Trouble Passing array to a template function

[Rivaskh]

There is a template function called insertionSort defined as follows:

template < class Comparable >
void insertionSort ( vector < Comparable > & a )
{
       for ( int p = 1; p < a.size ( ); p++ )
       {
               Comparable tmp = a [ p ];
               int j;
               
               for ( j = p; j > 0 && tmp > a [ j - 1 ]; j-- )
                      a [ j ] = a [ j - 1 ];
               a [ j ] = tmp;
       }

}

There is a abstract class called shapes and several derived classes who implement the virtual functions within shape. Now if I want to sort an array of Shape Objects based on area of the objects, I cannot pass an array of Shape because this would just do insertion sort on pointers.

So I create a struct which is used to dereference pointers to Shape.

struct PtrToShape
{
       Shape *ptr;

       bool operator= ( const PtrToShape & rhs ) const
       { return *ptr < *rhs.ptr; }

       const Shape & operator* ( ) const
       { return *ptr; }
}

In the main function, then I write:

vector < PtrToShape > ary ( numShapes );
// Read into PtrToShape
insertionSort ( ary );

This gives me a warning : signed/unsigned mismatch inside insertionSort at the first for statement and a whole lot of errors after that ( probably caused due to this error ).

Now my question is: Is this warning caused because I am passing an array of PtrToShape to the template variable Comparable? Or when you pass an array to Comparable (the template variable), do you need to do anything?

I am using Visual Studio 2005.

[ropez]

The signed/unsigned warning is because vector::size returns a size_t, which is an unsigned int. You get rid of it by declaring p and j as size_t instead of int.

However the reason why it won't work is probably because PtrToShape is not comparable. Did you mean to write operator< instead of operator= in the PtrToShape class?

[ropez]

Actually, I think you need operator>, not operator<

It probably this that fails:

tmp > a [ j - 1 ]

Maybe you need this operator:

    bool operator> ( const PtrToShape & rhs ) const
    { return *ptr > *rhs.ptr; }