ition_failure</a></code></code>). This functor should capture variables by (constant) references (to access the values they will have at function exit). This functor must be a nullary functor if <code class="computeroutput">VirtualResult</code> is <code class="computeroutput">void</code>, otherwise it must be a unary functor accepting the return value as a parameter of type <code class="computeroutput">VirtualResult const&amp;</code> (to avoid extra copies of the return value, or of type <code class="comput