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Asked: 2026-06-16T15:07:08+00:00

There are tons of codes that sort an array of strings lexicographically but I

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There are tons of codes that sort an array of strings lexicographically but I can’t find one that does sorting characters in ONE string reverse lexicographically. So far I discovered that using std::sort() in <algorithm> might be the closest candidate.

This is what I tried:

template <typename T>
class ReverseComparator{
     bool operator()(T l, T r){return !(l < r);}
};
//.....later
std::sort(str.begin(), str.end(), comparator);

Here’s the question: how can I initialize my comparator so that it can compare the characters in the string? I tried ReverseComparator<char> comparator but compiler throws a ton of error msg. gcc-4.5.1 showed these errors:

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
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:2125:4: error: within this context
prog.cpp: In function 'void std::__heap_select(_RandomAccessIterator, _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:5100:7:   instantiated from 'void std::partial_sort(_RAIter, _RAIter, _RAIter, _Compare) [with _RAIter = __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:2297:8:   instantiated from 'void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Size = int, _Compare = ReverseComparator<char>]'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:5250:4:   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
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:1914:2: error: within this context
prog.cpp: In function 'void std::__move_median_first(_Iterator, _Iterator, _Iterator, _Compare) [with _Iterator = __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:2260:7:   instantiated from '_RandomAccessIterator std::__unguarded_partition_pivot(_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:2302:62:   instantiated from 'void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Size = int, _Compare = ReverseComparator<char>]'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:5250:4:   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
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:108:7: error: within this context
prog.cpp:6:11: error: 'bool ReverseComparator<T>::operator()(T, T) [with T = char]' is private
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:110:4: error: within this context
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:2260:7:   instantiated from '_RandomAccessIterator std::__unguarded_partition_pivot(_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:2302:62:   instantiated from 'void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Size = int, _Compare = ReverseComparator<char>]'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:5250:4:   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
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:112:9: error: within this context
prog.cpp:6:11: error: 'bool ReverseComparator<T>::operator()(T, T) [with T = char]' is private
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:115:12: error: within this context
prog.cpp:6:11: error: 'bool ReverseComparator<T>::operator()(T, T) [with T = char]' is private
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:117:12: error: within this context
prog.cpp: In function '_RandomAccessIterator std::__unguarded_partition(_RandomAccessIterator, _RandomAccessIterator, const _Tp&, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Tp = char, _Compare = ReverseComparator<char>]':
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:2261:78:   instantiated from '_RandomAccessIterator std::__unguarded_partition_pivot(_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:2302:62:   instantiated from 'void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Size = int, _Compare = ReverseComparator<char>]'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:5250:4:   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
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:2229:4: error: within this context
prog.cpp:6:11: error: 'bool ReverseComparator<T>::operator()(T, T) [with T = char]' is private
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:2232:4: error: within this context
prog.cpp: In function 'void std::__unguarded_linear_insert(_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:2133:6:   instantiated from '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
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:2083:7: error: within this context
In file included from /usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:62:0,
                 from /usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/algorithm:63,
                 from prog.cpp:1:
prog.cpp: In function 'void std::__adjust_heap(_RandomAccessIterator, _Distance, _Distance, _Tp, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Distance = int, _Tp = char, _Compare = ReverseComparator<char>]':
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_heap.h:434:4:   instantiated from 'void std::make_heap(_RAIter, _RAIter, _Compare) [with _RAIter = __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:1912:7:   instantiated from 'void std::__heap_select(_RandomAccessIterator, _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:5100:7:   instantiated from 'void std::partial_sort(_RAIter, _RAIter, _RAIter, _Compare) [with _RAIter = __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:2297:8:   instantiated from 'void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Size = int, _Compare = ReverseComparator<char>]'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:5250:4:   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
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_heap.h:303:4: error: within this context
prog.cpp: In function 'void std::__push_heap(_RandomAccessIterator, _Distance, _Distance, _Tp, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Distance = int, _Tp = char, _Compare = ReverseComparator<char>]':
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_heap.h:316:7:   instantiated from 'void std::__adjust_heap(_RandomAccessIterator, _Distance, _Distance, _Tp, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Distance = int, _Tp = char, _Compare = ReverseComparator<char>]'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_heap.h:434:4:   instantiated from 'void std::make_heap(_RAIter, _RAIter, _Compare) [with _RAIter = __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:1912:7:   instantiated from 'void std::__heap_select(_RandomAccessIterator, _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:5100:7:   instantiated from 'void std::partial_sort(_RAIter, _RAIter, _RAIter, _Compare) [with _RAIter = __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:2297:8:   instantiated from 'void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<char*, std::basic_string<char> >, _Size = int, _Compare = ReverseComparator<char>]'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:5250:4:   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
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_heap.h:180:7: error: within this context
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1 Answer

  1. Editorial Team

    While solving the compiler error simply requires the operator() to be public as according to most other answers to this question, you should note that your comparator isn’t a valid strict comparator. A strict comparator has to return false for elements which are treated as being equal. You should also use const references instead of values for the parameter types of your comparator.

    I want to present you multiple options to solve your problem (not the compiler-error but the original problem, sorting a string in reversed lexical order). All of them return false for two equal elements.

    The first option is your solution but I fixed the strictness:

    template <typename T>
class ReverseComparator{
public:
     bool operator()(const T& l, const T& r){return (r < l);}
};

std::sort(str.begin(), str.end(), ReverseComparator());

    The second option uses C++11’s lambda function, but is essentially the same:

    std::sort(str.begin(), str.end(), [](const T& l, const T& r){return (r < l);});

    As a third option, as first proposed by Mooning Duck, you could simply use the std functor for operator> on your type char:

    std::sort(str.begin(), str.end(), std::greater<char>());

    The fourth option (the simplest one) would be to use the default comparator on the reversed string, as first proposed by Peter Wood (note the r!):

    std::sort(str.rbegin(), str.rend());
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