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POType trait to obtain default argument promotions
 

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    <p>[Disclaimer: I know an answer to this question. I thought it might be of some general interest.]</p> <p><strong>Question:</strong> How can we have a type trait that produces the type that results from performing <em>default argument promotions</em>?</p> <p><strong>Motivation:</strong> I would like to be able to use variable arguments portably. For example:</p> <pre><code>void foo(char const * fmt, ...); // Please pass: * unsigned short // * bool // * char32_t // * unsigned char </code></pre> <p>When passing arguments to a function call without parameters, i.e. matching the ellipsis, the arguments undergo default argument promotion. So far so good, but those promotions are platform dependent. I can recover the arguments with <code>va_arg(ap, T)</code>, but what is <code>T</code>?</p> <p>Now, for some simple situations this is easy: For example, I can always say:</p> <pre><code>unsigned short n = va_args(ap, unsigned int); </code></pre> <p>The default promotion will result in either a <code>signed int</code> or an <code>unsigned int</code>, but according to, say, C11 7.16.1.1/3, va-casting to <code>unsigned int</code> is always fine, since even if the default promotion results in an <code>int</code>, the original value can be represented by both types.</p> <p>But what type should I cast to when I expect a <code>char32_t</code>? C++11 4.5/2 leaves the resulting type wide open. So I would like a trait that lets me write:</p> <pre><code>char32_t c = va_args(ap, default_promote&lt;char32_t&gt;::type); </code></pre> <p>How to do this?</p> <p>Bonus points for a trait that produces a static assertion when the parameter type must not be passed as a variable argument.</p>
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    1. COI was looking to find information about the default promotions, but couldn't find anything that mentioned it is platform-dependent (at least, explicitly). Do you mean that based on the fact that sizeof(int) is platform-dependent?
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      1. USTamás Szelei
    2. CO@fish: For example, it is platform dependent what the default promotion of `unsigned short` is.
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    3. CO@KerrekSB Is my solution too complicated? Is there a simpler one?
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      1. USScarletAmaranth
 

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