Question

i am using different classes and also objects relavent to those different classes. Now, i want to write a seperate function which use to joint two vector to another vector. where should i write these type of seperate functions. Can i use a new header file for that? actually, i made a new header file (ConnectedSegment.h) and put my function under that. but, i got this error.

43 D:myex\ConnectedSegment.h non-member function `std::vector<int>& NeighboursToGivenValue(std::map<int, std::vector<int> >&, int, int)' cannot have `const' method qualifier 

D:\myex\Detects\Makefile.win [Build Error]  [Detects.o] Error 1 

Here is my code of the function:

vector<int> & NeighboursToGivenValue(map<int, vector<int> > &adjacency, 
                                              int s1, int s2) const
{
  vector<int>::const_iterator any;
  vector<int>::iterator is_any;
  vector<int> *neighbours2both = new vector<int>;
  int i;

  neighbours2both->reserve(adjacency[s1].size() + adjacency[s2].size());
  neighbours2both->insert(neighbours2both->end(), adjacency[s1].begin(),adjacency[s1].end ());

  vector<int>& neighbours2s2=adjacency[s2];        
  for (any=neighbours2s2.begin(); any!=neighbours2s2.end(); any++){            
       is_any = find (neighbours2both->begin(), neighbours2both->end(), *any);           
       if(is_any == neighbours2both->end()){
          if(s1 != *any) neighbours2both->push_back(*any);
       }
  }
  for (i=0; i<neighbours2both->size(); i++){
       neighbours2both->erase(neighbours2both->begin() + i);
  }
  return(*neighbours2both);
}

In here, I got the values for the adjacency() by using another class called MyPoint.
So I used myxyz.adjacency() to accomadate the values for this adjacency. Now I dont want to call the same class MyPoint for calling function NeighboursToGivenValue. So,
could you tell me where I should write this function. Alternatively, if I write this function inside the MyPoint class, how can I call this function without an object of that class.

Answer 1

These free functions can be in the header (will need to be declared inline as Pedro highlights in his comment) – or they can be declared in the header and defined in an implementation file – for example,

#header

vector<int>& join_int_vector(vector<int> const& lhs, vector<int> const& rhs);

# cpp

vector<int>& join_int_vector(vector<int> const& lhs, vector<int> const& rhs)
{
 // do stuff
}

The problem you face is that you can only have const member functions – i.e. free functions (and static functions) cannot be const – it doesn’t mean anything.

btw. don’t construct a vector dynamically and return a reference to it, i.e:

vector<int> *neighbours2both = new vector<int>;
:
return (*neighbours2both);

The callee has no awareness that this object needs to be manually cleaned up – in this case, return by value.

vector<int> join_int_vector(vector<int> const& lhs, vector<int> const& rhs)
{
  vector<int> neighbours2both;
  :
  return neighbours2both;
}

EDIT: for functions defined in the header itself, something like this:

inline vector<int> join_int_vector(vector<int> const& lhs, vector<int> const& rhs)
{
  vector<int> neighbours2both;
  :
  return neighbours2both;
}

note the use of the keyword inline – this prevents multiple definition errors if this header is included in multiple translation units.