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POWhy do C++ templates let me circumvent incomplete types (forward declarations)?
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  1. POWhy do C++ templates let me circumvent incomplete types (forward declarations)?
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    copied!<p>I tried three iterations of the following simple program. This is a highly simplified attempt to write a container-and-iterator pair of classes, but I was running into issues with incomplete types (forward declarations). I discovered that this was in fact possible once I templatized everything - but only if I actually used the template parameter! (I realized this by looking at the <a href="http://code.google.com/p/google-sparsehash/source/browse/trunk/src/google/sparsehash/densehashtable.h" rel="nofollow noreferrer">Google sparsetable code</a>.)</p> <p>Any hints explaining why the second works while the third doesn't? (I know why the first one doesn't work - the compiler needs to know the memory layout of the container.)</p> <p>Thanks in advance.</p> <pre><code>// This doesn't work: invalid use of incomplete type. #if 0 struct container; struct iter { container &amp;c; int *p; iter(container &amp;c) : c(c), p(&amp;c.value()) {} }; struct container { int x; int &amp;value() { return x; } iter begin() { return iter(*this); } }; int main() { container c; c.begin(); return 0; } #endif // This *does* work. template&lt;typename T&gt; struct container; template&lt;typename T&gt; struct iter { container&lt;T&gt; &amp;c; T *p; iter(container&lt;T&gt; &amp;c) : c(c), p(&amp;c.value()) {} }; template&lt;typename T&gt; struct container { T x; T &amp;value() { return x; } iter&lt;T&gt; begin() { return iter&lt;T&gt;(*this); } }; int main() { container&lt;int&gt; c; c.begin(); return 0; }; // This doesn't work either. #if 0 template&lt;typename T&gt; struct container; template&lt;typename T&gt; struct iter { container&lt;int&gt; &amp;c; int *p; iter(container&lt;int&gt; &amp;c) : c(c), p(&amp;c.value()) {} }; template&lt;typename T&gt; struct container { int x; int &amp;value() { return x; } iter&lt;int&gt; begin() { return iter&lt;int&gt;(*this); } }; int main() { container&lt;int&gt; c; c.begin(); return 0; } #endif </code></pre>
 

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