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