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<DIV class="header"> <A href="../../index.html">Boost.Test</A> > <A href="../index.html">Components</A> > <A href="index.html">The
Test Tools</A> > <SPAN class="current_article">custom predicate support </SPAN></DIV>
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<H1 class="subtitle">The custom predicate support</H1>
<H2 class="first-line-indented">Synopsis</H2>
<PRE class="code"><SPAN class="reserv-word">class</SPAN> <SPAN class="new-term">extended_predicate_value</SPAN> {
<SPAN class="reserv-word">public</SPAN>:
extended_predicate_value( <SPAN class="cpp-type">bool</SPAN> predicate_value_ );
extended_predicate_value( extended_predicate_value const& rhs );
<SPAN class="cpp-type">bool</SPAN> operator!() <SPAN class="reserv-word">const</SPAN>;
<SPAN class="cpp-type">void</SPAN> operator=( <SPAN class="cpp-type">bool</SPAN> predicate_value_ );
readonly_property<bool> p_predicate_value;
boost::shared_ptr<wrap_stringstream> p_message;
};</PRE>
<H3>Description</H3>
<P class="first-line-indented">There two layers of custom predicate support
implemented by the Test Tools:</P>
<UL>
<LI>Simple predicate one or two arity without custom error message support</LI>
<LI>Arbitrary predicate with custom error message support</LI>
</UL>
<P class="first-line-indented">To use first layer use BOOST_CHECK_PREDICATE
tool. You could implement any custom check and report the result by returning
boolean value from your predicate. For detailed description of this see <A href="reference/index.html">reference</A>.</P>
<P class="first-line-indented">To
use second layer you predicate should be defined to return boost::test_tools::<SPAN class="new-term">extended_predicate_value</SPAN>.
This class encapsulate both boolean result value and error/info message
in case if result is true/false. Be aware that message returned in extended_predicate_value
will be the only one logged by the framework in case of error (actually
plus error location). Usual error notification gets skipped. </P>
<P class="first-line-indented">Usually you construct the instance of extended_predicate_value
inside your predicate and return it by value. The constructor expects one
argument - the boolean result value. The constructor is implicit, meaning
that you could simply return boolean value from your predicate - extended_predicate_value
get implicitly constructed to hold your value and empty error message. In
case if you want to update this value after construction use method <SPAN class="new-term">operator=(
bool )</SPAN>.
Would you need to check the current predicate value use method <SPAN class="new-term">operator!()</SPAN> or
directly access public read only property <SPAN class="code"><SPAN class="new-term">p_predicate_value</SPAN></SPAN>. To
create/modify error message you should use public read/write property <SPAN class="new-term">p_message</SPAN>.
p_message is a shared pointer to the <SPAN class="code">class
wrap_stringstream</SPAN>. You will need to allocate instance of it dynamically
in you predicate. It gets destroyed after error is logged. Use usual ostream
interface to create the error message.</P>
<H3>Example</H3>
<PRE class="code"><SPAN class="comment">// custom comparison predicate that only checks for lists size</SPAN>
extended_predicate_value
compare_lists( <SPAN class="cpp-type">std::list</SPAN><<SPAN class="cpp-type">int</SPAN>> <SPAN class="reserv-word">const</SPAN>& l1, <SPAN class="cpp-type">std::list</SPAN><<SPAN class="cpp-type">int</SPAN>> <SPAN class="reserv-word">const</SPAN>& l2 )
{
<SPAN class="reserv-word">if</SPAN>( l1.size() != l2.size() ) {
extended_predicate_value res( <SPAN class="reserv-word">false</SPAN> );
res.p_message.reset( <SPAN class="reserv-word">new</SPAN> boost::wrap_stringstream );
*res.p_message << <SPAN class="literal">" Different sizes ["</SPAN> << l1.size() << <SPAN class="literal">"!="</SPAN> << l2.size() << <SPAN class="literal">"]"</SPAN>;
<SPAN class="reserv-word">return</SPAN> res;
}
<SPAN class="reserv-word">return</SPAN> <SPAN class="reserv-word">true</SPAN>;
}
...
<SPAN class="cpp-type">void</SPAN> my_test_case()
{
...
BOOST_CHECK( compare_lists( l1, l2 ) );
}
</PRE>
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<P> © <A name="Copyright">Copyright</A> <A href="mailto:boost-test%20at%20emailaccount%20dot%20com%20%28please%20unobscure%29">Gennadiy
Rozental</A> 2001-2006. <BR>
Distributed under the Boost Software License, Version 1.0.
(See accompanying file <A href="../../../../../LICENSE_1_0.txt">LICENSE_1_0.txt</A> or copy at
<A href="http://www.boost.org/LICENSE_1_0.txt">www.boost.org/LICENSE_1_0.txt</A>)</P>
<P>Revised:
<!-- #BeginDate format:Sw1 -->28 February, 2006<!-- #EndDate -->
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