Question

I have a std::vector<std::pair<int,std::pair<Bone,std::string> > >

I’m trying to sort it with std sort, with the expectation that it is sorted by int, but instead I get 21 errors related to the sort call.

What could be wrong?

Code:

std::vector<std::pair<int,std::pair<Bone,std::string> > > tempBones;
std::sort(tempBones.begin(),tempBones.end());

Errors:

Error 1 error C2784: 'bool std::operator <(const std::basic_string<_Elem,_Traits,_Alloc> &,const _Elem *)' : could not deduce template argument for 'const std::basic_string<_Elem,_Traits,_Alloc> &' from 'const skl::Bone' c:\Program Files\Microsoft Visual Studio 9.0\VC\include\utility 102

Error 2 error C2784: 'bool std::operator <(const _Elem *,const std::basic_string<_Elem,_Traits,_Alloc> &)' : could not deduce template argument for 'const _Elem *' from 'const skl::Bone' c:\Program Files\Microsoft Visual Studio 9.0\VC\include\utility 102

Error 3 error C2784: 'bool std::operator <(const std::basic_string<_Elem,_Traits,_Alloc> &,const std::basic_string<_Elem,_Traits,_Alloc> &)' : could not deduce template argument for 'const std::basic_string<_Elem,_Traits,_Alloc> &' from 'const skl::Bone' c:\Program Files\Microsoft Visual Studio 9.0\VC\include\utility 102

Error 4 error C2784: 'bool std::operator <(const std::_Tree<_Traits> &,const std::_Tree<_Traits> &)' : could not deduce template argument for 'const std::_Tree<_Traits> &' from 'const skl::Bone' c:\Program Files\Microsoft Visual Studio 9.0\VC\include\utility 102

Error 5 error C2784: 'bool std::operator <(const std::vector<_Ty,_Alloc> &,const std::vector<_Ty,_Alloc> &)' : could not deduce template argument for 'const std::vector<_Ty,_Alloc> &' from 'const skl::Bone' c:\Program Files\Microsoft Visual Studio 9.0\VC\include\utility 102

Error 6 error C2784: 'bool std::operator <(const std::list<_Ty,_Ax> &,const std::list<_Ty,_Ax> &)' : could not deduce template argument for 'const std::list<_Ty,_Ax> &' from 'const skl::Bone' c:\Program Files\Microsoft Visual Studio 9.0\VC\include\utility 102

Error 7 error C2784: 'bool std::operator <(const std::reverse_iterator<_RanIt> &,const std::reverse_iterator<_RanIt2> &)' : could not deduce template argument for 'const std::reverse_iterator<_RanIt> &' from 'const skl::Bone' c:\Program Files\Microsoft Visual Studio 9.0\VC\include\utility 102

Error 8 error C2784: 'bool std::operator <(const std::_Revranit<_RanIt,_Base> &,const std::_Revranit<_RanIt2,_Base2> &)' : could not deduce template argument for 'const std::_Revranit<_RanIt,_Base> &' from 'const skl::Bone' c:\Program Files\Microsoft Visual Studio 9.0\VC\include\utility 102

Error 9 error C2784: 'bool std::operator <(const std::pair<_Ty1,_Ty2> &,const std::pair<_Ty1,_Ty2> &)' : could not deduce template argument for 'const std::pair<_Ty1,_Ty2> &' from 'const skl::Bone' c:\Program Files\Microsoft Visual Studio 9.0\VC\include\utility 102

Error 10 error C2676: binary '<' : 'const skl::Bone' does not define this operator or a conversion to a type acceptable to the predefined operator c:\Program Files\Microsoft Visual Studio 9.0\VC\include\utility 10

Error 11 error C2784: 'bool std::operator <(const std::basic_string<_Elem,_Traits,_Alloc> &,const _Elem *)' : could not deduce template argument for 'const std::basic_string<_Elem,_Traits,_Alloc> &' from 'const skl::Bone' c:\Program Files\Microsoft Visual Studio 9.0\VC\include\utility 103

Error 12 error C2784: 'bool std::operator <(const _Elem *,const std::basic_string<_Elem,_Traits,_Alloc> &)' : could not deduce template argument for 'const _Elem *' from 'const skl::Bone' c:\Program Files\Microsoft Visual Studio 9.0\VC\include\utility 103

Error 13 error C2784: 'bool std::operator <(const std::basic_string<_Elem,_Traits,_Alloc> &,const std::basic_string<_Elem,_Traits,_Alloc> &)' : could not deduce template argument for 'const std::basic_string<_Elem,_Traits,_Alloc> &' from 'const skl::Bone' c:\Program Files\Microsoft Visual Studio 9.0\VC\include\utility 103

Error 14 error C2784: 'bool std::operator <(const std::_Tree<_Traits> &,const std::_Tree<_Traits> &)' : could not deduce template argument for 'const std::_Tree<_Traits> &' from 'const skl::Bone' c:\Program Files\Microsoft Visual Studio 9.0\VC\include\utility 103

Error 15 error C2784: 'bool std::operator <(const std::vector<_Ty,_Alloc> &,const std::vector<_Ty,_Alloc> &)' : could not deduce template argument for 'const std::vector<_Ty,_Alloc> &' from 'const skl::Bone' c:\Program Files\Microsoft Visual Studio 9.0\VC\include\utility 103

Error 16 error C2784: 'bool std::operator <(const std::list<_Ty,_Ax> &,const std::list<_Ty,_Ax> &)' : could not deduce template argument for 'const std::list<_Ty,_Ax> &' from 'const skl::Bone' c:\Program Files\Microsoft Visual Studio 9.0\VC\include\utility 103

Error 17 error C2784: 'bool std::operator <(const std::reverse_iterator<_RanIt> &,const std::reverse_iterator<_RanIt2> &)' : could not deduce template argument for 'const std::reverse_iterator<_RanIt> &' from 'const skl::Bone' c:\Program Files\Microsoft Visual Studio 9.0\VC\include\utility 103

Error 18 error C2784: 'bool std::operator <(const std::_Revranit<_RanIt,_Base> &,const std::_Revranit<_RanIt2,_Base2> &)' : could not deduce template argument for 'const std::_Revranit<_RanIt,_Base> &' from 'const skl::Bone' c:\Program Files\Microsoft Visual Studio 9.0\VC\include\utility 103

Error 19 error C2784: 'bool std::operator <(const std::pair<_Ty1,_Ty2> &,const std::pair<_Ty1,_Ty2> &)' : could not deduce template argument for 'const std::pair<_Ty1,_Ty2> &' from 'const skl::Bone' c:\Program Files\Microsoft Visual Studio 9.0\VC\include\utility 103

Error 20 error C2676: binary '<' : 'const skl::Bone' does not define this operator or a conversion to a type acceptable to the predefined operator c:\Program Files\Microsoft Visual Studio 9.0\VC\include\utility 103

Answer 1

Why would you expect it to be sorted by int? Any container<T> is sorted by T, and sorting requires that there be a less-than comparison for T. Now for pair<S,T>, there is an operator<, defined as lexicographic ordering, but this requires comparability of both S and T:

// lexicographic compare:
bool operator<(pair<S,T> a, pair<S,T> b)
{
  return a.first < b. first || (!(b.first < a.first) && (a.second < b.second));
  //                                                    ^^^^^^^^^^^^^^^^^^^^^^^
} //                                                     compare second type!

Moving down recursively, you need Bone to be less-than comparable. Do check, any bet that you didn’t implement a comparison operator for that type!

Edit: According to your update that is precisely your problem.

Remark: You might wonder why lexicographic compare is the default: It’s the only sensible comparison for which !(x < y) && !(y < x) is identical to x == y, provided the same holds for both sub-types individually.

Update: If you only want ordering by the first member, you should write a custom comparator predicate, or use a multi-map instead.

Custom comparator:

template <typename S, typename T>
bool PairCompareOnlyFirst(const std::pair<S,T> & a, const std::pair<S,T> & b)
{
  return a.first < b.first;
}

std::sort(v.begin(), v.end(), PairCompareOnlyFirst);

Note that sort may rearrange elements with the same key. If you want those to maintain their relative ordering, use stable_sort instead.

Multimap:

typedef std::multimap<int, std::pair<Bone,std::string> > myMapType;

The multimap is a more complicated structure than the vector, and its elements are always kept in sorted order by key value, i.e. your int. Which one is preferable depends on what you’re doing.