<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html><head><title> Concepts and External Concepts </title><metahttp-equiv="Content-Type"content="text/html; charset=iso-8859-1"></head><body><table><tr><td><imgsrc="../../../../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>
<br>Use, modification and distribution is subject to the Boost
Software License, Version 1.0. (See accompanying file
<codeclass="filename">LICENSE_1_0.txt</code> or copy at <ahref="http://www.boost.org/LICENSE_1_0.txt"target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
