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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html><head><title> Concepts and External Concepts </title><meta http-equiv="Content-Type"content="text/html; charset=iso-8859-1"></head> <body><table ><tr ><td ><img src="../../../../boost.png" width="100%" border="0"></td><td ><h1 >Concepts and External Concepts</h1></td></tr></table><p >Generic programming in C++ is characterized by the use of function and class templates where
the template parameter(s) must satisfy certain requirements.Often these
requirements are so important that we give them a name: we call
such a set of type requirements a <b>concept</b>. We say that a type <i>
conforms to a concept</i> or that it <i>is a model of a concept</i> if it
satisfies all of those requirements. The concept can be specified as a set
of member functions with well-defined semantics
and a set of nested typedefs with well-defined properties.</p><p >Often it much more flexible to provide free-standing functions and typedefs
which provides the exact same semantics (but a different syntax) as
specified
by the concept. This allows generic code to treat different types <i> as if
</i> they fulfilled the concept. In this case we say that the concept has
been <b> externalized </b> or that the new requirements constitutes an <b>external
concept </b>. We say that a type <i> conforms to an external concept </i>
or that it <i> is a model of an external concept </i>. A concept may exist
without a corresponding external concept and conversely.</p><p >Whenever a concept specifies a member function, the corresponding external
concept
must specify a free-standing function of the same name, same return type and
the same argument list except there is an extra first argument which must
be of the type (or a reference to that type) that is to fulfill the external
concept. If the corresonding member function has any cv-qulifiers, the
first argument must have the same cv-qualifiers. Whenever a concept
specifies a nested typedef, the corresponding external concept
specifies a <b>type-generator</b>, that is, a type with a nested typedef
named <code>type</code>. The type-generator has the name as the nested typedef with
<code>_of</code> appended.
The converse relationship of an external concept and its corresponding concept
also holds.</p><p ><b ><i >Example:</i></b></p><p >A type <code>T</code> fulfills the FooConcept if it
has the follwing public members:</p><code> void T::foo( int ) const; <br>
int T::bar(); <br>
typedef <i>implementation defined </i> foo_type;</code><p >The corresponding external concept is the ExternalFooConcept.</p><p >A type <code>T</code> fullfills the ExternalFooConcept if these
free-standing functions and type-generators exists:</p><code>void foo( const T&, int ); <br>
int bar( T& ); <br>
foo_type_of< T >::type;</code> <br> <br><hr size="1" ><h3 >Literature</h3><ul ><li > <a href="http://www.boost.org/more/generic_programming.html#type_generator" target="_self" >Type Generators</a> </li><li > <a href="http://www.boost.org/more/generic_programming.html#concept" target="_self" >Concepts</a> </li><li > <a href="http://www.sgi.com/tech/stl/stl_introduction.html" target="_self" >Concepts and SGI STL</a> </li></ul><hr size="1" ><p >© Thorsten Ottosen 2003-2004 (nesotto_AT_cs.auc.dk).
Permission to copy, use, modify, sell and distribute this software is granted provided this copyright notice appears
in all copies. This software is provided "as is" without express or implied warranty, and with no
claim as to its suitability for any purpose.</p><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br></body></html>
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