changed ex7_23_24_25.cpp to ex7_23.h/cpp and ex7_24.h

1. `const pos h`'s `const` is unnecessary.
2. the second contructs get contents by a `while` loop. unnecessary.
3. the third contructs may not get the point of question. (**a character**)
4. we should add inline member functions in this exercise.

ch07/README.md

@@ -81,5 +81,10 @@ the interface should be defined as public, the data shouldn't expose to outside
 - code verbosity, declarations inside the class, outside the class.
 
 ## [Exercise 7.21](ex7_21.h)
-
 ## [Exercise 7.22](ex7_22.h)
+## Exercise 7.23 [Header](ex7_23.h)|[CPP](ex7_23.cpp)
+## [Exercise 7.24](ex7_24.h)
+
+## Exercise 7.25
+
+The class below can rely on it. It goes in Section 7.1.5:"..the synthesized versions are unlikely to work correctly for classes that allocate resources that reside outside the class objects themselves. " and "Moreover, the synthesized versions for copy, assignment, and destruction work correctly for classes that have vector or string members. " Hence the class below which used only built-in type and strings can rely on the default version of copy and assignment. (@Mooophy)

ch07/ex7_23.cpp

@@ -0,0 +1,23 @@
+//
+//  ex7_23.cpp
+//  Exercise 7.23
+//
+//  Created by pezy on 11/14/14.
+//  Copyright (c) 2014 pezy. All rights reserved.
+//
+
+//#include "ex7_23.h"
+#include "ex7_24.h"
+
+char Screen::get(pos r, pos c) const
+{
+    pos row = r * width;
+    return contents[row + c];
+}
+
+Screen& Screen::move(pos r, pos c)
+{
+    pos row = r * width;
+    cursor = row + c;
+    return *this;
+}

ch07/ex7_23.h

@@ -0,0 +1,31 @@
+//
+//  ex7_23.h
+//  Exercise 7.23
+//
+//  Created by pezy on 11/14/14.
+//  Copyright (c) 2014 pezy. All rights reserved.
+//
+
+#ifndef CP5_ex7_23_h
+#define CP5_ex7_23_h
+
+#include <string>
+
+class Screen {
+    public:
+        using pos = std::string::size_type;
+
+        Screen() = default;
+        Screen(pos ht, pos wd, char c):height(ht),width(wd),contents(ht*wd, c){}
+
+        char get() const { return contents[cursor]; }
+        inline char get(pos r, pos c) const;
+        inline Screen& move(pos r, pos c);
+
+    private:
+        pos cursor = 0;
+        pos height = 0, width = 0;
+        std::string contents;
+};
+
+#endif

ch07/ex7_24.h

@@ -0,0 +1,32 @@
+//
+//  ex7_24.cpp
+//  Exercise 7.24
+//
+//  Created by pezy on 11/14/14.
+//  Copyright (c) 2014 pezy. All rights reserved.
+//
+
+#ifndef CP5_ex7_24_h
+#define CP5_ex7_24_h
+
+#include <string>
+
+class Screen {
+    public:
+        using pos = std::string::size_type;
+
+        Screen() = default; // 1 
+        Screen(pos ht, pos wd, pos n):height(ht),width(wd),contents(n, ' '){} // 2
+        Screen(pos ht, pos wd, char c):height(ht),width(wd),contents(ht*wd, c){} // 3
+
+        char get() const { return contents[cursor]; }
+        inline char get(pos r, pos c) const;
+        inline Screen& move(pos r, pos c);
+
+    private:
+        pos cursor = 0;
+        pos height = 0, width = 0;
+        std::string contents;
+};
+
+#endif