Boost C++ Libraries of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards


Generic macros for working with data types

Parsing sequences
Converting sequences
Accessing a sequence element
Getting the type of data
Testing for equality and inequality
Summing up the generic macros

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

In the normal use of the Boost Preprocessor library 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 )


( 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 which can be space separated as a single argument. The need to space separate the preprocessor data occurs when consecutive tokens are VMD identifiers. The 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. Furthermore if you use generic macros to parse a sequence, any top-level VMD type which is an identifier must be a specific identifier, meaning it must be an identifier that has been registered.

What the VMD sequence functionality 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 specific 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 generically 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.

A sequence that consists of 0 elements is an empty sequence, and we have already seen that emptiness can be tested with the specific macro BOOST_VMD_IS_EMPTY. A sequence which consists of a single element can be tested using the various identifying macros which test for "specific" VMD data types. But whether a sequence consists of 0 elemants, one element, or more than one element, we can also use "generic" macros, which will be further described, to parse a sequence.

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 models a forward iterator.

Working with a sequence is equivalent to using VMD macros 'generically', whereas previously we had documented using VMD macros "specifically" with identifying macros that tested each type of VMD data.

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 constraints in a sequence are that the top-level must consist of VMD data types, ie. preprocessor tokens which VMD understands, and that all top-level VMD identifiers must be specific identifiers when the sequence is parsed generically. 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 )


identifier "string literal"


identifier + number


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 )


identifier ( "string literal" )


identifier ( + ) number


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 identifying macro to test if the sequence is an empty sequence.

#include <boost/vmd/is_empty.hpp>

#define A_NON_EMPTY_SEQUENCE 23 ( some_identifier )


The type of an empty sequence is BOOST_VMD_TYPE_EMPTY. You can test for an empty sequence when a top-level identifier is not a specific identifier and the macro will correctly return 0. This is because BOOST_VMD_IS_EMPTY, which has been previously documented, is really an identifying macro which also works to parse any sequence and determine emptiness or not.

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 generic macro to test if the sequence is a single element sequence.

#include <boost/vmd/is_unary.hpp>

#define NOT_A_SINGLE_ELEMENT_SEQUENCE (1,2) (45)(name)


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>


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

The type of any sequence can be obtained 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>

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

Distinguishing consecutive seqs and tuples in a sequence

As has previously been mentioned a single element tuple is also a one element seq, so parsing a sequence which has seqs and tuples in them might be a problem as far as identify each element of the sequence. In a multi-element sequence if the data consists of a mixture of seqs and tuples consecutively we need to distinguish how VMD parses the data. The rule is that VMD always parses a single element tuple as a tuple unless it is followed by one or more single element tuples, in which case it is a seq. Here are some examples showing how the rule is applied.

#define ST_DATA (somedata)(element1,element2)

VMD parses the above data as 2 consecutive tuples. The first tuple is the single element tuple '(somedata)' and the second tuple is the multi element tuple '(element1,element2)'.

#define ST_DATA (element1,element2)(somedata)

VMD parses the above data as 2 consecutive tuples. The first tuple is the multi element tuple '(element1,element2)' and the second tuple is the single element tuple '(somedata)'.

#define ST_DATA (somedata)(some_other_data)(element1,element2)

VMD parses the above data as a seq followed by a tuple. The seq is '(somedata) (some_other_data)' and the tuple is '(element1,element2)'.

Using VMD to parse sequence input

For a VMD sequence 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.

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 SEQUENCE_MULTI (0,1) (2)(3)(4)

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))'

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.

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 A_SEQUENCE (1,2,3) 46 (list_data1,(list_data2,BOOST_PP_NIL)) BOOST_VMD_TYPE_SEQ ANAME

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(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.


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>