Update examples on friend class

Author: XuhuaHuang

FriendClass/CMakeLists.txt

@@ -8,6 +8,9 @@ set(CMAKE_CXX_STANDARD_REQUIRED ON)
 # Adding multiple executable to the project
 # Right-click on each item to set as start-up project
 add_executable(square_rectangle "square_rectangle.cpp")
+add_executable(modern_square_rectangle "modern_square_rectangle.cpp")
 add_executable(time_date "time_date.cpp")
 
+set_target_properties(modern_square_rectangle PROPERTIES CXX_STANDARD 23)
+
 add_subdirectory("TimeDate")

FriendClass/modern_square_rectangle.cpp

@@ -11,6 +11,8 @@
 
 #include <iostream>
 
+class square_t; // forward declaration
+
 class rectangle_t {
 private:
   int width{0};
@@ -46,6 +48,24 @@ class rectangle_t {
   rectangle_t double_in_size() const noexcept {
     return rectangle_t(width * 2, height * 2);
   }
+
+  void convert(const square_t& sq) noexcept;
+};
+
+class square_t {
+  friend class rectangle_t; // allow rectangle_t to access private data
+
+private:
+  int side{0};
+
+public:
+  explicit constexpr inline square_t(int s)
+    : side(s) {}
+
+  [[nodiscard]]
+  int get_side() const noexcept {
+    return side;
+  }
 };
 
 // Optional: keep a free function wrapper if needed
@@ -54,6 +74,11 @@ rectangle_t double_in_size(const rectangle_t& rect) noexcept {
   return rect.double_in_size();
 }
 
+void rectangle_t::convert(const square_t& sq) noexcept {
+  width  = sq.side;
+  height = sq.side;
+}
+
 int main() {
   rectangle_t r1;
   rectangle_t r2(2, 3);
@@ -62,5 +87,12 @@ int main() {
 
   std::cout << r1.area() << std::endl;
 
+  rectangle_t rect;
+  square_t    sqr(4);
+
+  rect.convert(sqr);
+
+  std::cout << __LINE__ << ": " << rect.area() << std::endl;
+
   return 0;
 }

FriendClass/square_rectangle.cpp

@@ -1,61 +1,61 @@
-/*****************************************************************/ /**
-                                                                     * \file   square_rectangle.cpp
-                                                                     * \brief  class "Square" made friend of class
-                                                                     *"rectangle"
-                                                                     *
-                                                                     * worth noting:
-                                                                     * 1) granting the other class to access the
-                                                                     *protected and private data members of the class
-                                                                     *made friend with. 2) class friendship is not
-                                                                     *bidirectional. Need to specify. 3) class
-                                                                     *friendship is not transmissive: The friend of a
-                                                                     *friend is not considered to be a friend unless
-                                                                     *explicitly specified.
-                                                                     *
-                                                                     * \author Xuhua
-                                                                     * \date   November 2020
-                                                                     *********************************************************************/
+// clang-format off
+/*****************************************************************//**
+ * \file   square_rectangle.cpp
+ * \brief  class "Square" made friend of class "rectangle"
+ *
+ * worth noting:
+ * 1) granting the other class to access the
+ * protected and private data members of the class
+ * made friend with. 2) class friendship is not
+ * bidirectional. Need to specify. 3) class
+ * friendship is not transmissive: The friend of a
+ * friend is not considered to be a friend unless
+ * explicitly specified.
+ *
+ * \author Xuhua
+ * \date   November 2020
+ *********************************************************************/
+// clang-format on
 
 #include <iostream>
 
 class Square {
 private:
-    int side;
+  int side;
 
 public:
-    inline void setSide(int a) { side = a; }
+  inline void setSide(int a) { side = a; }
 
-    // Square class allows Rectangle class to access its private data members.
-    friend class Rectangle;
-    // conflict the concept of class encapsulation
-    // could be avoided by using public get functions (getter)
+  // Square class allows Rectangle class to access its private data members.
+  friend class Rectangle;
+  // conflict the concept of class encapsulation
+  // could be avoided by using public get functions (getter)
 };
 
 class Rectangle {
 private:
-    int width, height;
+  int width, height;
 
 public:
-    constexpr inline int area() { return (width * height); }
-    void                 convert(const Square& rhs);
+  constexpr inline int area() const { return (width * height); }
+  void                 convert(const Square& rhs);
 };
 
 void Rectangle::convert(const Square& a) {
-    width  = a.side;
-    height = a.side;
-    // Rectangle class using Square's private data members (side)
-
-    return;
+  width  = a.side;
+  height = a.side;
+  // Rectangle class using Square's private data members (side)
+  return;
 }
 
 int main(void) {
-    Square    sqr;
-    Rectangle rect;
+  Square    sqr;
+  Rectangle rect;
 
-    sqr.setSide(4);
-    rect.convert(sqr); // converting a square to rectangle
+  sqr.setSide(4);
+  rect.convert(sqr); // converting a square to rectangle
 
-    std::cout << rect.area() << "\n";
+  std::cout << rect.area() << "\n";
 
-    return 0;
+  return 0;
 }