STL Map Iteration Question

[TSYS]

I have an STL map that uses a structure as a key:

typedef struct _RC_ {
  UINT  R;
  UINT  C;
} RC;

typedef struct _Data_ {
  int  data1;
  int  data2;
} DATA;
 
struct _RC_Compare {
  bool operator() ( const RC p1, const RC p2 ) const
  {
    return p1.R == p2.R? p1.C < p2.C : p1.R < p2.R;
  }
typedef std::map< RC, DATA, _RC_Compare >  MY_MAP

I want to access every member of the map without regard to the key value, so I tried:

MY_MAP::iterator  it;
for( it = MY_MAP.begin(); it != MY_MAP.end(); ++it )
{
  // Bunch of code having nothing to do with the map
}

When this executes, it termiantes with the message "map/set iterator not incrementable" when it tries to execute the "++it".

Why can't I sequentially access a map? Everything I've read indicates this is possible, although all the examples have C++ primitives as keys.