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POCapturing reference variable by copy in C++0x lambda
 

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    <p>According to the answers and comments for <a href="https://stackoverflow.com/q/5470174/320418">this question</a>, when a reference variable is captured by value, the lambda object should make a copy of the referenced object, not the reference itself. However, GCC doesn't seem to do this.</p> <p>Using the following test: </p> <pre class="lang-c++ prettyprint-override"><code>#include &lt;stddef.h&gt; #include &lt;iostream&gt; using std::cout; using std::endl; int main(int argc, char** argv) { int i = 10; int&amp; ir = i; [=] { cout &lt;&lt; "value capture" &lt;&lt; endl &lt;&lt; "i: " &lt;&lt; i &lt;&lt; endl &lt;&lt; "ir: " &lt;&lt; ir &lt;&lt; endl &lt;&lt; "&amp;i: " &lt;&lt; &amp;i &lt;&lt; endl &lt;&lt; "&amp;ir: " &lt;&lt; &amp;ir &lt;&lt; endl &lt;&lt; endl; }(); [&amp;] { cout &lt;&lt; "reference capture" &lt;&lt; endl &lt;&lt; "i: " &lt;&lt; i &lt;&lt; endl &lt;&lt; "ir: " &lt;&lt; ir &lt;&lt; endl &lt;&lt; "&amp;i: " &lt;&lt; &amp;i &lt;&lt; endl &lt;&lt; "&amp;ir: " &lt;&lt; &amp;ir &lt;&lt; endl &lt;&lt; endl; }(); return EXIT_SUCCESS; } </code></pre> <p>Compiling with GCC 4.5.1, using <code>-std=c++0x</code>, and running gives the following output:</p> <pre class="lang-none prettyprint-override"><code>value capture i: 10 ir: -226727748 &amp;i: 0x7ffff27c68a0 &amp;ir: 0x7ffff27c68a4 reference capture i: 10 ir: 10 &amp;i: 0x7ffff27c68bc &amp;ir: 0x7ffff27c68bc </code></pre> <p>When captured by copy, <code>ir</code> just references junk data. But it correctly references <code>i</code> when captured by reference.</p> <p>Is this a bug in GCC? If so, does anyone know if a later version fixes it? What is the correct behavior?</p> <h3>EDIT</h3> <p>If the first lambda function is changed to</p> <pre class="lang-c++ prettyprint-override"><code>[i, ir] { cout &lt;&lt; "explicit value capture" &lt;&lt; endl &lt;&lt; "i: " &lt;&lt; i &lt;&lt; endl &lt;&lt; "ir: " &lt;&lt; ir &lt;&lt; endl &lt;&lt; "&amp;i: " &lt;&lt; &amp;i &lt;&lt; endl &lt;&lt; "&amp;ir: " &lt;&lt; &amp;ir &lt;&lt; endl &lt;&lt; endl; }(); </code></pre> <p>then the output looks correct:</p> <pre class="lang-none prettyprint-override"><code>explicit value capture i: 10 ir: 10 &amp;i: 0x7fff0a5b5790 &amp;ir: 0x7fff0a5b5794 </code></pre> <p>This looks more and more like a bug.</p>
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    1. COIntel 11.1 behaves as expected (prints 10). GCC appears to still have the bug in 4.6.0 and 4.6.1, but 4.7.0-20110618 fails to compile the program with an error about "non-trivial conversion in assignment", they must be working on that.
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    2. COCurious off-topic: Why would you ever capture "everything" `=` rather than specifically say what you want? Wouldn't that be more efficient?
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    3. CO@Kerrek: As long as the same type of capture is used for all variables, capturing everything and specifically listing names should be equivalent, shouldn't they? From what I understand, only the used variables from the immediate enclosing scope are captured.
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