inserting an element in to a std::set causes several errors

Question

Consider the following class

#include <set>
#include <vector>
using namespace std;
class foo {
public:
  struct spatial {
    bool block;
    char status;  // H, M, Z
  };

  typedef pair< int, vector<spatial> > way;  // tag + spatial vector
  typedef set< way > one_set;


  void bar() 
  {
     way theWay;
     theWay.first = 10;

     one_set theSet;
     one_set::iterator sit = theSet.end();
     if (theSet.size() == 16) {
        sit = theSet.begin();
     }
     theSet.insert(sit, theWay);
  }
};

For the insert function, I receive these errors and I don't know what does that mean

error C2784: 'bool std::operator <(const std::vector<_Ty,_Ax> &,const std::vector<_Ty,_Ax> &)' : could not deduce template argument for 'const std::vector<_Ty,_Ax> &' from 'const foo::spatial'    c:\program files (x86)\microsoft visual studio 10.0\vc\include\xutility

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 foo::spatial'   c:\program files (x86)\microsoft visual studio 10.0\vc\include\xutility 3144

error C2784: 'bool std::operator <(const std::unique_ptr<_Ty,_Dx> &,const std::unique_ptr<_Ty2,_Dx2> &)' : could not deduce template argument for 'const std::unique_ptr<_Ty,_Dx> &' from 'const foo::spatial'  c:\program files (x86)\microsoft visual studio 10.0\vc\include\xutility 3144

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 foo::spatial'    c:\program files (x86)\microsoft visual studio 10.0\vc\include\xutility 3144

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 foo::spatial'  c:\program files (x86)\microsoft visual studio 10.0\vc\include\xutility 3144    

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 foo::spatial'    c:\program files (x86)\microsoft visual studio 10.0\vc\include\xutility 3144

error C2676: binary '<' : 'const foo::spatial' does not define this operator or a conversion to a type acceptable to the predefined operator    c:\program files (x86)\microsoft visual studio 10.0\vc\include\xutility 3144

Stuck at this point. Appreciate any help.

Answer 1

Your way type must have an operator< defined to be used in an std::set .

std::pair has operator< if both types has operator< . int has operator< , that is ok, but vector<spatial> has operator< only if its element type has operator< . Your spatial class has none and therefore your way typedef has no operator< as well.

Create an operator< for your spatial class and you're ok.

If you think that spatial classes should not be comparable but insist on having way in a set, you can also create your own comparator (a functor/lambda with two const way& parameters that return true if the first is less than the second) and pass that as the second template parameter of your set.

Anyway, because std::set stores its elements sorted, to use it you have to define the ordering of your elements at some point.

Answer 2

Depending on your use-case you can also use unordered_set . There you need a operator== and a hash-function:

#include <unordered_set>
#include <vector>
using namespace std;

class foo {
public:
  struct spatial {
    bool block;
    char status;  // H, M, Z

    bool operator== (spatial const & that) const {
       return block==that.block && status==that.status;
    }

    size_t hash() const {
       return status & static_cast<int>(block) << 9;
    }
  };

  typedef pair< int, vector<spatial> > way;  // tag + spatial vector

  struct way_hash {
    size_t operator()(const way &w) const{
      return w.first;
    }
  };

  typedef unordered_set< way, foo::way_hash > one_set;

  void bar() 
  {
     way theWay;
     theWay.first = 10;

     one_set theSet;
     one_set::iterator sit = theSet.end();
     if (theSet.size() == 16) {
        sit = theSet.begin();
     }
     theSet.insert(sit, theWay);
  }
};

int main() {
   foo f;
   f.bar();

   return 0;
}