std::string text = "......";
char->freq

std::map< char, int > freq;

for ( char const & ch : string ) {
	++freq[ ch ];
}

freq[ '\' ]

class C {
        int i;
    public:
        C( int i = 0 ) : i(i) {}
        void Print() const { std::cout << i; }
        bool operator<( C const& arg2 ) const { return i<arg2.i; }
};

class C_pointer_comparison {
public:
	bool operator() (C* p1, C* p2) {
		return *p1 < *p2;
	}
};
std::map< C*, int, C_pointer_comparison > freq;

std::map< C*, int > freq;
by default std::map will use comparison defined by STL like this
deafult: std::less<C*>
will use
namespace std {
  template <typename T>
  struct less {
    bool operator() ( T const p1, T const  p2 ) const {
        return p1 < p2;
    }
   };
}
which will be instatiated as (syntax below is illegal)

struct std::less<C*> {
    bool operator() ( C* const p1, C* const  p2 ) const {
        return p1 < p2; // pointer comparison
    }
};


Solution:
=========
client's  code
----------- 
namespace std {         // open std namespace
  template <>           // and defined specialization for C*
  struct less<C*> {
    bool operator() ( C const * p1, C const * p2 ) const {
        return *p1 < *p2; // object comparison
    }
   };
}

std::map< C*, int > freq;