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POWhy does this code call different template function in vs2005?
 

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  1. POWhy does this code call different template function in vs2005?
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    <p>The code is:</p> <pre><code>#include &lt;iostream&gt; using namespace std; // compares two objects template &lt;typename T&gt; void compare(const T&amp;, const T&amp;){ cout&lt;&lt;"T"&lt;&lt;endl; }; // compares elements in two sequences template &lt;class U, class V&gt; void compare(U, U, V){ cout&lt;&lt;"UV"&lt;&lt;endl; }; // plain functions to handle C-style character strings void compare(const char*, const char*){ cout&lt;&lt;"ordinary"&lt;&lt;endl; }; int main() { cout&lt;&lt;"-------------------------char* --------------------------"&lt;&lt; endl; char* c="a"; char* d="b"; compare(c,d); cout&lt;&lt;"------------------------- char [2]---------------------------"&lt;&lt; endl; char e[]= "a"; char f[]="b"; compare(e,f); system("pause"); } </code></pre> <p>The result is:</p> <blockquote> <p>-------------------------char* --------------------------</p> <p>T</p> <p>------------------------- char [2]-----------------------</p> <p>ordinary</p> </blockquote> <p>And my question is: Why does compare(c,d) call compare(const T&amp;, const T&amp;) and compare(e,f) call the ordinary function even though the arguments of the two functions are char*s?</p>
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    1. COThe correct behavior is to call `T` in both cases, as they become perfect matches. `const char*` would be preferred if it was a perfect match to either, but it's not. Anything else is non-conforming, and you'll have to consult your compiler's documentation or chalk it up as a bug. *I would recommend using a newer compiler.*
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      1. USGManNickG
    2. CO@GManNickG: I disagree: the array-to-pointer conversion and qualification conversion both yield an exact match, so the nontemplate should be preferred in both cases. The latest versions of Clang and g++ both say I am wrong, though, so perhaps I am missing something. (I do agree with your advice to use a newer compiler, though Visual C++ 2012 exhibits the same behavior in this case.)
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      1. USJames McNellis
    3. CO@James et al, I've updated my answer based on more info in the standard (regarding selecting best viable function where ranks are equal) - the extra stuff has to do with whether one conversion sequence is a subset of another (the subset is then preferred). So there's no preference between array-to-pointer and qualification, but there IS between array-to-pointer/qualification and qualification (the latter is preferred). Crikey, this has been an education on the C++ standard for me :-)
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