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POWhat are the rules regarding initialization of non-local statics?
 

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  1. POWhat are the rules regarding initialization of non-local statics?
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    <p>Suppose I have a class whose only purpose is the side-effects caused during construction of its objects (e.g., registering a class with a factory):</p> <pre><code>class SideEffectCauser { public: SideEffectCauser() { /* code causing side-effects */ } }; </code></pre> <p>Also suppose I'd like to have an object create such side-effects once for each of several translation units. For each such translation unit, I'd like to be able to just put an a <code>SideEffectCauser</code> object at namespace scope in the .cpp file, e.g.,</p> <pre><code>SideEffectCauser dummyGlobal; </code></pre> <p>but 3.6.2/3 of the C++03 standard suggests that this object need not be constructed at all unless an object or function in the .cpp file is used, and articles such as <a href="http://accu.org/index.php/journals/597" rel="nofollow noreferrer">this</a> and online discussions such as <a href="http://www.gamedev.net/topic/612598-self-registering-factory-in-c/" rel="nofollow noreferrer">this</a> suggest that such objects are sometimes not initialized. </p> <p>On the other hand, <a href="https://stackoverflow.com/questions/582331/is-there-a-way-to-instantiate-objects-from-a-string-holding-their-class-name/582456#582456">Is there a way to instantiate objects from a string holding their class name? </a> has a solution that is claimed to work, and I note that it's based on using an object of a type like <code>SideEffectCauser</code> as a static data member, not as a global, e.g., </p> <pre><code>class Holder { static SideEffectHolder dummyInClass; }; SideEffectHolder Holder::dummyInClass; </code></pre> <p>Both <code>dummyGlobal</code> and <code>dummyInClass</code> are non-local statics, but a closer look at 3.6.2/3 of the C++03 standard shows that that passage applies only to objects at namespace scope. I can't actually find anything in the C++03 standard that says when non-local statics at class scope are dynamically initialized, though 9.4.2/7 suggests that the same rules apply to them as to non-local statics at namespace scope.</p> <p>Question 1: In C++03, is there any reason to believe that <code>dummyInClass</code> is any more likely to be initialized than <code>dummyGlobal</code>? Or may both go uninitialized if no functions or objects in the same translation unit are used?</p> <p>Question 2: Does anything change in C++11? The wording in 3.6.2 and 9.4.2 is not the same as the C++03 versions, but, from what I can tell, there is no behavioral difference specified for the scenarios I describe above.</p> <p>Question 3: Is there a reliable way to use objects of a class like <code>SideEffectHolder</code> outside a function body to force side-effects to take place?</p>
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      1. POWhat are the rules regarding initialization of non-local statics?
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    1. COI think we discussed this some time ago, and neither C++03 nor C++11 strictly require that the statics are constructed if no function in the TU is called, but no compiler would exploit that because it would break everyone's code...
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      1. POWhat are the rules regarding initialization of non-local statics?
      1. USKerrek SB
    2. COTo answer your first question: In C++03, the [Schwarz counter](http://en.wikibooks.org/wiki/More_C++_Idioms/Nifty_Counter) will guarantee that static classes, `dummyInClass` and `dummyGlobal` are initialized, whether or not their objects are called. This approach removes any uncertainty relating to initialization or its absence.
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      1. POWhat are the rules regarding initialization of non-local statics?
      1. USdamienh
    3. CO@KerrekSB: Unfortunately, my question is prompted by the experience of a colleague who finds that with static linking, everything works, but with dynamic linking, it does not. One would like to think that when a TU is brought in via dynamic linking, the non-local statics in that TU would be dynamically initialized, but this does not seem to be happening, because nothing in that TU is used (and won't be until the non-local statics have their side-effects).
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