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POC++ and the const reference to temporary binding issue (Implementing the D language pass by value and by reference rules in C++0X)
 

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  1. POC++ and the const reference to temporary binding issue (Implementing the D language pass by value and by reference rules in C++0X)
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    <p>I was wondering to what extend it's possible to mimic the D language rules for pass by value and pass by reference rules in C++. For background please see the following two references (mostly Alexandrescu):</p> <p><a href="http://bartoszmilewski.wordpress.com/category/d-programming-language/page/2/" rel="nofollow">http://bartoszmilewski.wordpress.com/category/d-programming-language/page/2/</a></p> <p>and</p> <p><a href="http://groups.google.com/group/comp.std.c++/msg/303e3bf2407a7609" rel="nofollow">http://groups.google.com/group/comp.std.c++/msg/303e3bf2407a7609</a>?</p> <p>One of the key differences is that in D const references does not bind (as non const ones) to temporaries.</p> <p>However, I am not aware of any way to define a generic class X in such a way that the following code will fail to compile:</p> <pre><code>void f(const X&amp; x) {...} f( X() ); //Cannot disable binding of const ref to X </code></pre> <p>One possibility could be to make f a template function, check rvalue/lvalue-ness of the argument passed (possible in C++0X) and use disable_if but this clutters the code too much and does not scale well.</p> <p>Another possibility could be to introduce a template class like</p> <pre><code>template&lt;class T&gt; Ref&lt;T&gt; : public T {...} //D-style ref, does not bind to temporaries! </code></pre> <p>and then use </p> <pre><code>void f(Ref&lt;const X&gt; x) {...} //Does not look bad.... f( X() ); //Compile error here is doable, I checked a similar example already... </code></pre> <p>However, this way I loose the ability to write template functions taking Ref as the following will not compile...</p> <pre><code>template&lt;class T&gt; void ft(Ref&lt;const T&gt; x) {...} ft( X() ); //Template deduction error </code></pre> <p>What are your thoughts? Any suggestion/comment/help is appreciated!</p>
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    1. COMy thought: if you like D then use it!
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      1. USCrazy Eddie
    2. COI probably would, but practical considerations make this not an option at this time :)
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    3. COinteresting question. I'd really like to be able to disable this behavior too. It's created a number of bugs when references to those "bound" temporaries have been returned, and some of them cannot be detected by anything else than whole program analysis...
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      1. USMatthieu M.
 

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