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POInitializing mutually-referencing objects
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copied!<p>Consider the following pair of mutually referencing types:</p> <pre><code>struct A; struct B { A& a; }; struct A { B& b; }; </code></pre> <p>This can be initialized with aggregate initialization in GCC, Clang, Intel, MSVC, but not SunPro which insists that user-defined ctors are required.</p> <pre><code>struct {A first; B second;} pair = {pair.second, pair.first}; </code></pre> <p>Is this initialization legal? </p> <p>slightly more elaborate demo: <a href="http://ideone.com/P4XFw" rel="nofollow noreferrer">http://ideone.com/P4XFw</a></p> <p>Now, heeding Sun's warning, what about classes with user-defined constructors? The following works in GCC, clang, Intel, SunPro, and MSVC, but is it legal?</p> <pre><code>struct A; struct B { A& ref; B(A& a) : ref(a) {} }; struct A { B& ref; A(B& b) : ref(b) {} }; struct {B first; A second;} pair = {pair.second, pair.first}; </code></pre> <p>demo: <a href="http://ideone.com/QQEpA" rel="nofollow noreferrer">http://ideone.com/QQEpA</a></p> <p>And finally, what if the container is not trivial either, e.g. (works in G++, Intel, Clang (with warnings), but not MSVC ("pair" unknown in initializer) or SunPro ("pair is not a structure")</p> <pre><code>std::pair<A, B> pair(pair.second, pair.first); </code></pre> <p>From what I can see, <code>§3.8[basic.life]/6</code> forbids access to a non-static data member before lifetime begins, but is lvalue evaluation of pair.second "access" to second? If it is, then are all three initializations illegal? Also, <code>§8.3.2[dcl.ref]/5</code> says "reference shall be initialized to refer to a valid object" which probably makes all three illegal as well, but perhaps I'm missing something and the compilers accept this for a reason.</p> <p>PS: I realize these classes are not practical in any way, hence the language-lawyer tag. Related and marginally more practical old discussion here: <a href="https://stackoverflow.com/questions/1329223/circular-reference-in-c-without-pointers/1329257#1329257">Circular reference in C++ without pointers</a></p>

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