友元的用法

#include <cstdio>#include <cstring>#include <cstdlib> #include <algorithm> #include <iostream>#include <vector>#include <string>using namespace std;class base;class derive;class base{public:    base():n_(0)//初始化,不是赋值    {        printf("base\n");    }    private:    int n_;    friend class derive;};class derive{public:    derive()    {        printf("derive\n");    }    void print(base b)    {        printf("%d\n", b.n_);    }};int main(int argc, char* argv[]){    derive d;    base b;    d.print(b);    return 0;}

友元声明在public和private都不影响使用