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POHandling literal zero in unit-safe code
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  1. POHandling literal zero in unit-safe code
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    copied!<p>With a very few exceptions (looking at you, Fahrenheit and Celsius temperature scales), units are linear, and the value zero is simultaneously the additive identity for all units at once.</p> <p>So given</p> <pre><code>auto speed = dimensioned(20, _meter/_second); auto power = dimensioned(75, _watt); </code></pre> <p>then</p> <pre><code>if (speed &lt; 6) ... if (power &gt; 17) ... </code></pre> <p>makes no more sense than</p> <pre><code>if (speed &gt; power) ... </code></pre> <p>you should write</p> <pre><code>if (speed &lt; dimensioned(6, _mile/_hour)) ... </code></pre> <p>However, this DOES make sense:</p> <pre><code>if (speed &lt; 0) </code></pre> <p>because 0 m/s == 0 mph == 0 A.U./fortnight or any other units that you care to use (for velocity). The question then is how to enable this and only this usage.</p> <p>C++11 explicit operators and contextual conversion to <code>bool</code> got rid of the need of the "safe-bool" idiom. It appears this problem can be solved with a comparable "safe-zero" idiom:</p> <pre><code>struct X { int a; friend bool operator&lt;(const X&amp; left, const X&amp; right) { return left.a &lt; right.a; } private: struct safe_zero_idiom; public: friend bool operator&lt;(const X&amp; left, safe_zero_idiom*) { return left.a &lt; 0; } }; </code></pre> <ul> <li>Demo: <a href="http://ideone.com/vUxmxi" rel="noreferrer">http://ideone.com/vUxmxi</a></li> </ul> <p>Unfortunately it seems that deployed dimension/unit libraries aren't doing this. (This question arose because I actually wanted to test whether a <code>std::chrono::duration</code> was negative). Is this useful? Are there cases that would cause it to fail? Is there any easier way to permit comparison to zero?</p> <p>One suspects that instead of implementing this for individual operators, there ought to exist an implicit conversion from literal zero to unit-tagged types.</p> <hr> <p>I do note that it allows</p> <pre><code> X{1} &lt; nullptr </code></pre> <p>as a valid expression :(, and unfortunately providing an inaccessible overload of type <code>std::nullptr_t</code> doesn't fix this, since the Standard says in section 4.10</p> <blockquote> <p>A null pointer constant of integral type can be converted to a prvalue of type <code>std::nullptr_t</code>.</p> </blockquote>
 

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