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POC++: force the compiler to use one of two competing operators
 

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  1. POC++: force the compiler to use one of two competing operators
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    <p>I've been playing with C++ recently, and I just stumbled upon an interesting precedence issue. I have one class with two operators: "cast to double" and "+". Like so:</p> <pre><code>class Weight { double value_; public: explicit Weight(double value) : value_(value) {} operator double() const { return value_; } Weight operator+(const Weight&amp; other) { return Weight(value_ + other.value_); } }; </code></pre> <p>When I try to add two instances of this class...</p> <pre><code>class Weighted { Weight weight_; public: Weighted(const Weight&amp; weight) : weight_(weight) {} virtual Weighted twice() const { Weight w = weight_ + weight_; return Weighted(w); } }; </code></pre> <p>...something unexpected happens: the compiler sees the "+" sign and casts the two <code>weight_</code>s to <code>double</code>. It then spits out a compilation error, because it can't implicitly cast the resulting double back to a <code>Weight</code> object, due to my <code>explicit</code> one-argument constructor.</p> <p>The question: how can I tell the compiler to use my own <code>Weight::operator+</code> to add the two objects, and to ignore the cast operator for this expression? Preferably without calling <code>weight_.operator+(weight_)</code>, which defeats the purpose.</p> <hr> <p><strong>Update</strong>: Many thanks to chris for pointing out that the compiler is right not to use my class's <code>operator+</code> because that operator is not <code>const</code> and the objects that are being <code>+</code>ed are <code>const</code>.</p> <p>I now know of three ways to fix the above in VS2012. Do see the accepted answer from chris for additional information.</p> <ol> <li>Add the <code>explicit</code> qualifier to <code>Weight::operator double()</code>. This doesn't work in VS 2012 (no support), but it stands to reason that it's a good solution for compilers that do accept this approach (from the accepted answer).</li> <li>Remove the <code>virtual</code> qualifier from method <code>Weighted::twice</code>, but don't ask me why this works in VS.</li> <li>Add the <code>const</code> qualifier to method <code>Weight::operator+</code> (from the accepted answer).</li> </ol>
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    1. USMihai Danila
    1. USMihai Danila
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    1. COYou could make your conversion operator explicit.
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      1. POC++: force the compiler to use one of two competing operators
      1. USchris
    2. COOK, a good idea, and it's probably OK to explicitly cast to `double` when I need one anyway. Why don't you make this into an answer? Nonetheless, I am having some troubles marking the operator explicit (the compiler now cries "illegal storage class"). Let me look into this a little closer.
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      1. POC++: force the compiler to use one of two competing operators
      1. USMihai Danila
    3. CO@MihaiDanila: `explicit` for conversion operators was added in C++11. Depending on the compiler/version you're using, it may not be supported at all, or you may have to give a flag to tell it to support C++11.
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      1. POC++: force the compiler to use one of two competing operators
      1. USJerry Coffin
 

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