What are security risks with friend functions?

Question

What are security risks with friend functions? Will it compromise encapsulation and data-hiding in C++?

I am not able to get the proper answer in-spite a lots of research. Can someone give a concrete answer with example?

Answer 1

Here is an example ,the function FUNC will destroy the protection of the data in multiple thread enviroment.

# include <windows.h>
# include <iostream>

using namespace std;

void func();

class DataAdapter
{
    friend void func();
private:
    static volatile LONG _index;
public:
    static void incr() 
    { 
        InterlockedIncrement(&_index);
    }
};

void func()
{
    DataAdapter::_index += 1;
}

DWORD WINAPI threadproc(void *pdata)
{
    pdata = pdata;

    DataAdapter::incr();

    return 0;
}

int _tmain(int argc, _TCHAR* argv[])
{  
    HANDLE hThread = CreateThread(NULL , 0 , threadproc , 0 , 0 , 0);
    WaitForSingleObject(hThread , 5000);
    return 0;
}