如何为私有(private)类(class)定义 friend ?
#include <iostream>
class Base_t{
private:
struct Priv_t{
friend std::ostream & operator<<(std::ostream &os, const Priv_t& obj);
} p;
friend std::ostream & operator<<(std::ostream &os, const Base_t& obj);
};
std::ostream & operator<<(std::ostream &os, const Base_t& obj) {
return os << "base: " << obj.p;
}
std::ostream & operator<<(std::ostream &os, const Base_t::Priv_t& obj) {
return os << "priv";
}
int main() {
Base_t b;
std::cout << b << std::endl;
}
错误:
:!make t17 |& tee /tmp/vB5G5ID/54
g++ t17.cpp -o t17
t17.cpp: In function 'std::ostream& operator<<(std::ostream&, const Base_t::Priv_t&)':
t17.cpp:5:16: error: 'struct Base_t::Priv_t' is private
struct Priv_t{
^
t17.cpp:15:59: error: within this context
std::ostream & operator<<(std::ostream &os, const Base_t::Priv_t& obj) {
^
make: *** [t17] Error 1
shell returned 2
当我直接在 Priv_t 中定义 friend 时它有效
friend std::ostream & operator<<(std::ostream &os, const Priv_t& obj) { return os << "priv"; }
如何在类/结构定义之外做到这一点?
最佳答案
虽然Priv_t
是私有(private)声明,但你应该移动
friend std::ostream & operator<<(std::ostream &os, const Base_t::Priv_t& obj);
进入Base_t
:
class Base_t
{
private:
struct Priv_t
{
} p;
friend std::ostream & operator<<(std::ostream &os, const Base_t& obj);
friend std::ostream & operator<<(std::ostream &os, const Base_t::Priv_t& obj);
};
关于c++ - 私有(private)类的 friend ,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/23626045/