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copied!<p>Usually, only the first error is relevant, so we have this:</p> <pre><code>In file included from /usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/algorithm:63:0, from prog.cpp:1: prog.cpp: In function 'void std::__insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]': /usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:2192:4: instantiated from 'void std::__final_insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]' /usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:5252:4: instantiated from 'void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Compare = ReverseComparator<char>]' prog.cpp:12:49: instantiated from here prog.cpp:6:11: error: 'bool ReverseComparator<T>::operator()(T, T) [with T = char]' is private </code></pre> <p>Most of that is simply context, so here's the key part:</p> <pre><code>'bool ReverseComparator<T>::operator()(T, T) [with T = char]' is private </code></pre> <p>Well that's easy to fix:</p> <pre><code>#include <algorithm> #include <string> template <typename T> class ReverseComparator{ public: //forgot this bool operator()(T l, T r){return !(l < r);} }; int main() { std::string str = "HELLO"; ReverseComparator<char> comparator; std::sort(str.begin(), str.end(), comparator); } </code></pre> <p><a href="http://ideone.com/KWtasS" rel="nofollow noreferrer">This compiles fine</a>, however, sorts are based on what's called a "strict weak ordering", and <code>!(l < r)</code> is <em>not</em> a strict weak ordering. Namely, <code>comparitor(a,a)</code> should return <code>false</code>, but yours returns <code>true</code>. Luckily, the idea of sorting this way is already in the standard library under the name <code>std::greater</code>. (<code>std::sort</code> without a comparitor uses <code>std::less</code> by default)</p>

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