From f1e2b3fbef84caa6943cde627e1fd101d8741e25 Mon Sep 17 00:00:00 2001
From: light-city <455954986@qq.com>
Date: Tue, 6 Aug 2019 15:30:17 +0800
Subject: [PATCH] update dir

---
 README.md               |   1 +
 explicit/README.md      |   2 +-
 friend/README.md        | 101 ++++++++++++++++++++++++++++++++++++++++
 friend/friend_class     | Bin 0 -> 9344 bytes
 friend/friend_class.cpp |  28 +++++++++++
 friend/friend_func      | Bin 0 -> 9336 bytes
 friend/friend_func.cpp  |  33 +++++++++++++
 7 files changed, 164 insertions(+), 1 deletion(-)
 create mode 100644 friend/README.md
 create mode 100755 friend/friend_class
 create mode 100644 friend/friend_class.cpp
 create mode 100755 friend/friend_func
 create mode 100644 friend/friend_func.cpp

diff --git a/README.md b/README.md
index 64156ea..b70efaf 100644
--- a/README.md
+++ b/README.md
@@ -23,6 +23,7 @@
 - [x] [union那些事](./union)
 - [x] [c实现c++多态那些事](./c_poly)
 - [x] [explicit那些事](./explicit)
+- [x] [friend那些事](./friend)
 
 代码运行：
  全部在linux下用vim编写，使用gcc/g++调试！全部可正常运行！
diff --git a/explicit/README.md b/explicit/README.md
index 8fbe13f..05dda6f 100644
--- a/explicit/README.md
+++ b/explicit/README.md
@@ -4,7 +4,7 @@
 - explicit 修饰转换函数时，可以防止隐式转换，但按语境转换除外
 
 
-代码参见:[./explict.cpp](./explicit.cpp)
+代码参见:[.explicit.cpp](./explicit.cpp)
 
 参考链接：
 > https://stackoverflow.com/questions/4600295/what-is-the-meaning-of-operator-bool-const
diff --git a/friend/README.md b/friend/README.md
new file mode 100644
index 0000000..768b95c
--- /dev/null
+++ b/friend/README.md
@@ -0,0 +1,101 @@
+# 友元函数与友元类
+## 0.概述
+
+友元提供了一种 普通函数或者类成员函数 访问另一个类中的私有或保护成员 的机制。也就是说有两种形式的友元：
+
+（1）友元函数：普通函数对一个访问某个类中的私有或保护成员。
+
+（2）友元类：类A中的成员函数访问类B中的私有或保护成员
+
+优点：提高了程序的运行效率。
+
+缺点：破坏了类的封装性和数据的透明性。
+
+## 1.友元函数
+
+在类声明的任何区域中声明，而定义则在类的外部。
+
+```
+friend <类型><友元函数名>(<参数表>);
+```
+
+注意，友元函数只是一个普通函数，并不是该类的类成员函数，它可以在任何地方调用，友元函数中通过对象名来访问该类的私有或保护成员。
+
+具体代码见：[friend_func.cpp](friend_func.cpp)
+
+```c++
+#include <iostream>
+
+using namespace std;
+
+class A
+{
+public:
+    A(int _a):a(_a){};
+    friend int geta(A &ca);  ///< 友元函数
+private:
+    int a;
+};
+
+int geta(A &ca) 
+{
+    return ca.a;
+}
+
+int main()
+{
+    A a(3);    
+    cout<<geta(a)<<endl;
+
+    return 0;
+}
+```
+
+## 2.友元类
+友元类的声明在该类的声明中，而实现在该类外。
+
+```
+friend class <友元类名>;
+```
+
+类B是类A的友元，那么类B可以直接访问A的私有成员。
+
+具体代码见：[friend_class.cpp](friend_class.cpp)
+```c++
+#include <iostream>
+
+using namespace std;
+
+class A
+{
+public:
+    A(int _a):a(_a){};
+    friend class B;
+private:
+    int a;
+};
+
+class B
+{
+public:
+    int getb(A ca) {
+        return  ca.a; 
+    };
+};
+
+int main() 
+{
+    A a(3);
+    B b;
+    cout<<b.getb(a)<<endl;
+    return 0;
+}
+```
+
+
+## 3.注意
+- 友元关系没有继承性
+假如类B是类A的友元，类C继承于类A，那么友元类B是没办法直接访问类C的私有或保护成员。
+
+- 友元关系没有传递性
+假如类B是类A的友元，类C是类B的友元，那么友元类C是没办法直接访问类A的私有或保护成员，也就是不存在“友元的友元”这种关系。
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diff --git a/friend/friend_class.cpp b/friend/friend_class.cpp
new file mode 100644
index 0000000..bfa2372
--- /dev/null
+++ b/friend/friend_class.cpp
@@ -0,0 +1,28 @@
+#include <iostream>
+
+using namespace std;
+
+class A
+{
+public:
+    A(int _a):a(_a){};
+    friend class B;
+private:
+    int a;
+};
+
+class B
+{
+public:
+    int getb(A ca) {
+        return  ca.a; 
+    };
+};
+
+int main() 
+{
+    A a(3);
+    B b;
+    cout<<b.getb(a)<<endl;
+    return 0;
+}
diff --git a/friend/friend_func b/friend/friend_func
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HcmV?d00001

diff --git a/friend/friend_func.cpp b/friend/friend_func.cpp
new file mode 100644
index 0000000..8ef67cc
--- /dev/null
+++ b/friend/friend_func.cpp
@@ -0,0 +1,33 @@
+/**
+ * @file friend_func.cpp
+ * @brief 友元函数
+ * @author 光城
+ * @version v1
+ * @date 2019-08-06
+ */
+
+#include <iostream>
+
+using namespace std;
+
+class A
+{
+public:
+    A(int _a):a(_a){};
+    friend int geta(A &ca);  ///< 友元函数
+private:
+    int a;
+};
+
+int geta(A &ca) 
+{
+    return ca.a;
+}
+
+int main()
+{
+    A a(3);    
+    cout<<geta(a)<<endl;
+
+    return 0;
+}