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### Functionality for "empty" seqs and tuples

Boost PP arrays and lists can be empty but seqs and tuples cannot. The form of an empty array is '(0,())' and the form of an empty list is 'BOOST_PP_NIL'. But the form of '()' does not represent an empty seq or an empty tuple. Instead for a seq it represents a one element seq whose data is empty, while for a tuple it represents a tuple with a size of 1 whose single element is empty.

Not having a way to represent an empty seq or tuple represents a small problem for Boost PP users. For a tuple, especially, not being able to be "empty" is the only reason, when variadic macros are supported, why an end-user might prefer to use an array rather than a tuple as Boost PP preprocessor data. Otherwise, when using variadic macros, using a tuple is easier to notate and subsequently use than an array since no effort is needed to specify the number of elements of a tuple.

VMD as we have seen has functionality to tell when preprocessor data is "empty" through its BOOST_VMD_IS_EMPTY macro. Because of this it is possible to treat emptiness, when applied to a seq or tuple, as an empty seq or an empty tuple respectively, thus allowing seqs and tuples to be empty, just as arrays and lists are.

However there is one large problem when treating emptiness as an empty seq or empty tuple; Boost PP functionality for a seq or tuple knows nothing about it. In other words if you passed emptiness to a Boost PP macro which expects a seq or tuple, such as:

    BOOST_PP_SEQ_SIZE()

or

    BOOST_PP_TUPLE_PUSH_BACK(,1)

you would get UB, undefined behavior.

In order to use seqs and tuples which can be empty VMD defines a number of macros which mimic many of those in Boost PP, but begin with BOOST_VMD_ rather than BOOST_PP_. The VMD macros work with what I will call a 'VMD seq' or a 'VMD tuple', while the Boost PP equivalents work with the traditional seq or tuple. A VMD seq is a seq which may be empty and a VMD tuple is a tuple which may be empty. A VMD seq is therefore a superset of a normal seq and a VMD tuple is therefore a superset of a normal tuple. The VMD functionality therefore can accept a VMD seq or tuple or a normal seq or tuple whereas the equivalent Boost PP functionality only accepts a normal seq or tuple. To be more more precise the Boost PP functionality can accept a VMD seq or a VMD tuple as long as it is not empty. In other words there is no difference between a non-empty VMD seq or a Boost PP seq, or between a non-empty VMD tuple and a Boost PP tuple.

The particular macros which VMD supplies, supporting VMD seqs and VMD tuples, are divided between array, list, seq, and tuple functionality. The actual header files for these macros are in sub-directories of the VMD directory, which start respectively with 'array','list','seq' and 'tuple'. The header files are all also included in the general boost/vmd/vmd.hpp header file.

##### Array macros

The array macros provide conversions from an array to a VMD seq or a VMD tuple. The array macros are:

Table 1.1. Array macros

Macro | Parameter | Return | Functionality | Header
--- | --- | --- | --- | ---
BOOST_VMD_ARRAY_TO_SEQ | array = a Boost PP array | A VMD seq | Converts an array to a VMD seq. If the array is empty the seq is empty, otherwise the conversion is the same as BOOST_PP_ARRAY_TO_SEQ | boost/vmd/array/to_seq.hpp
BOOST_VMD_ARRAY_TO_TUPLE | array = a Boost PP array | A VMD tuple | Converts an array to a VMD tuple. If the array is empty the tuple is empty, otherwise the conversion is the same as BOOST_PP_ARRAY_TO_TUPLE | boost/vmd/array/to_tuple.hpp

The difference between the array conversion macros and their equivalent ones in Boost PP is that if the array is empty the VMD macros return emptiness whereas the Boost PP macros have UB.

You can include the array macros either using their individual header files, the general 'boost/vmd/array.hpp' header file for all array macros, or the general header file 'boost/vmd/vmd.hpp' for all macros.

##### List macros

The list macros provide conversions from a list to a VMD seq or a VMD tuple. The list macros are:

Table 1.2. List macros

Macro | Parameter | Return | Functionality | Header
--- | --- | --- | --- | ---
BOOST_VMD_LIST_TO_SEQ | list = a Boost PP list | A VMD seq | Converts a list to a VMD seq. If the list is empty the seq is empty, otherwise the conversion is the same as BOOST_PP_LIST_TO_SEQ | boost/vmd/list/to_seq.hpp
BOOST_VMD_LIST_TO_TUPLE | list = a Boost PP list | A VMD tuple | Converts a list to a VMD tuple. If the list is empty the tuple is empty, otherwise the conversion is the same as BOOST_PP_LIST_TO_TUPLE | boost/vmd/list/to_tuple.hpp

The difference between the list conversion macros and their equivalent ones in Boost PP is that if the list is empty the VMD macros return emptiness whereas the Boost PP macros have UB.

You can include the list macros either using their individual header files, the general 'boost/vmd/list.hpp' header file for all list macros, or the general header file 'boost/vmd/vmd.hpp' for all macros.

##### Seq macros

The seq macros either work with a VMD seq or return A VMD seq. The seq macros are:

Table 1.3. Seq macros

Macro | Parameter | Return | Functionality | Header
--- | --- | --- | --- | ---
BOOST_VMD_IS_VMD_SEQ | sequence = a VMD sequence | 1 if the VMD sequence is a VMD seq, else 0 | Tests a sequence as a VMD seq. If the sequence is empty returns 1, otherwise returns the same as BOOST_VMD_IS_SEQ | boost/vmd/seq/is_vmd_seq.hpp
BOOST_VMD_SEQ_POP_BACK | seq = a Boost PP seq | A VMD seq | Pops an element from the end of a seq. If the seq has a single element returns an empty seq, otherwise works exactly the same as BOOST_PP_SEQ_POP_BACK | boost/vmd/seq/pop_back.hpp
BOOST_VMD_SEQ_POP_FRONT | seq = a Boost PP seq | A VMD seq | Pops an element from the beginning of a seq. If the seq has a single element returns an empty seq, otherwise works exactly the same as BOOST_PP_SEQ_POP_FRONT | boost/vmd/seq/pop_front.hpp
BOOST_VMD_SEQ_PUSH_BACK | seq = a VMD seq, elem = element to push onto the end of the VMD seq | A Boost PP seq | Pushes an element onto the end of a VMD seq. If the VMD seq is empty the returned seq consists of that element, otherwise works exactly the same as BOOST_PP_SEQ_PUSH_BACK | boost/vmd/seq/push_back.hpp
BOOST_VMD_SEQ_PUSH_FRONT | seq = a VMD seq, elem = element to push onto the beginning of the VMD seq | A Boost PP seq | Pushes an element onto the beginning of a VMD seq. If the VMD seq is empty the returned seq consists of that element, otherwise works exactly the same as BOOST_PP_SEQ_PUSH_FRONT | boost/vmd/seq/push_front.hpp
BOOST_VMD_SEQ_REMOVE | seq = a Boost PP seq, i = index of element to remove | A VMD seq | Removes an element from a Boost PP seq. If the seq has a single element and the index to be removed is 0 returns an empty seq, otherwise works exactly the same as BOOST_PP_SEQ_REMOVE | boost/vmd/seq/remove.hpp
BOOST_VMD_SEQ_SIZE | seq = a VMD seq | The number of elements in the VMD seq | Returns the number of elements in the VMD seq. If the seq is empty returns 0, otherwise works exactly the same as BOOST_PP_SEQ_SIZE | boost/vmd/seq/size.hpp
BOOST_VMD_SEQ_TO_ARRAY | seq = a VMD seq | A Boost PP array | Converts a VMD seq to a Boost PP array. If the seq is empty returns an empty array whose form is '(0,())', otherwise works exactly the same as BOOST_PP_SEQ_TO_ARRAY | boost/vmd/seq/to_array.hpp
BOOST_VMD_SEQ_TO_LIST | seq = a VMD seq | A Boost PP list | Converts a VMD seq to a Boost PP list. If the seq is empty returns an empty list whose form is 'BOOST_PP_NIL', otherwise works exactly the same as BOOST_PP_SEQ_TO_LIST | boost/vmd/seq/to_list.hpp
BOOST_VMD_SEQ_TO_TUPLE | seq = a VMD seq | A VMD tuple | Converts a VMD seq to a VMD tuple. If the seq is empty returns an empty tuple, otherwise works exactly the same as BOOST_PP_SEQ_TO_TUPLE | boost/vmd/seq/to_tuple.hpp

The difference between the seq macros and their equivalent ones in Boost PP is that working with a VMD seq or returning a VMD seq is valid with the seq macros but with the Boost PP equivalent macros working with an empty seq causes UB and returning an empty seq can never occur.

You can include the seq macros either using their individual header files, the general 'boost/vmd/seq.hpp' header file for all seq macros, or the general header file 'boost/vmd/vmd.hpp' for all macros.

##### Tuple macros

The tuple macros either work with a VMD tuple or return a VMD tuple. The tuple macros are:

Table 1.4. Tuple macros

Macro | Parameter | Return | Functionality | Header
--- | --- | --- | --- | ---
BOOST_VMD_IS_VMD_TUPLE | sequence = a VMD sequence | 1 if the VMD sequence is a VMD tuple, else 0 | Tests a sequence as a VMD tuple. If the sequence is empty returns 1, otherwise returns the same as BOOST_VMD_IS_TUPLE | boost/vmd/tuple/is_vmd_tuple.hpp
BOOST_VMD_TUPLE_POP_BACK | tuple = a Boost PP tuple | A VMD tuple | Pops an element from the end of a tuple. If the tuple's size is 1 returns an empty tuple, otherwise works exactly the same as BOOST_PP_TUPLE_POP_BACK | boost/vmd/tuple/pop_back.hpp
BOOST_VMD_TUPLE_POP_FRONT | tuple = a Boost PP tuple | A VMD tuple | Pops an element from the beginning of a tuple. If the tuple's size is 1 returns an empty tuple, otherwise works exactly the same as BOOST_PP_TUPLE_POP_FRONT | boost/vmd/tuple/pop_front.hpp
BOOST_VMD_TUPLE_PUSH_BACK | tuple = a VMD tuple, elem = element to push onto the end of the VMD tuple | A Boost PP tuple | Pushes an element onto the end of a VMD tuple. If the VMD tuple is empty the returned tuple consists of that element, otherwise works exactly the same as BOOST_PP_TUPLE_PUSH_BACK | boost/vmd/tuple/push_back.hpp
BOOST_VMD_TUPLE_PUSH_FRONT | tuple = a VMD tuple, elem = element to push onto the beginning of the VMD tuple | A Boost PP tuple | Pushes an element onto the beginning of a VMD tuple. If the VMD tuple is empty the returned tuple consists of that element, otherwise works exactly the same as BOOST_PP_TUPLE_PUSH_FRONT | boost/vmd/tuple/push_front.hpp
BOOST_VMD_TUPLE_REMOVE | tuple = a Boost PP tuple, i = index of element to remove | A VMD tuple | Removes an element from a Boost PP tuple. If the tuple has a single element and the index to be removed is 0 returns an empty tuple, otherwise works exactly the same as BOOST_PP_TUPLE_REMOVE | boost/vmd/tuple/remove.hpp
BOOST_VMD_TUPLE_SIZE | tuple = a VMD tuple | The number of elements in the VMD tuple | Returns the number of elements in the VMD tuple. If the tuple is empty returns 0, otherwise works exactly the same as BOOST_PP_TUPLE_SIZE | boost/vmd/tuple/size.hpp
BOOST_VMD_TUPLE_TO_ARRAY | tuple = a VMD tuple | A Boost PP array | Converts a VMD tuple to a Boost PP array. If the tuple is empty returns an empty array whose form is '(0,())', otherwise works exactly the same as BOOST_PP_TUPLE_TO_ARRAY | boost/vmd/tuple/to_array.hpp
BOOST_VMD_TUPLE_TO_LIST | tuple = a VMD tuple | A Boost PP list | Converts a VMD tuple to a Boost PP list. If the tuple is empty returns an empty list whose form is 'BOOST_PP_NIL', otherwise works exactly the same as BOOST_PP_TUPLE_TO_LIST | boost/vmd/tuple/to_list.hpp
BOOST_VMD_TUPLE_TO_SEQ | tuple = a VMD tuple | A VMD seq | Converts a VMD tuple to a VMD seq. If the tuple is empty returns an empty seq, otherwise works exactly the same as BOOST_PP_TUPLE_TO_SEQ | boost/vmd/tuple/to_seq.hpp

The difference between the tuple macros and their equivalent ones in Boost PP is that working with a VMD tuple or returning a VMD tuple is valid with the tuple macros but with the Boost PP equivalent macros working with an empty tuple causes UB and returning an empty tuple can never occur.

You can include the tuple macros either using their individual header files, the general 'boost/vmd/tuple.hpp' header file for all tuple macros, or the general header file 'boost/vmd/vmd.hpp' for all macros.

##### Seq and tuple functionality

The set of macros for seq and tuple functionality which work with VMD seqs and VMD tuples are largely only a subset of the seq and tuple functionality in Boost PP. This reflects the fact that a number of macros in Boost PP for working with a seq or tuple make no sense when extended to work with a possible empty seq or empty tuple. For instance BOOST_PP_SEQ_FIRST_N could not mean anything when passed an empty seq and BOOST_PP_TUPLE_REM_CTOR could not mean anything when passed an empty tuple. Likewise for other Boost PP seq and tuple macros which do not have a VMD equivalent.

The set of functionality in VMD for working with a possibly empty seq or empty tuple makes it possible to add or remove elements in a seq or tuple which could start out or end up empty, take the seq or tuple size even when the seq or tuple is empty, convert between arrays, lists, seqs, or tuples which could start out or end up empty, and test for a VMD seq or a VMD tuple. This functionality should allow macro programmers the ability to work with empty seqs and tuples while still using other Boost PP seq and tuple functuionality to work with non-empty seqs and tuples. The easiest way to do this is to use the VMD seq and VMD tuple equivalent functions when choosing between VMD and Boost PP, and use the Boost PP seq and tuple functionality otherwise. Just remember that Boost PP seq and tuple functionality can never work with empty seqs or empty tuples.