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       ExtUtils::Constant::Base - base class for ExtUtils::Constant objects


           require ExtUtils::Constant::Base;
           @ISA = 'ExtUtils::Constant::Base';


       ExtUtils::Constant::Base provides a base implementation of methods to generate C code to
       give fast constant value lookup by named string. Currently it's mostly used
       ExtUtils::Constant::XS, which generates the lookup code for the constant() subroutine
       found in many XS modules.


       ExtUtils::Constant::Base exports no subroutines. The following methods are available

           A method returning a scalar containing definitions needed, typically for a C header

       memEQ_clause args_hashref
           A method to return a suitable C "if" statement to check whether name is equal to the C
           variable "name". If checked_at is defined, then it is used to avoid "memEQ" for short
           names, or to generate a comment to highlight the position of the character in the
           "switch" statement.

           If i<checked_at> is a reference to a scalar, then instead it gives the characters pre-
           checked at the beginning, (and the number of chars by which the C variable name has
           been advanced. These need to be chopped from the front of name).

       dump_names arg_hashref, ITEM...
           An internal function to generate the embedded perl code that will regenerate the
           constant subroutines.  default_type, types and ITEMs are the same as for C_constant.
           indent is treated as number of spaces to indent by.  If "declare_types" is true a
           $types is always declared in the perl code generated, if defined and false never
           declared, and if undefined $types is only declared if the values in types as passed in
           cannot be inferred from default_types and the ITEMs.

       assign arg_hashref, VALUE...
           A method to return a suitable assignment clause. If type is aggregate (eg PVN expects
           both pointer and length) then there should be multiple VALUEs for the components. pre
           and post if defined give snippets of C code to proceed and follow the assignment. pre
           will be at the start of a block, so variables may be defined in it.

       return_clause arg_hashref, ITEM
           A method to return a suitable "#ifdef" clause. ITEM is a hashref (as passed to
           "C_constant" and "match_clause". indent is the number of spaces to indent, defaulting
           to 6.

       switch_clause arg_hashref, NAMELEN, ITEMHASH, ITEM...
           An internal method to generate a suitable "switch" clause, called by "C_constant"
           ITEMs are in the hash ref format as given in the description of "C_constant", and must
           all have the names of the same length, given by NAMELEN.  ITEMHASH is a reference to a
           hash, keyed by name, values being the hashrefs in the ITEM list.  (No parameters are
           modified, and there can be keys in the ITEMHASH that are not in the list of ITEMs
           without causing problems - the hash is passed in to save generating it afresh for each

       params WHAT
           An "internal" method, subject to change, currently called to allow an overriding class
           to cache information that will then be passed into all the "*param*" calls. (Yes,
           having to read the source to make sense of this is considered a known bug). WHAT is be
           a hashref of types the constant function will return. In ExtUtils::Constant::XS this
           method is used to returns a hashref keyed IV NV PV SV to show which combination of
           pointers will be needed in the C argument list generated by
           C_constant_other_params_definition and C_constant_other_params

       dogfood arg_hashref, ITEM...
           An internal function to generate the embedded perl code that will regenerate the
           constant subroutines.  Parameters are the same as for C_constant.

           Currently the base class does nothing and returns an empty string.

       normalise_items args, default_type, seen_types, seen_items, ITEM...
           Convert the items to a normalised form. For 8 bit and Unicode values converts the item
           to an array of 1 or 2 items, both 8 bit and UTF-8 encoded.

       C_constant arg_hashref, ITEM...
           A function that returns a list of C subroutine definitions that return the value and
           type of constants when passed the name by the XS wrapper.  ITEM... gives a list of
           constant names. Each can either be a string, which is taken as a C macro name, or a
           reference to a hash with the following keys

           name    The name of the constant, as seen by the perl code.

           type    The type of the constant (IV, NV etc)

           value   A C expression for the value of the constant, or a list of C expressions if
                   the type is aggregate. This defaults to the name if not given.

           macro   The C pre-processor macro to use in the "#ifdef". This defaults to the name,
                   and is mainly used if value is an "enum". If a reference an array is passed
                   then the first element is used in place of the "#ifdef" line, and the second
                   element in place of the "#endif". This allows pre-processor constructions such

                       #if defined (foo)
                       #if !defined (bar)

                   to be used to determine if a constant is to be defined.

                   A "macro" 1 signals that the constant is always defined, so the "#if"/"#endif"
                   test is omitted.

           default Default value to use (instead of "croak"ing with "your vendor has not
                   defined...") to return if the macro isn't defined. Specify a reference to an
                   array with type followed by value(s).

           pre     C code to use before the assignment of the value of the constant. This allows
                   you to use temporary variables to extract a value from part of a "struct" and
                   return this as value. This C code is places at the start of a block, so you
                   can declare variables in it.

           post    C code to place between the assignment of value (to a temporary) and the
                   return from the function. This allows you to clear up anything in pre.  Rarely

                   Equivalents of pre and post for the default value.

           utf8    Generated internally. Is zero or undefined if name is 7 bit ASCII, "no" if the
                   name is 8 bit (and so should only match if SvUTF8() is false), "yes" if the
                   name is utf8 encoded.

                   The internals automatically clone any name with characters 128-255 but none
                   256+ (ie one that could be either in bytes or utf8) into a second entry which
                   is utf8 encoded.

           weight  Optional sorting weight for names, to determine the order of linear testing
                   when multiple names fall in the same case of a switch clause.  Higher comes
                   earlier, undefined defaults to zero.

           In the argument hashref, package is the name of the package, and is only used in
           comments inside the generated C code. subname defaults to "constant" if undefined.

           default_type is the type returned by "ITEM"s that don't specify their type. It
           defaults to the value of "default_type()". types should be given either as a comma
           separated list of types that the C subroutine subname will generate or as a reference
           to a hash. default_type will be added to the list if not present, as will any types
           given in the list of ITEMs. The resultant list should be the same list of types that
           "XS_constant" is given. [Otherwise "XS_constant" and "C_constant" may differ in the
           number of parameters to the constant function. indent is currently unused and ignored.
           In future it may be used to pass in information used to change the C indentation style
           used.]  The best way to maintain consistency is to pass in a hash reference and let
           this function update it.

           breakout governs when child functions of subname are generated.  If there are breakout
           or more ITEMs with the same length of name, then the code to switch between them is
           placed into a function named subname_len, for example "constant_5" for names 5
           characters long.  The default breakout is 3.  A single "ITEM" is always inlined.


       Not everything is documented yet.

       Probably others.


       Nicholas Clark <> based on the code in "h2xs" by Larry Wall and others