StackOverflow2013

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copied!<blockquote> <p>Have any of you tried Expected; in practice?</p> </blockquote> <p>It's quite natural, I used it even before I saw this talk.</p> <blockquote> <p>How would you apply this idiom to functions returning nothing (that is, void functions)?</p> </blockquote> <p>The form presented in the slides has some subtle implications:</p> <ul> <li>The exception is bound to the value.</li> <li>It's ok to handle the exception as you wish.</li> <li>If the value ignored for some reasons, the exception is suppressed.</li> </ul> <p>This does not hold if you have <code>expected<void></code>, because since nobody is interested in the <code>void</code> value the exception is always ignored. I would force this as I would force reading from <code>expected<T></code> in Alexandrescus class, with assertions and an explicit <code>suppress</code> member function. Rethrowing the exception from the destructor is not allowed for good reasons, so it has to be done with assertions.</p> <pre><code>template <typename T> struct expected; #ifdef NDEBUG // no asserts template <> class expected<void> { std::exception_ptr spam; public: template <typename E> expected(E const& e) : spam(std::make_exception_ptr(e)) {} expected(expected&& o) : spam(std::move(o.spam)) {} expected() : spam() {} bool valid() const { return !spam; } void get() const { if (!valid()) std::rethrow_exception(spam); } void suppress() {} }; #else // with asserts, check if return value is checked // if all assertions do succeed, the other code is also correct // note: do NOT write "assert(expected.valid());" template <> class expected<void> { std::exception_ptr spam; mutable std::atomic_bool read; // threadsafe public: template <typename E> expected(E const& e) : spam(std::make_exception_ptr(e)), read(false) {} expected(expected&& o) : spam(std::move(o.spam)), read(o.read.load()) {} expected() : spam(), read(false) {} bool valid() const { read=true; return !spam; } void get() const { if (!valid()) std::rethrow_exception(spam); } void suppress() { read=true; } ~expected() { assert(read); } }; #endif expected<void> calculate(int i) { if (!i) return std::invalid_argument("i must be non-null"); return {}; } int main() { calculate(0).suppress(); // suppressing must be explicit if (!calculate(1).valid()) return 1; calculate(5); // assert fails } </code></pre>

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