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## Generic macros for working with data types

Besides the specific macros for working with data types VMD has a number of generic macros for parsing sequences.

### Parsing sequences

In the normal use of Boost PP data is passed as arguments to a macro in discrete units so that each parameter expects a single data type. A typical macro might be:

    #define AMACRO(anumber,atuple,anidentifier) someoutput

where the 'atuple', having the form of ( data1, data2, data3 ), itself may contain different data types of elements.

This is the standard macro design and internally it is the easiest way to pass macro data back and forth. The Boost PP library has a rich set of functionality to deal with all of its high-level data types, and variadic data, with its own simpler functionality, also offers another alternative to representing data.

Occasionally designers of macros, especially for the use of others programmers within a particular library, have expressed the need for a macro parameter to allow a more C/C++ like syntax where a single parameter might mimic a C++ function-call or a C-like type modification syntax, or some other more complicated construct. Something along the lines of:

    areturn afunction ( aparameter1, aparameter2, aparameter3 )

or

    ( type ) data

etc. etc.

In other words, from a syntactical level when designing possible macro input, is it possible to design parameter data to look more like C/C++ when macros are used in a library and still do a certain amount of preprocessor metaprogramming with such mixed token input ?

VMD has functionality which allows more than one type of preprocessing token, excluding an 'empty' token which always refers to some entire input, to be part of a single parameter of input data. These preprocessing tokens as a single parameter are syntactically a consecutive series of data. The single limitation of this consecutive series of data is that each top-level part of the data of this series is of some VMD data type. What this means is that if some input consists of a series of data types it is possible to extract the data for each data type in that series.

In practicality what this means is that, given the examples just above, if 'areturn', 'afunction', and 'data' are identifiers it would be possible to parse either of the two inputs above so that one could identify the different data types involved and do preprocessor metaprogramming based on those results.

##### Sequence definition

I will be calling such input data, which consists of all top-level data types in a series, by the term of a 'sequence'. Each separate data type in the sequence is called an 'element'. In this definition of a 'sequence' we can have 0 or more elements, so that a sequence is a general name for any VMD input. A sequence is therefore any input VMD can parse, whether it is emptiness, a single element, or more than one element in a series. Therefore when we speak of VMD macros parsing input data we are really speaking of VMD macros parsing a sequence. A sequence can therefore also be part of a Boost PP composite data type, or variadic data, and VMD can still parse such an embedded sequence if asked to do so.

##### Sequence parsing

Parsing a sequence means that VMD can step through each element of a sequence sequentially, determine the type and data of each element, then move on to the next element. Parsing is sequential and can only be done in a forward direction, but it can be done any number of times. In C++ iterator terms parsing of a sequence is a forward iterator.

Working with a sequence is equivalent to using VMD macros 'generically'.

Before I give an explanation of how to use a sequence using VMD generic functionality I would like to make two points:

- The possibility of working with a sequence which contains more than one data type can be easily abused. In general keeping things simple is usually better than making things overly complicated when it comes to the syntactical side of things in a computer language. A macro parameter syntactical possibility has to be understandable to be used.
- Using VMD to parse the individual data types of a sequence takes more preprocessing time than functionality offered with Boost PP data types, because it is based on forward access through each top-level type of the sequence.

The one constraint in a sequence is that the top-level must consist of VMD data types, in other words preprocessor tokens which VMD understands. By top-level it is meant that a Boost PP composite data may have elements which VMD cannot parse but as long as the input consists of the composite data types and not the inner unparsable elements, VMD can parse the input. Therefore if preprocessor data is one of the examples above, you will be successful in using VMD. However if your preprocessor data takes the form of:

    &name identifier ( param )

or

    identifier "string literal"

or

    identifier + number

or

    identifier += 4.3

etc. etc.

you will not be able to parse the data using VMD since '&', "string literal", '+', '+=', and "4.3" are preprocessor tokens which are not VMD top-level data types and therefore VMD cannot handle them at the parsing level. You can still of course pass such data as preprocessing input to macros but you cannot use VMD to recognize the parts of such data.

This is similar to the fact that VMD cannot tell you what type preprocessor data is as a whole, using any of the VMD identifying macros already discussed, if the type is not one that VMD can handle.

On the other hand you can still use VMD to parse such tokens in the input if you use Boost PP data types as top-level data types to do so. Such as:

    ( &name ) identifier ( param )

or

    identifier ( "string literal" )

or

    identifier ( + ) number

or

    identifier ( += ) 4 ( . ) 3

The succeeding topics explain the VMD functionality for parsing a sequence for each individual VMD data type in that sequence.

##### Sequence types

A VMD sequence can be seen as one of either three general types:

1. An empty sequence
2. A single element sequence
3. A multi-element sequence

An empty sequence is merely input that is empty, what VMD calls "emptiness". Use the previously explained BOOST_VMD_IS_EMPTY macro to test for an empty sequence.

    #include <boost/vmd/is_empty.hpp>

    #define AN_EMPTY_SEQUENCE

    BOOST_VMD_IS_EMPTY(AN_EMPTY_SEQUENCE) will return 1

The type of an empty sequence is BOOST_VMD_TYPE_EMPTY.

A single element sequence is a single VMD data type. This is what we have been previously discussing as data which VMD can parse in this documentation with our identifying macros. You can use the BOOST_VMD_IS_UNARY macro to test for a single element sequence.

    #include <boost/vmd/is_unary.hpp>

    #define A_SINGLE_ELEMENT_SEQUENCE (1,2)

    BOOST_VMD_IS_UNARY(A_SINGLE_ELEMENT_SEQUENCE) will return 1

The type of a single element sequence is the type of the individual data type. In our example above the type of A_SINGLE_ELEMENT_SEQUENCE is BOOST_VMD_TYPE_TUPLE.

A multi-element sequence consists of more than one data type. This is the "new" type which VMD can parse. You can use the BOOST_VMD_IS_MULTI macro to test for a multi-element sequence.

    #define A_MULTI_ELEMENT_SEQUENCE (1,2) (1)(2) 45

The A_MULTI_ELEMENT_SEQUENCE consists of a tuple followed by a seq followed by a number.

    #include <boost/vmd/is_multi.hpp>

    BOOST_VMD_IS_MULTI(A_MULTI_ELEMENT_SEQUENCE) will return 1

The type of a multi-element sequence is always BOOST_VMD_TYPE_SEQUENCE.

The type of a sequence can be obtained generically with the BOOST_VMD_GET_TYPE macro. We will be explaining this further in the documentation.

##### Sequence size

The size of any sequence can be accessed using the BOOST_VMD_SIZE macro. For an empty sequence the size is always 0. For a single element sequence the size is always 1. For a multi-element sequence the size is the number of individual top-level data types in the sequence.

    #include <boost/vmd/size.hpp>

    BOOST_VMD_SIZE(AN_EMPTY_SEQUENCE) will return 0
    BOOST_VMD_SIZE(A_SINGLE_ELEMENT_SEQUENCE) will return 1
    BOOST_VMD_SIZE(A_MULTI_ELEMENT_SEQUENCE) will return 3

##### Using VMD to parse sequence input

For a VMD sequence essentially two ways of parsing into individual data types are offered by the VMD library:

1. The sequence can be converted to any of the Boost PP data types, or to variadic data, where each individual data type in the sequence becomes a separate element of the particular composite data type chosen. The conversion to a particular Boost PP data type or variadic data is slow, because it is based on forward access through each top-level type of the sequence, but afterwards accessing any individual element is as fast as accessing any element in the Boost PP data type or among variadic data.
2. The sequence can be accessed directly through its individual elements. This is slower than accessing an element of a Boost PP data type or variadic data but offers conceptual access to the original sequence as a series of elements.

These two techniques will be discussed in succeeding topics.

#### Converting sequences

The easiest way to work with a sequence is to convert it to a Boost PP data type. Likewise you can also convert a sequence to variadic data even though the Boost PP data types have much greater functionality than variadic data in Boost PP.

To convert a sequence to a Boost PP data type or variadic data the macros to be used are:

- BOOST_VMD_TO_ARRAY(sequence) to convert the sequence to an array
- BOOST_VMD_TO_LIST(sequence) to convert the sequence to a list
- BOOST_VMD_TO_SEQ(sequence) to convert the sequence to a seq
- BOOST_VMD_TO_TUPLE(sequence) to convert the sequence to a tuple
- BOOST_VMD_ENUM(sequence) to convert the sequence to variadic data

After the conversion the elements of a sequence become the elements of the corresponding composite data type.

Once the elements of the sequence have been converted to the elements of the composite data type the full power of that composite data type can be used to process each element. Furthermore the programmer can use VMD to discover the type of an individual element for further processing.

For single element sequences the result is always a single element composite data type. For multi-element sequences the result is always a composite data type of more than one element.

For a sequence that is empty the result is emptiness when converting to a seq, tuple, or variadic data; the result is an empty array or list when converting to each of those composite data types respectively.

    #include <boost/vmd/enum.hpp>
    #include <boost/vmd/to_array.hpp>
    #include <boost/vmd/to_list.hpp>
    #include <boost/vmd/to_seq.hpp>
    #include <boost/vmd/to_tuple.hpp>

    #define BOOST_VMD_REGISTER_ANID (ANID)

    #define SEQUENCE_EMPTY
    #define SEQUENCE_SINGLE 35
    #define SEQUENCE_SINGLE_2 ANID
    #define SEQUENCE_MULTI (0,1) (2)(3)(4)
    #define SEQUENCE_MULTI_2 BOOST_VMD_TYPE_SEQ (2,(5,6))

    BOOST_VMD_TO_ARRAY(SEQUENCE_EMPTY) will return an empty array '(0,())'
    BOOST_VMD_TO_LIST(SEQUENCE_SINGLE) will return a one-element list '(35,BOOST_PP_NIL)'
    BOOST_VMD_TO_SEQ(SEQUENCE_SINGLE_2) will return a one-element seq '(ANID)'
    BOOST_VMD_TO_TUPLE(SEQUENCE_MULTI) will return a multi-element tuple '((0,1),(2)(3)(4))'
    BOOST_VMD_ENUM(SEQUENCE_MULTI_2) will return multi-element variadic data 'BOOST_VMD_TYPE_SEQ,(2,(5,6))'

###### Usage

You can use the general header file:

    #include <boost/vmd/vmd.hpp>

or you can use individual header files for each of these macros. The individual header files are:

    #include <boost/vmd/to_array.hpp> //  for the BOOST_VMD_TO_ARRAY macro
    #include <boost/vmd/to_list.hpp> //  for the BOOST_VMD_TO_LIST macro
    #include <boost/vmd/to_seq.hpp> //  for the BOOST_VMD_TO_SEQ macro
    #include <boost/vmd/to_tuple.hpp> // for the BOOST_VMD_TO_TUPLE macro.
    #include <boost/vmd/enum.hpp> // for the BOOST_VMD_ENUM macro.

#### Accessing a sequence element

It is possible to access an individual element of a sequence. The macro to do this is called BOOST_VMD_ELEM. The macro takes two required parameters. The required parameters are the element number to access and the sequence, in that order. The element number is a 0-based number and its maximum value should be one less than the size of the sequence.

The BOOST_VMD_ELEM macro returns the actual sequence element. If the first required parameter is greater or equal to the size of the sequence the macro returns emptiness. Because of this using BOOST_VMD_ELEM on an empty sequence, whose size is 0, always returns emptiness.

    #include <boost/vmd/elem.hpp>

    #define BOOST_VMD_REGISTER_ANAME (ANAME)
    #define A_SEQUENCE (1,2,3) 46 (list_data1,(list_data2,BOOST_PP_NIL)) BOOST_VMD_TYPE_SEQ ANAME
    #define AN_EMPTY_SEQUENCE

    BOOST_VMD_ELEM(0,A_SEQUENCE) will return (1,2,3)
    BOOST_VMD_ELEM(1,A_SEQUENCE) will return 46
    BOOST_VMD_ELEM(2,A_SEQUENCE) will return (list_data1,(list_data2,BOOST_PP_NIL))
    BOOST_VMD_ELEM(3,A_SEQUENCE) will return BOOST_VMD_TYPE_SEQ
    BOOST_VMD_ELEM(4,A_SEQUENCE) will return ANAME

    BOOST_VMD_ELEM(5,A_SEQUENCE) will return emptiness
    BOOST_VMD_ELEM(0,AN_EMPTY_SEQUENCE) will return emptiness

Accessing an element of a sequence directly is slower than accessing an element of a Boost PP data type or even variadic data, since each access has to directly cycle through each element of the sequence to get to the one being accessed. The process of sequentially parsing each element again each time is slower than accessing a Boost PP data type element.

###### Usage

You can use the general header file:

    #include <boost/vmd/vmd.hpp>

or you can use the individual header file:

    #include <boost/vmd/elem.hpp>