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POWhy std::sort doesn't accept Compare classes declared within a function
 

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  1. POWhy std::sort doesn't accept Compare classes declared within a function
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    <p>I was at work, writing the Comparators in a function (to move later, when I decided where was best), and noticed this peculiarity. I thought about it for a while, and realized I do not understand exactly why the code will not compile if I use the inner comparators, but the outer one is fine. </p> <p>Any explanations?</p> <p>Quick Test harness: </p> <pre><code>#include &lt;iostream&gt; #include &lt;vector&gt; #include &lt;algorithm&gt; class CompareMe { public: CompareMe(int in) : toCompare(in){} int toCompare; }; class Comparators { public: bool operator()(CompareMe * first, CompareMe * second) { return first-&gt;toCompare &lt; second-&gt;toCompare; } }; class ComparatorsOuter : public Comparators{}; int main() { class ComparatorsInner : public Comparators{}; std::vector&lt;CompareMe *&gt; compares; compares.push_back(new CompareMe(0)); compares.push_back(new CompareMe(1234)); compares.push_back(new CompareMe(163)); compares.push_back(new CompareMe(6)); compares.push_back(new CompareMe(12)); //This works, and properly sorts the array ComparatorsOuter comparator; std::sort(compares.begin(), compares.end(), comparator); //Uncomment out the sort below and it will not compile. ComparatorsInner comparatorInner; //std::sort(compares.begin(), compares.end(), comparatorInner); std::vector&lt;CompareMe *&gt;::iterator it; for(it = compares.begin(); it != compares.end(); ++it) { std::cout &lt;&lt; (*it)-&gt;toCompare &lt;&lt; std::endl; } } </code></pre> <blockquote> <p>error: no matching function for call to '<code>sort(__gnu_cxx::__normal_iterator&lt;CompareMe**, std::vector&lt;CompareMe*, std::allocator&lt;CompareMe*&gt; &gt; &gt;, __gnu_cxx::__normal_iterator&lt;CompareMe**, std::vector&lt;CompareMe*, std::allocator&lt;CompareMe*&gt; &gt; &gt;, main()::ComparitorInner&amp;)</code>' </p> </blockquote>
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    1. COWhat do you mean by `breaks down`? Perhaps you are using an older compiler?
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      1. USJesse Good
    2. COI mean, it does not compile. I will edit to clarify.
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    3. COBTW it's spelt "comparator".
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