why I cannot access private data of a friend

the only thing wrong about the following code is friendship, once I chance the data from private to public, it compiles. so why the friend declaration did let me go through?

Code:

#include <iostream>
#include <vector>
#include <string>
#include <fstream>
#include <sstream>
using namespace std;

class READER{
class DATA {
	vector<string> rowtypes;
	vector<string> rownames;
       class rows;
       friend class rows;
};

class state {
public:
  virtual bool parse(const char* a, DATA& r) = 0;
};

class rows: public state {
public:
  bool parse(const char* a, DATA& r) {
		istringstream is(a);
		string s1, s2;

		is >> s1;
		is >> s2;
		if (is.fail())
			return false;
		r.rowtypes.push_back(s1);
		r.rownames.push_back(s2);
		return true;
	}
};
};

The READER::DATA class declared a DATA::rows class, and then declared it as a friend. What you want to do is to declare the READER::rows class as a friend. One way is to simply remove that declaration of the rows from from the DATA class definition, possibly moving it to before the DATA class definition.

By the way, fully capitalised names are conventionally reserved for macro names. You might want to use the more common conventions of lower case names, or capitalised names (my personal preference), or a C prefix, for class names.

Also, it is good that you used code tags, but please indent your code consistently.

thanks, it works!