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POWhy is using a NON-decimal data type bad for money?
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  1. POWhy is using a NON-decimal data type bad for money?
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    copied!<p><strong>tl;dr:</strong> What's wrong with my <code>Cur</code> (currency) structure?</p> <p><strong>tl;dr 2:</strong> <em>Read the rest of the question please, before giving an example with <code>float</code> or <code>double</code>.</em> :-)</p> <hr> <p>I'm aware that this question has come up numerous times before all around the internet, but <em>I have not yet seen a convincing answer</em>, so I thought I'd ask again.</p> <p>I fail to understand why using a non-decimal data type is bad for handling money. (That refers to data types that store binary digits instead of decimal digits.)</p> <p>True, it's not wise to compare two <code>double</code>s with <code>a == b</code>. But you can easily say <code>a - b &lt;= EPSILON</code> or something like that.</p> <p><em>What is wrong with this approach?</em></p> <p>For instance, I just made a <code>struct</code> in C# that I believe handles money correctly, without using any decimal-based data formats:</p> <pre><code>struct Cur { private const double EPS = 0.00005; private double val; Cur(double val) { this.val = Math.Round(val, 4); } static Cur operator +(Cur a, Cur b) { return new Cur(a.val + b.val); } static Cur operator -(Cur a, Cur b) { return new Cur(a.val - b.val); } static Cur operator *(Cur a, double factor) { return new Cur(a.val * factor); } static Cur operator *(double factor, Cur a) { return new Cur(a.val * factor); } static Cur operator /(Cur a, double factor) { return new Cur(a.val / factor); } static explicit operator double(Cur c) { return Math.Round(c.val, 4); } static implicit operator Cur(double d) { return new Cur(d); } static bool operator &lt;(Cur a, Cur b) { return (a.val - b.val) &lt; -EPS; } static bool operator &gt;(Cur a, Cur b) { return (a.val - b.val) &gt; +EPS; } static bool operator &lt;=(Cur a, Cur b) { return (a.val - b.val) &lt;= +EPS; } static bool operator &gt;=(Cur a, Cur b) { return (a.val - b.val) &gt;= -EPS; } static bool operator !=(Cur a, Cur b) { return Math.Abs(a.val - b.val) &lt; EPS; } static bool operator ==(Cur a, Cur b) { return Math.Abs(a.val - b.val) &gt; EPS; } bool Equals(Cur other) { return this == other; } override int GetHashCode() { return ((double)this).GetHashCode(); } override bool Equals(object o) { return o is Cur &amp;&amp; this.Equals((Cur)o); } override string ToString() { return this.val.ToString("C4"); } } </code></pre> <p>(Sorry for changing the name <code>Currency</code> to <code>Cur</code>, for the poor variable names, for omitting the <code>public</code>, and for the bad layout; I tried to fit it all onto the screen so that you could read it without scrolling.) :)</p> <p>You can use it like:</p> <pre><code>Currency a = 2.50; Console.WriteLine(a * 2); </code></pre> <p>Of course, C# has the <code>decimal</code> data type, but that's beside the point here -- the question is about why the above is dangerous, not why we shouldn't use <code>decimal</code>.</p> <p>So would someone mind providing me with <em>a real-world counterexample</em> of a dangerous statement that would fail for this in C#? I can't think of any.</p> <p>Thanks!</p> <hr> <p>Note: I am <strong><em>not</em></strong> debating whether <code>decimal</code> is a good choice. I'm asking why a binary-based system is said to be inappropriate.</p>
 

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