Using sort within template class (C++)

Question

I would like to sort my list with the Standard algorithm std::sort.

Here is my try:

#include <list>
#include <algorithm>

template<typename T>
class MyList {
private:
    std::list<T> myList;        
public:
    void add(T item) {
        myList.push_back(item);
    }

    void mySort() {
        std::sort(myList.begin(), myList.end());
    }
};

Compile error:

    c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\bits\stl_algo.h:5475:22: note: candidates are:
In file included from c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\bits\stl_algobase.h:67:0,
                 from c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\bits\char_traits.h:39,
                 from c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\ios:40,
                 from c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\ostream:38,
                 from c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\iostream:39,
                 from ..\src\firstone.cpp:1:
c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\bits\stl_iterator.h:327:5: note: template<class _Iterator> typename std::reverse_iterator<_Iterator>::difference_type std::operator-(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_Iterator>&)
     operator-(const reverse_iterator<_Iterator>& __x,
     ^
c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\bits\stl_iterator.h:327:5: note:   template argument deduction/substitution failed:
In file included from c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\algorithm:62:0,
                 from ..\src\mylist.h:5,
                 from ..\src\firstone.cpp:2:
c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\bits\stl_algo.h:5475:22: note:   'std::_List_iterator<int>' is not derived from 'const std::reverse_iterator<_Iterator>'
     std::__lg(__last - __first) * 2);
                      ^
In file included from c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\bits\stl_algobase.h:67:0,
                 from c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\bits\char_traits.h:39,
                 from c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\ios:40,
                 from c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\ostream:38,
                 from c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\iostream:39,
                 from ..\src\firstone.cpp:1:
c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\bits\stl_iterator.h:384:5: note: template<class _IteratorL, class _IteratorR> typename std::reverse_iterator<_Iterator>::difference_type std::operator-(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_IteratorR>&)
     operator-(const reverse_iterator<_IteratorL>& __x,
     ^
c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\bits\stl_iterator.h:384:5: note:   template argument deduction/substitution failed:
In file included from c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\algorithm:62:0,
                 from ..\src\mylist.h:5,
                 from ..\src\firstone.cpp:2:
c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\bits\stl_algo.h:5475:22: note:   'std::_List_iterator<int>' is not derived from 'const std::reverse_iterator<_Iterator>'
     std::__lg(__last - __first) * 2);
                      ^

18:57:32 Build Finished (took 986ms)

What am I doing wrong?

You may use std::vector instead of std::list.

Jarod42
– Jarod42

2014-01-01 18:09:12 +00:00
Commented Jan 1, 2014 at 18:09 — Jarod42
– Jarod42, Commented Jan 1, 2014 at 18:09

ScarletAmaranth · Accepted Answer · 2014-01-01 18:18:16Z

6

You're trying to sort a list. Lists don't support random access iterators which are required for the std::sort to work. (Lists support only bidirectional iterators.)

In my humble opinion, this is faulty logic coming from the "great gods of C++".

You can use the list::sort member function to sort a list.

edited Jan 1, 2014 at 18:18

answered Jan 1, 2014 at 18:04

ScarletAmaranth

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3 Comments

juanchopanza Over a year ago

Well, the great gods of C++ provide std::list::sort. I guess std::sort could have been specialized to use that.

ScarletAmaranth Over a year ago

@juanchopanza doesn't matter when writing generic code, I would need to provide a silly specialization for lists (and forward lists for that matter) just because of a silly implementation detail that for some reason made its way to semantics of sorting

ScarletAmaranth Over a year ago

@juanchopanza container concept (or something like sortable which is actually slightly fuzzy) should surely not require a random access iterator

P0W · Accepted Answer · 2014-01-01 18:07:14Z

1

The standard algorithm std::sort requires random access iterators,std::list<>::iterators are bidirectional iterators)

Use member function std::list<>::sort to sort the list

answered Jan 1, 2014 at 18:07

P0W

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Comments

Jerry Coffin · Accepted Answer · 2014-01-01 18:09:16Z

1

You basically have two choices: either switch to using std::list's sort() member function:

myList.sort();

...or else switch to a different container type such as vector or deque, that supports std::sort by providing random access iterators.

In nearly every possible case, the latter is the correct choice; std::list is (in my opinion) nearly always a mistake.

answered Jan 1, 2014 at 18:09

Jerry Coffin

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juanchopanza · Accepted Answer · 2014-01-01 18:17:00Z

1

std::list does not have random access iterators, which is a requirement for std::sort. But you can use the std::list::sort member function instead.

Note that this assumes that you need an std::list in the first place. In my experience this is rarely the case in real code. It could well be that an std::vector would suffice.

edited Jan 1, 2014 at 18:17

answered Jan 1, 2014 at 18:07

juanchopanza

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Digital_Reality · Accepted Answer · 2014-01-01 18:10:11Z

0

Use code below:

  void mySort() 
  {
    myList.sort();
    //std::sort(myList.begin(), myList.end());
  }

answered Jan 1, 2014 at 18:10

Digital_Reality

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