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POStrange behavior of copy-initialization, doesn't call the copy-constructor!
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copied!<p>I was reading the difference between direct-initialization and copy-initialization (§8.5/12):</p> <pre><code>T x(a); //direct-initialization T y = a; //copy-initialization </code></pre> <p>What I understand from reading about <a href="https://stackoverflow.com/questions/1051379/is-there-a-difference-in-c-between-copy-initialization-and-direct-initializatio">copy-initialization</a> is that it needs <a href="https://stackoverflow.com/questions/4293596/when-should-you-use-direct-initialization-and-when-copy-initialization">accessible & non-explicit copy-constructor</a>, or else the program wouldn't compile. I verified it by writing the following code:</p> <pre><code>struct A { int i; A(int i) : i(i) { std::cout << " A(int i)" << std::endl; } private: A(const A &a) { std::cout << " A(const A &)" << std::endl; } }; int main() { A a = 10; //error - copy-ctor is private! } </code></pre> <p>GCC gives an error (<a href="http://www.ideone.com/fB9K5" rel="nofollow noreferrer">ideone</a>) saying:</p> <blockquote> <p>prog.cpp:8: error: ‘A::A(const A&)’ is private</p> </blockquote> <p>So far everything is fine, <a href="http://www.gotw.ca/gotw/036.htm" rel="nofollow noreferrer">reaffirming what Herb Sutter says</a>,</p> <blockquote> <p>Copy initialization means the object is initialized <strong>using the copy constructor</strong>, after first calling a user-defined conversion if necessary, and is equivalent to the form "T t = u;":</p> </blockquote> <hr> <p>After that I made the copy-ctor accessible by commenting the <code>private</code> keyword. Now, naturally I would expect the following to get printed:</p> <blockquote> <p>A(const A&)</p> </blockquote> <p>But to my surprise, it prints this instead (<a href="http://www.ideone.com/oEXUT" rel="nofollow noreferrer">ideone</a>):</p> <blockquote> <p>A(int i)</p> </blockquote> <p>Why?</p> <p>Alright, I understand that first a temporary object of type <code>A</code> is created out of <code>10</code> which is <code>int</code> type, by using <code>A(int i)</code>, applying the conversion rule as its needed here (§8.5/14), and then it was supposed to call copy-ctor to initialize <code>a</code>. But it didn't. Why? </p> <p>If an implementation is permitted to eliminate the need to call copy-constructor (§8.5/14), then why is it not accepting the code when the copy-constructor is declared <code>private</code>? After all, its not calling it. Its like a spoiled kid who first irritatingly asks for a <em>specific</em> toy, and when you give him one, the <em>specific</em> one, he throws it away, behind your back. :|</p> <p>Could this behavior be dangerous? I mean, I might do some other <em>useful</em> thing in the copy-ctor, but if it doesn't call it, then does it not alter the behavior of the program?</p>

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