There are macros for the Boost PP data types that can parse the particular data type at the beginning of a v-sequence. Unlike the equivalent functionality for v-identifiers and v-numbers the only parameter that is allowed is the v-sequence itself.

The macros are:

Each of these macros expand to a tuple of two elements. If the particular data type is found at the beginning of the v-sequence the first tuple element is that data type and the second tuple element is the rest of the input v-sequence. If the particular data type is not found at the beginning of the input v-sequence of both tuple elements is empty.

Usage

In order to use any of the macros you can include the general header:

#include <boost/vmd/vmd.hpp>

or include the appropriate specific header:

#include <boost/vmd/array.hpp> // BOOST_VMD_ARRAY
#include <boost/vmd/list.hpp> // BOOST_VMD_LIST
#include <boost/vmd/seq.hpp> // BOOST_VMD_SEQ
#include <boost/vmd/tuple.hpp> // BOOST_VMD_TUPLE

Data types and tuples

An array and a non-empty list are also tuples. A seq is a series of single element tuples. Therefore if a v-sequence begins with an array, non-empty list, or seq, and you use BOOST_VMD_TUPLE passing that v-sequence, you will successfully find the beginning tuple. In the case of an array or a non-empty list what is returned will be exactly the same as a tuple. In the case of a seq what is returned will only be the same if the seq is a single one-element tuple.

As an example:

#define INPUT_D1 (3,(a,b,c)) data

BOOST_VMD_ARRAY(INPUT_D1)
BOOST_VMD_TUPLE(INPUT_D1)

expands to ((3,(a,b,c)),data)

#define INPUT_D2 (a,(b,(c,BOOST_PP_NIL))) 204

BOOST_VMD_LIST(INPUT_D2)
BOOST_VMD_TUPLE(INPUT_D2)

expands to ((a,(b,(c,BOOST_PP_NIL))),204)

#define INPUT_D3 (a)(b)(c) anything

BOOST_VMD_SEQ(INPUT_D3)

expands to ((a)(b)(c),anything)

BOOST_VMD_TUPLE(INPUT_D3)

expands to ((a),(b)(c) anything)

On the other hand not every tuple is a non-empty list or array while only a single element tuple is also a seq.

As an example:

#define INPUT_D4 (z,(a,b,c)) data

BOOST_VMD_ARRAY(INPUT_D4)

expands to (,) since the beginning tuple is not an array

BOOST_VMD_TUPLE(INPUT_D4)

expands to ((z,(a,b,c)),data) since there is a beginning tuple

#define INPUT_D5 (a,(b,(c))) 204

BOOST_VMD_LIST(INPUT_D5)

expands to (,) since the beginning tuple is not a list ( missing BOOST_PP_NIL terminator )

BOOST_VMD_TUPLE(INPUT_D5)

expands to ((a,(b,(c))),204) since there is a beginning tuple

#define INPUT_D6 (a,b)(c) anything

BOOST_VMD_SEQ(INPUT_D6)

expands to (,) since the beginning tuple here cannot start a seq

BOOST_VMD_TUPLE(INPUT_D6)

expands to ((a,b),(c) anything) since there is a beginning tuple

#define INPUT_D7 (abc) anything

BOOST_VMD_SEQ(INPUT_D7)
BOOST_VMD_TUPLE(INPUT_D7)

expands to ((abc),anything) since the beginning tuple is also a beginning seq