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POWhat am I allowed to do with a static, constexpr, in-class initialized data member?
 

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  1. POWhat am I allowed to do with a static, constexpr, in-class initialized data member?
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    <p>This is probably a bit of an unusual question, in that it asks for a fuller explanation of a short answer given to <a href="https://stackoverflow.com/questions/14547370/confusion-about-in-class-initialization-of-static-data-members/14547498#comment20292860_14547498">another question</a> and of some aspects of the C++11 Standard related to it. </p> <p>For ease of reference, I shall sum up the referenced question here. The OP defines a class:</p> <pre><code>struct Account { static constexpr int period = 30; void foo(const int &amp;) { } void bar() { foo(period); } //no error? }; </code></pre> <p>and is wondering why he gets no error about his usage of an in-class initialized static data member (a book mentioned this to be illegal). Johannes Schaub's answer states, that:</p> <ol> <li>This violates the <strong>One Definition Rule</strong>;</li> <li>No diagnostics is required.</li> </ol> <p>As much as I rely the source and validity of this answer, I honestly dislike it because I personally find it too cryptic, so I tried to work out a more meaningful answer myself, with only partial success. Relevant seems to be § 9.4.2/4: </p> <p><em>"There shall be exactly one definition of a static data member that is <strong>odr-used</strong> (3.2) in a program; <strong>no diagnostic is required</strong>"</em> [Emphases are mine]</p> <p>Which gets me a bit closer to the point. And this is how § 3.2/2 defines an <strong>odr-used</strong> variable:</p> <p><em>"A variable whose name appears as a potentially-evaluated expression is odr-used <strong>unless</strong> it is an object that satisfies the requirements for appearing in a constant expression (5.19) <strong>and</strong> the lvalue-to-rvalue conversion (4.1) is immediately applied"</em> [Emphases are mine]</p> <p>In the OP's question, variable <code>period</code> clearly satisfies the requirements for appearing in a constant expression, being a <code>constexpr</code> variable. So the reason must be certainly found in the <em>second</em> condition: <em>"<strong>and the lvalue-to-rvalue conversion (4.1) is immediately applied</strong>"</em>.</p> <p>This is where I have troubles interpreting the Standard. <strong>What does this second condition actually mean?</strong> What are the situations it covers? Does it mean that a static <code>constexpr</code> variable is <strong>not</strong> odr-used (and therefore can be in-class initialized) if it is returned from a function? </p> <p>More generally: <strong>What are you allowed to do with a static <code>constexpr</code> variable so that you can in-class initialize it?</strong></p>
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    1. COI see that we have very [relevant yet distinct questions](http://stackoverflow.com/questions/14547923/why-passing-a-static-in-class-initialized-member-to-a-function-taking-const-refe). Both arising from the same base Q and answer :)
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    2. CO@AlokSave: indeed it seems so. At least I'm not the only one finding this whole thing a bit confusing.
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    3. COTo be honest I did a fair bit of head scratching trying to make sense of the standerdese but I couldn't really see through it clearly.You actually managed to express the thought process quite well throughout in your question.Me I made a more generic question fearing my perceptions of the standardese might actually influence someone reading it to make sense. +1 for documenting your research :)
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