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    <p>Current version:</p> <p>First of all, the <code>virtual</code> should have nothing to do with it. I'll bet that's a problem with MSVC, especially considering there's no difference on Clang. Anyway, your <code>twice</code> function is marked <code>const</code>. This means that members will be <code>const Weight</code> instead of <code>Weight</code>. This is a problem for <code>operator+</code> because it only accepts a non-const <code>this</code>. Therefore, the only way the compiler can go is to convert them to <code>double</code> and add those.</p> <p>Another problem is that adding <code>explicit</code> causes it to compile. In fact, this should <em>remove</em> the compiler's last resort of converting to <code>double</code>. This is indeed what happens on Clang:</p> <blockquote> <p>error: invalid operands to binary expression ('const Weight' and 'const Weight')<br> Weight w = weight_ + weight_;<br> note: candidate function not viable: 'this' argument has type 'const Weight', but method is not marked const</p> </blockquote> <p>Finally, making <code>operator+</code> <code>const</code> (or a free function) is the correct solution. When you do this, you might think you'll add back this route and thus have another error from ambiguity between this and the <code>double</code> route, but <code>Weight</code> to <code>const Weight &amp;</code> is a standard conversion, whereas <code>Weight</code> to <code>double</code> is a user-defined conversion, so the standard one is used and everything works.</p> <hr> <p>As of the updated code in the question, this is fine. The reason it won't compile is the fault of MSVC. For reference, it <a href="http://coliru.stacked-crooked.com/a/1ac9fd187279295d" rel="nofollow">does compile</a> on Clang. It also compiles on MSVC12 and the 2013 CTP.</p> <hr> <p>You may be storing the result in a <code>Foo</code>, but there is still an implicit conversion from <code>double</code> to <code>Foo</code> needed. You should return <code>Foo(value_ + other.value_)</code> in your addition operator so that the conversion is explicit. I recommend making the operator a free function as well because free operators are (almost) always at least as good as members. While I'm at it, a constructor initializer list would be a welcome change, too.</p> <p>In addition, from C++11 onward, a generally preferred choice is to make your conversion operator explicit:</p> <pre><code>explicit operator double() const {return value_;} </code></pre> <p>Also note the <code>const</code> added in because no state is being changed.</p>
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    1. CONice, and `operator+` can also be made `const`.
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    2. COChris, you are right about the return, but it seems the solution is to make `operator+` `const`. That alone removes my compilation error, without making the cast operator `explicit`.
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    3. CO@MihaiDanila, Not in Clang it doesn't. I'm willing to bet it's another small problem with MSVC.
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