On 9/13/2014 7:05 PM, Paul Mensonides wrote:

On 9/13/2014 1:51 PM, Edward Diener wrote:

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On 9/13/2014 9:44 AM, Paul Mensonides wrote:

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Perhaps Edward can enlighten me about the use-cases for which this is ideal. I do see some use-cases, but in most cases it requires dispatch based on what the particular "v-identifier" it is anyway.

The use case is to be able to determine if some preprocessor input matches a v-identifier and then to logically work off of that decision. The idea is to parse some input and, for instance, if the input is X, create output A, and if the input is Y, create output B etc. etc., where X and Y are v-identifiers.

In any given situation since the v-keys are being supplied, the end-user knows that if 1 is returned identifier X has been matched and if 2 is returned identifier Y has been matched, and of course if 0 is returned no identifier match has been found. This enables the end-user to dispatch on the result and to know which identifier has been found. Admittedly it does not extract the identifier itself as output from the macro.

What I meant WRT dispatching directly off of the v-identifier something along the lines of extracting the identifier and then using it to form the suffix of another macro. I.e. a conditional without going through an intermediate integer.

I understood that once you mentioned it.

However, there are still cases where something like

#define COMPARE_ABC_ABC ()

is useful

I understand this. You concatenate your result to 'COMPARE_ABC_' and if it expands to an empty single element tuple you know your result is 'ABC'. Ditto with any identifier 'XXX' for which you have '#define COMPARE_XXX_XXX ()' macros.

and direct comparison can be implemented with something like:

#include

#define IDENTIFIER_CIRCLE (CIRCLE), #define IDENTIFIER_SQUARE (SQUARE), #define IDENTIFIER_TRIANGLE (TRIANGLE), #define IDENTIFIER_RECTANGLE (RECTANGLE),

#define COMPARE_CIRCLE_CIRCLE () #define COMPARE_RECTANGLE_RECTANGLE () #define COMPARE_TRIANGLE_TRIANGLE () #define COMPARE_RECTANGLE_RECTANGLE ()

#define IS_IDENTIFIER(...) \ CHAOS_PP_QUICK_OVERLOAD(IS_IDENTIFIER_, __VA_ARGS__)(__VA_ARGS__) \ /**/ #define IS_IDENTIFIER_1(vseq) \ CHAOS_PP_BITAND \ (CHAOS_PP_COMPL(CHAOS_PP_IS_VARIADIC(vseq))) \ (IS_IDENTIFIER_1A(vseq)) \ /**/ #define IS_IDENTIFIER_1A(vseq) \ CHAOS_PP_BITAND \ (CHAOS_PP_IS_VARIADIC(IDENTIFIER_ ## vseq)) \ (CHAOS_PP_IS_EMPTY_NON_FUNCTION( \ CHAOS_PP_SPLIT(1, IDENTIFIER_ ## vseq) \ )) \ /**/ #define IS_IDENTIFIER_2(vseq, tuple) \ IS_IDENTIFIER_3(CHAOS_PP_STATE(), vseq, tuple) \ /**/ #define IS_IDENTIFIER_3(s, vseq, tuple) \ CHAOS_PP_BITAND \ (IS_IDENTIFIER_1(vseq)) \ (CHAOS_PP_VARIADIC_ELEM(0, \ CHAOS_PP_EXPR_S(s)(CHAOS_PP_TUPLE_FOLD_LEFT_S( \ s, IS_IDENTIFIER_3A, tuple, 0, \ CHAOS_PP_REM_CTOR( \ CHAOS_PP_SPLIT(0, IDENTIFIER_ ## vseq) \ ) \ )) \ )) \ /**/ #define IS_IDENTIFIER_3A(s, e, b, id) \ CHAOS_PP_BITOR \ (b) \ (CHAOS_PP_IS_VARIADIC(COMPARE_ ## e ## _ ## id)), id \ /**/

IS_IDENTIFIER(RECTANGLE, (CIRCLE, SQUARE, RECTANGLE))

Note this isn't finding the particular index, but it could be made to do so.

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One could do better for this low-level part with registrations such as (without a BOOST_VMD_ prefix for brevity)

#define IDENTIFIER_CIRCLE (CIRCLE), #define IDENTIFIER_SQUARE (SQUARE), #define IDENTIFIER_TRIANGLE (TRIANGLE), #define IDENTIFIER_RECTANGLE (RECTANGLE),

Is the ending comma there for a reason ?

Yes, though I think you already figured it out. The parentheses are there so that you can detect that your prefix token-pasting did something, the "v-identifier" is there so you can extract it, and the comma is there to separate the "v-identifier" from the rest of a "v-sequence".

When I saw what the comma did that light in my brain went on <g>.

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These definitions don't allow one to directly test for specific "v-identifiers", but they do allow one to extract registered "v-identifiers" from a "v-sequence".

If all such registrations are required to be the same, there is no need for separate per-library prefixes. Macros can be defined multiple times if they are defined the same way.

Similarly, the "v-numbers" could be pre-registered by the library as

#define NUMBER_0 (0), #define NUMBER_1 (1), #define NUMBER_2 (2), // ...

Ditto about the ending commas.

As above, only difference is prefix allowing a type-dispatch as opposed to a value-dispatch.

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Tests for specific "v-identifiers" can be done via extraction from the above and using macro definitions such as

#define COMPARE_CIRCLE_CIRCLE () #define COMPARE_SQUARE_SQUARE () // ...

Sure.

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In many cases this sort of comparison is not needed, as one would directly dispatch off of the "v-identifier" itself.

I appreciate your technique of matching a v-identifier/v-number by returning a registered identifier/number and then dispatching off of the identifier itself. It appears to make parsing a v-sequence much easier, but I need to consider this further.

One of the big things about it (IMO) is to make the registrations reusable. If something is defined the same, it can be defined more than once. All C/C++ keywords can also be registered by the library.

Understood. I agree this is a nice feature. My own key-value v-identifier implementation was to try to make things as simple as possible for the end-user. I now realize that if I keep it, possibly along with your own methodology, as a primary or alternative implementation of finding a v-identifier, it would have been better to have the key-value macros expand to a single comma (',') rather than expand to emptiness. Then the whole complication of parsing a single input sequence for an identifier will be much easier. I really appreciate your alerting me to the 'comma' technique. Substituting a comma into an input sequence is easy to detect and will tell me if I have matched an identifier or a number wthout having to recursively parse the rest of the v-sequence or have to know what type of input comes next. That is a wonderful improvement to VMD's overly complicated v-sequence parsing macros, and I thank you very much for pointing it out. Do realize that your own v-identifier technique requires dispatching off of the v-identifier itself using concatenation, a technique some macro programmers may not know as well as dispatching off of a Boost PP number using BOOST_PP_EQUAL. I agree that dispatching off of the identifier itself is more elegant and a bit faster as far as the preprocessor is concerned.

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In any case, one could "parse" a "v-sequence" into "v-types" without many of the limitations of the current design. The following example uses Chaos, but I do not think there is anything in the design that fundamentally cannot be made to work on (e.g.) VC++.

This is a bit unfair. You are asking me to understand Chaos code as opposed to Boost PP and VMD. But I think I get the idea from your original suggestion above and will study it further.

It is just a demonstration of a possibility. I did it quickly so had to reuse some existing stuff, but essentially it is just a while loop. It terminates when its (constrained) input is empty. Its "operator" (which iterates the data) tests the front the the "v-sequence" for a parenthetic expression. If it has one, the operator extracts it and puts it into the result. If it does not, it tests it (via token-pasting) for starting with a "v-number". If it does, the operator extracts it and puts it into the result. If it does not, it tests it (again via token-pasting) for starting with a registered "v-identifier". If it does, the operator extracts it and puts it into the result. If it does not, it puts the entire remainder of the "v-sequence" into the result.

Thanks for the explanation. I can follow it.

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#include

#define NUMBER_1 (1), #define NUMBER_2 (2), #define NUMBER_3 (3), #define NUMBER_4 (4), #define NUMBER_5 (5), // ...

// parser implementation... #define PARSE(vseq) PARSE_S(CHAOS_PP_STATE(), vseq) #define PARSE_S(s, vseq) \ CHAOS_PP_SPLIT(0, CHAOS_PP_EXPR_S(s)(CHAOS_PP_WHILE_S( \ s, PARSE_P, PARSE_O,, vseq \ ))) \ /**/ #define PARSE_P(s, seq, vseq) \ CHAOS_PP_COMPL(CHAOS_PP_IS_EMPTY_NON_FUNCTION(vseq)) \ /**/ #define PARSE_O(s, seq, vseq) \ seq CHAOS_PP_IIF(CHAOS_PP_IS_VARIADIC(vseq))( \ PARSE_O_A, PARSE_O_B \ )(vseq) \ /**/ #define PARSE_O_A(vseq) PARSE_O_A_A vseq #define PARSE_O_A_A(...) (3, (__VA_ARGS__)), #define PARSE_O_B(vseq) \ CHAOS_PP_IIF(CHAOS_PP_IS_VARIADIC(NUMBER_ ## vseq))( \ PARSE_O_B_A, \ CHAOS_PP_IIF(CHAOS_PP_IS_VARIADIC(IDENTIFIER_ ## vseq))( \ PARSE_O_B_B, PARSE_O_B_C \ ) \ )(vseq) \ /**/ #define PARSE_O_B_A(vseq) \ (1, CHAOS_PP_REM_CTOR( \ CHAOS_PP_SPLIT(0, NUMBER_ ## vseq) \ )), \ CHAOS_PP_SPLIT(1, NUMBER_ ## vseq) \ /**/ #define PARSE_O_B_B(vseq) \ (2, CHAOS_PP_REM_CTOR( \ CHAOS_PP_SPLIT(0, IDENTIFIER_ ## vseq) \ )), \ CHAOS_PP_SPLIT(1, IDENTIFIER_ ## vseq) \ /**/ #define PARSE_O_B_C(vseq) (0, vseq),

// registrations... #define IDENTIFIER_CIRCLE (CIRCLE), #define IDENTIFIER_SQUARE (SQUARE), #define IDENTIFIER_TRIANGLE (TRIANGLE), #define IDENTIFIER_RECTANGLE (RECTANGLE),

PARSE( CIRCLE RECTANGLE (a, b, c) TRIANGLE 3 (xyz) UNREGISTERED SQUARE )

(Apologies for any mistakes in the above. I wrote it up quickly.)

This particular example results in key-value pairs of the form: (0, <unrecognized>) (1, <v-number>) (2, <registered-identifier>) (3, <tuple>)

(2, CIRCLE) (2, RECTANGLE) (3, (a, b, c)) (2, TRIANGLE) (1, 3) (3, (xyz)) (0, UNREGISTERED SQUARE)

There are a number of things that could be done instead of producing a binary sequence like this (which Boost.Preprocessor cannot handle). One could fold the results via a user-supplied macro, for example.

Aside from the massive amount of clutter introduced by workarounds, many of the pieces of the above are already available either in Boost.Preprocessor or in other parts of the VMD library. For example, BOOST_VMD_IS_EMPTY is essentially CHAOS_PP_IS_EMPTY_NON_FUNCTION.

FYI

The CHAOS_PP_IS_VARIADIC macro is equivalent to your BOOST_VMD_IS_BEGIN_TUPLE macro. The CHAOS_PP_REM macro is just a parentheses remover (e.g. #define REM(...) __VA_ARGS__). The CHAOS_PP_REM_CTOR macro is just a "constructed" parentheses remover (e.g. #define REM_CTOR(...) REM __VA_ARGS__). The CHAOS_PP_SPLIT macro is basically just a head/tail macro of a comma separated list.

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-- Implementation --

I have not looked closely at the implementation, but I do know what a herculean effort it is to get this type of stuff working with VC++ in particular.

Thanks ! VC++ is horrible to make it work "correctly". But you already know that.

Yes.

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-- Documentation --

There are few inaccuracies in the documentation related to motivations for various Boost.Preprocessor things, but those could be fixed.

Please feel free to point them out and I will update the documentation accordingly.

They are minor. They have to do with the supposition that Boost.Preprocessor is designed to not produce erroneous results given erroneous inputs--which it isn't designed to do at all--

I don't know where in the doc I stated the above. I was trying to emphasize that Boost PP emphasized safety over the sort of functionality which VMD provides, which depends more on end-users following certain constraints on VMD input. Obviously depending on detecting emptiness as a core functionality of VMD, where the test for emptiness, largely taken from your own Internet code, can never be perfect, is an example of the sort of riskier use of macros which VMD represents. A number of people and a few reviewers also wanted to know why VMD, if accepted, should not just be an addition to Boost PP and I was expressing in my remarks to them a difference I perceived in philosophy between Boost PP and VMD.

and the reason why the low-level detection macros (such as BOOST_PP_IS_NULLARY and BOOST_PP_IS_EMPTY) is not because of their constrained input requirements, but rather because using them frequently leads to the need for hacks and workarounds in the code surrounding their use. They are "non-public" because they cannot fully encapsulate preprocessor workarounds.

OK. My interpretation of why you did not document them publicly was wrong. I really thought it was along the lines of "Because these macros could be dangerous or erroneous given certain input I don't want to document them for you." But if you are concerned with other issues related to their use I understand.

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The documentation for BOOST_VMD_IS_EMPTY is decent except that essentially any input that ends with a function-like macro name will cause weird results or errors. So much so that the documentation should just disallow input that ends with a function-like macro name across the board.

I don't think that input that ends with a function-like macro name, when that macro takes 0 or 1 parameter, causes incorrect results on a C++ standard conforming compiler. Can you explain why it would cause an incorrect result or an error ?

Contrived but an easy demonstration:

#define MACRO(x) CAT(+, x C) // valid input to MACRO is + or =

MACRO(+) // ++ C MACRO() // error

BOOST_VMD_IS_EMPTY(MACRO) // error

You think of things I haven't even considered <g>. Anyway I will also document this sort of situation in my discussion of emptiness in VMD.

You can get erroneous results too if the macro BOOST_VMD_BEGIN_TUPLE used in the BOOST_VMD_DETAIL_IS_EMPTY_PROCESS macro somehow causes another scan to be applied to its input.

I really did already understand that ( I was bitten by it once when I was programming VMD ) but I do not see how that could happen unless I specifically change the BOOST_VMD_DETAIL_IS_EMPTY_PROCESS and mess it up. But thanks for alerting me about it.

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Other than that, the documentation needs some clean-up and organization (particularly of definitions), but does a reasonable job of documenting the library. I do feel that a lot of documentation complexity comes from the way that v-keys/v-identifiers (etc) are handled.

I agree that if I use your methodology of registering v-identifiers and v-numbers then the documentation, as well as the code, for parsing v-sequences might be much simplified and I may be automatically able to parse any v-sequence into its v-types. I will have to take a look at this a bit more.

I think it is possible. The main difficulty will be VC++ because you will have to bend over backwards to get those macro-generated commas to form an argument separator in a variety of situations (if I recall correctly).

I am on the alert. Thanks ! I do like the comma technique enormously if I can get it to work with the usually horrid VC++isms. All that recursive processing I was doing with v-sequences, along with the complicated input that was needed to parse v-identifiers and v-numbers in the v-sequences, was hard work but I was too stupid to realize your much better way.

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I had my skull opened up and had "preprocessor metaprogramming" tattooed directly onto my brain.

Who coined that term anyway <g> ?

:) I don't know.

Likely story <g>.

On 9/14/2014 10:58 AM, pfultz2 wrote:

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One could do better for this low-level part with registrations such as (without a BOOST_VMD_ prefix for brevity)

#define IDENTIFIER_CIRCLE (CIRCLE), #define IDENTIFIER_SQUARE (SQUARE), #define IDENTIFIER_TRIANGLE (TRIANGLE), #define IDENTIFIER_RECTANGLE (RECTANGLE),

Is the ending comma there for a reason ?

Yes, though I think you already figured it out. The parentheses are there so that you can detect that your prefix token-pasting did something, the "v-identifier" is there so you can extract it, and the comma is there to separate the "v-identifier" from the rest of a "v-sequence".

There is no need for the commas though. Its easy enough to insert them in. Since we take sequence of tokens first:

CIRCLE SQUARE

And then we concat the identifiers, which gives us this:

(CIRCLE) SQUARE

Then we could use this:

#define APPEND_COMMA_I(...) (__VA_ARGS__), #define APPEND_COMMA(...) APPEND_COMMA_I __VA_ARGS__

APPEND_COMMA((CIRCLE) SQUARE) // Expands to (CIRCLE), SQUARE

It does require a little bit more for the implementation, however, this helps to simplify the "interface", I think.

The point is, however, that you don't know that the concatenation will produce a tuple when you call your APPEND_COMMA. If it does not and just produces an identifier of the form IDENTIFIER_XXX then your APPEND_COMMA(IDENTIFIER_XXX SQUARE) will end up as 'APPEND_COMMA_I IDENTIFIER_XXX SQUARE' which you do not want. Actually I realized that the comma is not be needed at all in Paul's IDENTIFIER macros above. There is already code in VMD that can split a sequence beginning with a tuple into the beginning tuple and the rest of the sequence ( BOOST_VMD_TUPLE ). So the logic of parsing for an identifier at the beginning of a sequence is to start by looking for a beginning tuple in the sequence, if not found test for an identifier through concatenation, and if the subsequent sequence after concatenation now starts with a beginning tuple, the code can separate the beginning tuple from the remainder of the sequence and extract the v-identifier.

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I think it is possible. The main difficulty will be VC++ because you will have to bend over backwards to get those macro-generated commas to form an argument separator in a variety of situations (if I recall correctly).

I actually use this `MSVC_INVOKE` macro to help in those situations:

#define MSVC_INVOKE BOOST_PP_CAT(MSVC_INVOKE_, BOOST_PP_AUTO_REC(DETAIL_MSVC_INVOKE_P, 16))

#define DETAIL_MSVC_INVOKE_P(n) BOOST_PP_IS_NULLARY( MSVC_INVOKE_ ## n((),) )

#define MSVC_INVOKE_1(macro, args) MSVC_INVOKE_I_1(macro, args) #define MSVC_INVOKE_2(macro, args) MSVC_INVOKE_I_2(macro, args) ... #define MSVC_INVOKE_16(macro, args) MSVC_INVOKE_I_16(macro, args)

#define MSVC_INVOKE_I_1(macro, args) MSVC_INVOKE_X_1(macro args) #define MSVC_INVOKE_I_2(macro, args) MSVC_INVOKE_X_2(macro args) ... #define MSVC_INVOKE_I_16(macro, args) MSVC_INVOKE_X_16(macro args)

#define MSVC_INVOKE_X_1(x) x #define MSVC_INVOKE_X_2(x) x ... #define MSVC_INVOKE_X_16(x) x

Sorry I can't follow this or udnerstand what you are doing. I don't know what BOOST_PP_AUTO_REC does even when I look at its code <g>.

Of course, I made it re-entrant which could slow down the preprocessor for MSVC, however, it helps simplify when I need to workaround MSVC. So if I need to call a macro with vardiac args, I can just call it like this:

#define HEAD(x, ...) x MSVC_INVOKE(HEAD, (1,2,3,4)) // Expands to 1

Are you passing '1,2,3,4' to HEAD ?

Unfortunately, there are still times where this fails as well, but most of the time this will work.

Finally, when I have done this DSL like parser(like in LINQ for example), I would convert the string of tokens to a sequence and then process them afterwards using the algorithms for a sequence. So I would essentially transform this:

#define STRING_from (from) #define STRING_where (where) #define STRING_select (select)

STRING_TO_SEQ(from(x, numbers) where(x < 3) select(x * x))

That then expands to:

(from)((x, numbers))(where)((x < 3))(select)((x * x))

You might find this expansion easier with VMD. With Paul's suggestion and my realization that the final comma is not needed, I will be reprogramming the VMD v-sequence macros so that you should be able to take a v-sequence, like your 'string' of 'from(x, numbers) where(x < 3) select(x * x)' and break it down into its constituent v-types without the limitations current VMD incurs. In fact I intend to add a VMD macro, something like BOOT_VMD_SEQUENCE(vsequence) which will return all the v-types in some way ( probably a tuple, seq, or just variadic data, I have yet to decide ).

On 9/14/2014 5:10 PM, pfultz2 wrote:

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The point is, however, that you don't know that the concatenation will produce a tuple when you call your APPEND_COMMA. If it does not and just produces an identifier of the form IDENTIFIER_XXX then your APPEND_COMMA(IDENTIFIER_XXX SQUARE) will end up as 'APPEND_COMMA_I IDENTIFIER_XXX SQUARE' which you do not want.

Yes, of course, you will need to check that you have an identifier first.

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Actually I realized that the comma is not be needed at all in Paul's IDENTIFIER macros above. There is already code in VMD that can split a sequence beginning with a tuple into the beginning tuple and the rest of the sequence ( BOOST_VMD_TUPLE ). So the logic of parsing for an identifier at the beginning of a sequence is to start by looking for a beginning tuple in the sequence, if not found test for an identifier through concatenation, and if the subsequent sequence after concatenation now starts with a beginning tuple, the code can separate the beginning tuple from the remainder of the sequence and extract the v-identifier.

Yes, there is similiar logic in the `STRING_TOKEN` macro I showed, except my code doesn't handle custom namespaces(ie `STRING_`) nor multiple namespaces.

I am not sure what you mean by namespaces ? Do you mean the type of the token parsed ?

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Sorry I can't follow this or udnerstand what you are doing. I don't know what BOOST_PP_AUTO_REC does even when I look at its code <g>.

Well the macro is auto reentrant. So it has a macro for each recursion level up to 16(ie `MSVC_INVOKE_1`, `MSVC_INVOKE_2`, etc). The `BOOST_PP_AUTO_REC` then will search for a macro that can be called(it hasn't been painted blue). It does this by calling the `DETAIL_MSVC_INVOKE_P` predicate. Then `DETAIL_MSVC_INVOKE_P` wil pass in nullary parenthesis into `MSVC_INVOKE`, and if that result isn't nullary then it means that recursion level got painted blue. So the `BOOST_PP_AUTO_REC` will do a binary search and try to find the first predicate that is true. It will return that recursion level, which it will concat with `MSVC_INVOKE_` so the correct macro will be called. Does that make sense?

No, but that's OK.

Perhaps Paul can explain it better than me.

The BOOST_PP_AUTO_REC macro is not documented so maybe you can understand my being puzzled by it and what it is used for. Other undocumented macros in Boost PP are generally easier for me to follow.

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You might find this expansion easier with VMD. With Paul's suggestion and my realization that the final comma is not needed, I will be reprogramming the VMD v-sequence macros so that you should be able to take a v-sequence, like your 'string' of 'from(x, numbers) where(x < 3) select(x * x)' and break it down into its constituent v-types without the limitations current VMD incurs. In fact I intend to add a VMD macro, something like BOOT_VMD_SEQUENCE(vsequence) which will return all the v-types in some way ( probably a tuple, seq, or just variadic data, I have yet to decide ).

Of course for my case, I don't need to know what category or type it is(it may be useful for other people). So having it as a sequence like I've shown is very useful. Also, I was thinking it might be useful to have some kind of `BELONGS_TO` to query which namespace a token belong to.

The BOOST_VMD_SEQUENCE macro I plan will return for each v-type found not only its data by is v-type also. I am leaning toward returning the data as a Boost PP seq where every element of he seq is a tuple of (type,value) pairs. Each type will be a number identifying a distinct v-type (number,identifier,array,list,seq,tuple). Then you could iterate through that returned seq, probably with BOOST_PP_SEQ_FOR_EACH, and do what you want with the type/data to create your final construct. This will allow a v-sequence, what you call a 'string' in your example, to be parsed for its individual pieces of VMD v-type data. You could actually do something similar with VMD now but you would have to try each v-type separately to see if it begins the v-sequence. But after Paul Mensonides provided a better solution to identifying v-identifiers and v-numbers I can improve the parsing of VMD v-sequences greatly. In fact with VMD a v-sequence becomes another preprocessor "data type" similar to the high level Boost PP data types of array, list, seq, and tuple. I should really work to produce whatever operations with a v-sequence I can that would mimic some of those Boost PP data types. Its challenging to think about but it will be fun work except for the usual VC++isms.

On 9/14/2014 6:49 PM, Paul Mensonides wrote:

On 9/14/2014 3:12 PM, Edward Diener wrote:

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On 9/14/2014 5:10 PM, pfultz2 wrote:

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I am not sure what you mean by namespaces ? Do you mean the type of the token parsed ?

I believe he means macro prefixes such as BOOST_PP_, BOOST_VMD_, or BOOST_VMD_NAMESPACE_.

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Perhaps Paul can explain it better than me.

The BOOST_PP_AUTO_REC macro is not documented so maybe you can understand my being puzzled by it and what it is used for. Other undocumented macros in Boost PP are generally easier for me to follow.

The BOOST_PP_AUTO_REC macro is used to find the first macro of a set of macros which has not been painted (i.e. will expand). It requires a set of macros which are a power of 2 which are used in order.

Here is my understanding of what BOOST_PP_AUTO_REC does from your example. It takes a function-like macro name and a parameter which is a Boost PP number which is a power of 2, from a minimum of 2 to a maximum of 256. It then calls the function-like macro name passing a number starting with 1 through the number passed to it. If the function-like macro returns 1 it stops and returns the current number else it continues with the next number. I won't ask what BOOST_PP_AUTO_REC returns if its function-like macro never returns 1 before it finishes cycling through its numbers <g>. I assume the "painted" part is because it is usually used with BOOST_PP_NULLARY as in your example below.

For example:

#include #include

#define CAT_1(a, b) PRIMITIVE_CAT_1(a, b) #define CAT_2(a, b) PRIMITIVE_CAT_2(a, b) #define CAT_3(a, b) PRIMITIVE_CAT_3(a, b) #define CAT_4(a, b) PRIMITIVE_CAT_4(a, b)

#define PRIMITIVE_CAT_1(a, b) a ## b #define PRIMITIVE_CAT_2(a, b) a ## b #define PRIMITIVE_CAT_3(a, b) a ## b #define PRIMITIVE_CAT_4(a, b) a ## b

#define CAT \ BOOST_PP_CAT( \ CAT_, \ BOOST_PP_AUTO_REC(PRIMITIVE_CAT_P, 4) \ ) /**/ #define PRIMITIVE_CAT \ BOOST_PP_CAT( \ PRIMITIVE_CAT_, \ BOOST_PP_AUTO_REC(PRIMITIVE_CAT_P, 4) \ ) \ /**/ #define PRIMITIVE_CAT_P(n) \ BOOST_PP_IS_NULLARY(PRIMITIVE_CAT_ ## n(,)()) \ /**/

Here the CAT and PRIMITIVE_CAT macros are defined as object-like macros which expand to the next available CAT_n and PRIMITIVE_CAT_n macros which are found via the the BOOST_PP_AUTO_REC macro--which does a binary search from 1 to 4 in this case finding the first index for which the passed predicate PRIMITIVE_CAT_P expands to 1.

Essentially, it makes it look like CAT can be invoked recursively, but CAT itself is not really even a function-like macro. E.g.

#define A1 CAT(B, 1) // here CAT expands to CAT_2 #define B1 CAT(C, 1) // here CAT expands to CAT_3 #define C1 CAT(D, 1) // here CAT expands to CAT_4

CAT(A, 1) // here CAT expands to CAT_1

Thanks for the explanation.

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You could actually do something similar with VMD now but you would have to try each v-type separately to see if it begins the v-sequence. But after Paul Mensonides provided a better solution to identifying v-identifiers and v-numbers I can improve the parsing of VMD v-sequences greatly.

In fact with VMD a v-sequence becomes another preprocessor "data type" similar to the high level Boost PP data types of array, list, seq, and tuple. I should really work to produce whatever operations with a v-sequence I can that would mimic some of those Boost PP data types. Its challenging to think about but it will be fun work except for the usual VC++isms.

The alternate registration mechanism (excluding the comma in the registration itself) that you guys realized is good and is obvious is retrospect. That would make the registration much cleaner.

I agree. Your proposal is obvious in retrospect but it is like the legendary story of Christopher Columbus and the egg. I am now very glad I can make a v-sequence much easier with better functionality. Why I didn't think of your solution to being with ... <g>.

On 9/14/2014 10:34 PM, Paul Mensonides wrote:

On 9/14/2014 5:12 PM, Edward Diener wrote:

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On 9/14/2014 6:49 PM, Paul Mensonides wrote:

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The BOOST_PP_AUTO_REC macro is used to find the first macro of a set of macros which has not been painted (i.e. will expand). It requires a set of macros which are a power of 2 which are used in order.

Here is my understanding of what BOOST_PP_AUTO_REC does from your example. It takes a function-like macro name and a parameter which is a Boost PP number which is a power of 2, from a minimum of 2 to a maximum of 256. It then calls the function-like macro name passing a number starting with 1 through the number passed to it. If the function-like macro returns 1 it stops and returns the current number else it continues with the next number. I won't ask what BOOST_PP_AUTO_REC returns if its function-like macro never returns 1 before it finishes cycling through its numbers <g>.

Yes, except that it does a binary rather than linear search, so it can search a fairly large set with minimal invocations of the predicate.

Still better. Good to know.

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The alternate registration mechanism (excluding the comma in the registration itself) that you guys realized is good and is obvious is retrospect. That would make the registration much cleaner.

I agree. Your proposal is obvious in retrospect but it is like the legendary story of Christopher Columbus and the egg.

I am now very glad I can make a v-sequence much easier with better functionality. Why I didn't think of your solution to being with ... <g>.

Sorry, I meant excluding the comma in the registration was obvious to me in retrospect, but I didn't think of it at the time.

I think that removing the existing length (etc.) limitations on v-sequences will make things much more usable--and make the documentation much much simpler.

Absolutely. It will also make the implementation of v-sequences much simpler. The whole business of trying to identify identifiers and numbers in a v-sequence the way I was doing it, even though it worked, imposed limitations which I am now glad to do without. With your suggestion I can throw all that onerous code away. It was a burden to do it and I am glad to be able to get rid of it for a much better and easier implementation.

On 9/15/2014 12:17 AM, pfultz2 wrote:

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I am not sure what you mean by namespaces ? Do you mean the type of the token parsed ?

I mean the prefix for the tokens.

OK. I will choose something easy, starting with BOOST_VMD_

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Perhaps Paul can explain it better than me.

The BOOST_PP_AUTO_REC macro is not documented so maybe you can understand my being puzzled by it and what it is used for. Other undocumented macros in Boost PP are generally easier for me to follow.

Oops! Well it is documented in Chaos. I guess its more of an implementation detail in Boost.PP. Of course, its not at all obvious what it does from looking at the code(even though it is quite nicely formatted).

Paul Mensonides explained it in his reply.

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The BOOST_VMD_SEQUENCE macro I plan will return for each v-type found not only its data by is v-type also. I am leaning toward returning the data as a Boost PP seq where every element of he seq is a tuple of (type,value) pairs. Each type will be a number identifying a distinct v-type (number,identifier,array,list,seq,tuple).

There may be use cases for this. However, it would be nice just to map it directly to a sequence without the v-type. So you could have a mpping to a v-sequence which has the v-types(perhaps the tuple (type,value) pairs), but in addition have a mapping to pp sequence which will just have the value. I can see how there are uses for both of these.

Sure. Easy enough.