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| <h1>Boost Exception</h1> |
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| <!-- Copyright (c) 2006-2009 Emil Dotchevski and Reverge Studios, Inc. --> |
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| <div class="RenoIncludeDIV"><div class="RenoAutoDIV"><h3>Using Virtual Inheritance in Exception Types</h3> |
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| <p>Exception types should use virtual inheritance when deriving from other exception types. This insight is due to Andrew Koenig. Using virtual inheritance prevents ambiguity problems in the exception handler:</p> |
| <pre>#include <iostream> |
| struct my_exc1 : std::exception { char const* what() const throw(); }; |
| struct my_exc2 : std::exception { char const* what() const throw(); }; |
| struct your_exc3 : my_exc1, my_exc2 {}; |
| |
| int |
| main() |
| { |
| try { throw your_exc3(); } |
| catch(std::exception const& e) {} |
| catch(...) { std::cout << "whoops!" << std::endl; } |
| }</pre> |
| <p>The program above outputs "whoops!" because the conversion to std::exception is ambiguous.</p> |
| <p>The overhead introduced by virtual inheritance is always negligible in the context of exception handling. Note that virtual bases are initialized directly by the constructor of the most-derived-type (the type passed to the throw statement, in case of exceptions.) However, typically this detail is of no concern when boost::<span class="RenoLink"><a href="exception.html">exception</a></span> is used, because it enables exception types to be trivial structs with no members (there's nothing to initialize.) See <span class="RenoLink"><a href="exception_types_as_simple_semantic_tags.html">Exception Types as Simple Semantic Tags</a></span>.</p> |
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| See also: <span class="RenoPageList"><a href="boost-exception.html">Boost Exception</a> | <a href="frequently_asked_questions.html">Frequently Asked Questions</a></span> |
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| <!-- Copyright (c) 2006-2009 Emil Dotchevski and Reverge Studios, Inc. --> |
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| <small>Copyright (c) 2006-2009 by Emil Dotchevski and Reverge Studios, Inc.<br/> |
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