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POnon-static reference member ‘int& Property<int>::value’, can’t use default assignment operator
 

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    <p>In some C++ code, I am encountering a compile-time error that seems like it might be indicating that I am trying to reseat a reference. But I feel quite sure that I am not trying to reseat a reference. I do not understand the cause of the error. Perhaps I am being silly, and missing something obvious. Or perhaps the problem reflects deep principles of C++ template programming that I do not adequately understand. Either way, I hope some of you can help. This is the code:</p> <pre><code>// p.cpp - slimmed down demonstration of error in prop.cpp template&lt;class T&gt; class Property { protected: T&amp; value; public: explicit Property(T&amp; a) : value(a) { } // default "copy" setter virtual Property&lt;T&gt;&amp; operator=(T a) { value = a; return *this; } // default "copy" getter virtual operator T() const { return value; } template&lt;class U&gt; // must invoke U virtual getter and T virtual setter Property&lt;T&gt;&amp; operator=(const Property&lt;U&gt;&amp; newval) { return (*this = U(newval)); } /* // uncommenting this eliminates the error Property&lt;T&gt;&amp; operator=(const Property&lt;T&gt;&amp; newval) { return (*this = T(newval)); } /**/ }; int main() { //* // this code produces the error { int i_ = 10, j_; Property&lt;int&gt; i (i_), j (j_); j = i; } /**/ /* // this code does NOT produce the error { int i_ = 10; long j_; Property&lt;int&gt; i (i_); Property&lt;long&gt; j (j_); j = i; } /**/ return 0; } </code></pre> <p>When I compile this with <code>gcc (Ubuntu/Linaro 4.6.1-9ubuntu3) 4.6.1</code> using the command <code>g++ -o p p.cpp</code> I get this output:</p> <pre><code>p.cpp: In member function ‘Property&lt;int&gt;&amp; Property&lt;int&gt;::operator=(const Property&lt;int&gt;&amp;)’: p.cpp:4:7: error: non-static reference member ‘int&amp; Property&lt;int&gt;::value’, can’t use default assignment operator p.cpp: In function ‘int main()’: p.cpp:33:13: note: synthesized method ‘Property&lt;int&gt;&amp; Property&lt;int&gt;::operator=(const Property&lt;int&gt;&amp;)’ first required here </code></pre> <p>Before you tell me that I am trying to reseat a reference, please keep in mind that:</p> <ol> <li><p>Uncommenting the definition for <code>Property::operator=</code> that explicitly takes the same constructed type on the right side as the constructed type on the left side eliminates the error. So the problem is not, per se, that <code>Property&lt;int&gt;::value</code> is being accessed through <code>*this</code>.</p></li> <li><p>Independently, as shown in <code>main()</code>, the <code>operator=</code> method template inside the <code>Property</code> class template is instantiated and works fine when the data type of the value on the right side of <code>=</code> is different from the data type on the left side of <code>=</code>.</p></li> <li><p><code>value</code> is a reference, but there is nothing wrong with assigning a value to it with <code>=</code>, at least in general. It is non-<code>const</code>. So once it is instantiated (which is guaranteed, via the initialization list in <code>Property</code>'s only constructor), assigning a value to it does not attempt to reseat it, but instead assigns a new value to the memory location to which it refers.</p></li> </ol> <p>I will provide the larger file <code>prop.cpp</code> which contains (successful) unit tests for each of <code>Property</code>'s member functions, <em>if requested</em>.</p> <p>You may notice that this code is an attempt to (partially) implement "function-like" ("C#-style") properties in C++. But this question is not about whether or not it would actually be appropriate to do this a real-world C++ project, nor is it about whether or not the pattern I have chosen is the most appropriate. It's possible that I'll ask separate questions about that later. If you want to comment on or criticize my approach, I <em>am</em> interested. <strong>I only ask that you not do it here, because it would distract from the more specific purpose of this question.</strong> Instead, you could post your own question, possibly with your own answer, and post a brief comment here linking to your new question.</p>
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