buffer id, or a buffer age. Every time clear_queue() is called on a multi_pass iterator, it is possible that all other iterators become invalid. When clear_queue() is called, buf_id_check increments the buffer id. When an iterator is dereferenced, this policy checks that the buffer id of the iterator matches the shared buffer id. This policy is most effective when used together with the std_deque StoragePolicy. It should not be used with the fixed_size_queue StoragePolicy, because it will not detect iterator dereferences that are out of range.

#### full_check

This policy has not been implemented yet. When it is, it will keep track of all iterators and make sure that they are all valid.

### Predefined StoragePolicy classes

#### std_deque

This policy keeps all buffered data in a std::deque. All data is stored as long as there is more than one iterator. Once the iterator count goes down to one, and the queue is no longer needed, it is cleared, freeing up memory. The queue can also be forcibly cleared by calling multi_pass::clear_queue().

#### fixed_size_queue<N>

fixed_size_queue keeps a circular buffer that is size N+1 and stores N elements. fixed_size_queue is a template with a std::size_t parameter that specified the queue size. It is your responsibility to ensure that N is big enough for your parser. Whenever the foremost iterator is incremented, the last character of the buffer is automatically erased. Currently there is no way to tell if an iterator is trailing too far behind and has become invalid. No dynamic allocation is done by this policy during normal iterator operation, only on initial construction. The memory usage of this StoragePolicy is set at N+1 bytes, unlike std_deque, which is unbounded.

## Combinations: How to specify your own custom multi_pass

The beauty of policy based designs is that you can mix and match policies to create your own custom class by selecting the policies you want. Here's an example of how to specify a custom multi_pass that wraps an istream_iterator<char>, and is slightly more efficient than the default because it uses the first_owner OwnershipPolicy and the no_check CheckingPolicy:

        typedef multi_pass<
            istream_iterator<char>,
            multi_pass_policies::input_iterator,
            multi_pass_policies::first_owner,
            multi_pass_policies::no_check,
            multi_pass_policies::std_deque
        > first_owner_multi_pass_type;

The default template parameters for multi_pass are: input_iterator InputPolicy, ref_counted OwnershipPolicy, buf_id_check CheckingPolicy and std_deque StoragePolicy. So if you use multi_pass<istream_iterator<char> > you will get those pre-defined behaviors while wrapping an istream_iterator<char>.

There is one other pre-defined class called look_ahead. look_ahead has two template parameters: InputT, the type of the input iterator to wrap, and a std::size_t N, which specifies the size of the buffer to the fixed_size_queue policy. While the default multi_pass configuration is designed for safey, look_ahead is designed for speed. look_ahead is derived from a multi_pass with the following policies: input_iterator InputPolicy, first_owner OwnershipPolicy, no_check CheckingPolicy, and fixed_size_queue<N> StoragePolicy.

### How to write a functor for use with the functor_input InputPolicy

If you want to use the functor_input InputPolicy, you can write your own functor that will supply the input to multi_pass. The functor must satisfy two requirements. It must have a typedef result_type which specifies the return type of operator(). This is standard practice in the STL. Also, it must supply a static variable called eof which is compared against to know whether the input has reached the end. Here is an example:

        class my_functor
        {
        public:

            typedef char result_type;

            my_functor()
            : c('A') {}

            char operator()() const
            {
                if (c == 'M')
                    return eof;
                else
                    return c++;
            }

            static result_type eof;

        private:

            char c;
        };

        my_functor::result_type my_functor::eof = '\0';

        typedef multi_pass<
            my_functor,
            multi_pass_policies::functor_input,
            multi_pass_policies::first_owner,
            multi_pass_policies::no_check,
            multi_pass_policies::std_deque
        > functor_multi_pass_type;

        functor_multi_pass_type first = functor_multi_pass_type(my_functor());
        functor_multi_pass_type last;

### How to write policies for use with multi_pass

#### InputPolicy

An InputPolicy must have the following interface:

        class my_input_policy // your policy name
        {
        public:

        //  class inner will be instantiated with the type given
        //  as the InputT parameter to multi_pass.

            template <typename InputT>
            class inner
            {
            public:

                // these typedefs determine the iterator_traits for multi_pass
                typedef x value_type;
                typedef x difference_type;
                typedef x pointer;
                typedef x reference;

            protected:

                inner();
                inner(InputT x);
                inner(inner const& x);
                // delete or clean up any state
                void destroy();
                // return true if *this and x have the same input
                bool same_input(inner const& x) const;
                void swap(inner& x);

            public:

                // get an instance from the input
                result_type get_input() const;
                // advance the input
                void advance_input();
                // return true if the input is at the end
                bool input_at_eof() const;
            };
        };

Because of the way that multi_pass shares a buffer and input among multiple copies, class inner should keep a pointer to it's input. The copy constructor should simply copy the pointer. destroy() should delete it. same_input should compare the pointers. For more details see the various implementations of InputPolicy classes.

#### OwnershipPolicy

The OwnershipPolicy must have the following interface:

        class my_ownership_policy
        {
        protected:

            my_ownership_policy();
            my_ownership_policy(my_ownership_policy const& x);
            // clone is called when a copy of the iterator is made
            void clone();
            // called when a copy is deleted. Return true to indicate
            // resources should be released
            bool release();
            void swap(my_ownership_policy& x);

        public:
            // returns true if there is only one iterator in existence.
            // std_dequeue StoragePolicy will free it's buffered data if this
            // returns true.
            bool unique() const;
        };

#### CheckingPolicy

The CheckingPolicy must have the following interface:

        class my_check
        {
        protected:

            my_check();
            my_check(my_check const& x);
            void destroy();
            void swap(my_check& x);
            // check should make sure that this iterator is valid
            void check_if_valid() const;
            void clear_queue();
        };

#### StoragePolicy

A StoragePolicy must have the following interface:

        class my_storage_policy
        {
        public:

        // class inner will be instantiated with the value_type from the InputPolicy

            template <typename ValueT>
            class inner
            {
            protected:

                inner();
                inner(inner const& x);
                // will be called from the destructor of the last iterator.
                void destroy();
                void swap(inner& x);
                // This is called when the iterator is dereferenced. It's a template
                // method so we can recover the type of the multi_pass iterator
                // and access it.
                template <typename MultiPassT>
                static ValueT dereference(MultiPassT const& mp);
                // This is called when the iterator is incremented. It's a template
                // method so we can recover the type of the multi_pass iterator
                // and access it.
                template <typename MultiPassT>
                static void increment(MultiPassT& mp);
                void clear_queue();
                // called to determine whether the iterator is an eof iterator
                template <typename MultiPassT>
                static bool is_eof(MultiPassT const& mp);
                // called by operator==
                bool equal_to(inner const& x) const;
                // called by operator<
                bool less_than(inner const& x) const;
            };  // class inner
        };

A StoragePolicy is the trickiest policy to write. You should study and understand the existing StoragePolicy classes before you try and write your own.

Copyright © 2001-2002 Daniel C. Nuffer
Use, modification and distribution is subject to the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)