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Wiki page [typelists] by stephan 2008-05-28 16:13:32.
D 2008-05-28T16:13:32
L typelists
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<h1>Typelists</h1>

Typelists are an idiom popularized by Andrei Alexandrescu in his amazing
book entitled <em>Modern C++ Design</em> (that book triggered a re-awakening
in my interest in C++).

In an ideal world we could pass an arbitrary number of arguments to
a class or function template. In the current standard we can't do that,
but in C++0x we can (gcc 4.3 already supports this). Typelists help
us work around that by providing a partial workaround (albeit a slightly
unsightly one). Typelists typically are made up of a very small struct
type with two typedefs, then a whole boatload of generated code which
implements specializations to allow variadic-like arguments (but
there is an upper-limit to the number, typically set by the typelist
implementor).

parsepp's older brother, [http://wanderinghorse.net/computing/parse0x|parse0x],
is essentially identical to parsepp but takes advantage of C++0x's variadic
templates to provide a much small implementation than we can get with
current typelists. To work around that missing feature, parsepp uses two core
typelist-like types, called <tt>rule_list</tt> and <tt>char_list</tt>.
(Normally referred to by the names RuleList and CharList.) The upper
limit on the number of arguments is set at compile time, with some
hard upper limit (whatever was generated by the code generator and build time).
The default maximum is "some reasonable number" (e.g. 15, which "should"
be suitable for most uses of this library).


A RuleList is a pseudo-variadic type which is used like this:

<verbatim>
rule_list<Rule1,...RuleN>
</verbatim>

All types passed to a RuleList must be Rules. That is, they must
behave as described in the API documentation for the
<tt>RuleConcept</tt> class.

Only a few of the core Rules expect a RuleList. Most expect a single
Rule instead (a RuleList is not inherently a Rule). Rules which expect
a RuleList conventionally use the template typename RuleList for that
argument, whereas others conventionally use the name Rule (or
&lt;Rule1,Rule2>, or similar).

Aside from RuleList, some of the core Rules expect a list
of character literals (e.g. match an 'A' or a 'z'). This feature
is provided by the <tt>char_list</tt> pseudo-variadic type, which
is used like this:

<verbatim>
char_list<Char1,...CharN>
</verbatim>

where each Char must be a character literal or an integer constant
which is a legal character value. The framework reserves the null
character to mark end-of-input, and reserves the integer value -1
to mark the end of CharLists.


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