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+[\<- 01/12](01-12.md)
+
+### Header Files
+
+- What's the problem below?
+- What's the solution
+
+```
+//file "a.h"
+class foo
+{
+	...
+};
+```
+
+```
+//file "b.h"
+#include "a.h"
+```
+
+```
+//file "c.c"
+#include "a.h"
+#include "b.h"
+```
+
+- Problem: `a.h` is included twice
+- Solutions
+	- One (kinda bad) solution is to not include `a.h` in `c.c`
+	- A better solution is to provide namespace
+
+### Namespace
+
+- Namespaces provide method for preventing name conflicts in large project
+- A namespace is a name that a programmer selects to identify a portion of the her work
+- A single namespace, such as `scu_coen79_2`, may have several different namespace groupings
+	- e.g. one for class definition, one for implementation
+
+```
+namespace scu_coen79_2
+{
+	//any item that belongs to the namespace is written here
+}
+```
+
+### Global Namespace & std
+
+- **The Global Namespace:**
+	- Any items that are not explicitly placed in a namespace become part of the global namespace
+		- These items can be used at any point without any need for a `using` statement or a scope resolution operator
+
+- **C++ Standard Library (std namespace)**
+	- All of the items in the C++ Standard Library are automatically part of the `std` namespace
+		- When using C++ header file names i.e. `<iostream>` or `<cstdlib>`
+	- The simplest way is to add `using namespace std;`
+
+### Using Namespaces
+
+- How to make the items that are defined in the namespace:
+	1. Make all of the namespace available
+		- Syntax: `using namespace ns_name;`
+		- Example: `using namespace scu_coen79_2A;`
+		- Question: Why is this a bad idea to put a `using` statement in a header file?
+	2. If we need to **use only a specific item from the namespace**
+		- Syntax: `using ns_name::name;`
+		- Example: `using scu_coen79_2A::throttle;`
+	3. Use any item by **prefixing the item name with the namespace and "::"** at the point where the item is used
+		- Syntax: `ns_name::name`
+		- Example: `scu_coen79_2A::throttle apollo`
+
+---
+
+# Classes and Parameters
+
+- Classes can be used as the type of a **function's parameter**, or as the type of the **return value** from a function
+- For example:
+	- You may pass a data structure to a function to apply some changes to the data structure
+	- The return value of a function may be a data structure
+
+## Programming Example: The Point Class
+
+![point class demo](01-14_1.png)
+
+- The `point` class: Is a data structure to store and manipulate the location of a single point on a plane
+- The point class has the member function listed here
+	- There is a constructor to initialize a point
+	- There is a member function to shift a point by given amounts along the x and y axes
+	- There is a member function to rotate a point by 90 degrees in a clockwise direction around the origin
+	- There are two constant member functions that allow us to retrieve the current x and y coordinates of a point
+
+- **Header file** of the Point class:
+
+```
+namespace scu_coen79_2A{
+
+	class point{
+
+		public:
+			//CONSTRUCTOR
+			//with default arguments (discussed later)
+			point(double initial_x = 0.0, double initial_y = 0.0);
+
+			//MODIFICATION MEMBER FUNCTIONS
+			void shift(double x_amount, double y_amount);
+			void rotate90();
+
+			//CONSTANT MEMBER FUNCTIONS
+			double get_x() const { return x; };
+			double get_y() const { return y; };
+
+		private:
+			double x; //x coordinate of this point
+			double y; //y coordinate of this point
+	};
+}
+```
+
+## Default Arguments
+
+- **Default Arguments** is a value that will be used for an argument when a programmer does not provide an actual argument
+- Example:
+	- `point(double initial_x = 0.0, double initial_y = 0.0);`
+	- Look at the header file of the point class
+	- `point a(-1, 0.8);`
+		- Uses the usual constructor with two arguments
+	- `point b(-1);`
+		- Uses -1 for the first argument and uses the default argument, `initial_y = 0.0`, for the second argument
+	- `point c;`
+		- Uses default arguments for both `initial_x = 0.0` and `initial_y = 0.0`
+
+- The default argument is specified only once (in the prototype) and not in the function's implementation
+- A function with several arguments does not need to specify default arguments for every argument
+- If only some of the arguments have defaults, `then those arguments must be right-most in the parameter list`
+- In a function call, arguments with default values **may be omitted from the right end** of the actual argument list
+- Example:
+
+```
+int date_check(int year, int month=1, int date=1);
+
+//...
+date_check(2000);
+date_check(2000, 7);
+date_check(2000, 7, 22);
+```
+
+## The Point Class (Cont'd)
+
+- **Implementation file** of the Point class
+
+```
+#include "point.hpp"
+
+namespace scu_coen79_2A{
+
+	point::point(double initial_x, double initial_y){
+		x = initial_x;
+		y = initial_y;
+	}
+
+	void point::shift(double x_amount, double y_amount){
+		x += x_amount;
+		y += y_amount;
+	}
+
+	void point::rotate90(){
+		double new_x;
+		double new_y;
+
+		//For a 90 degree clockwise rotation, the new x is the original y
+		new_x = y;
+		//and the new y is -1 times the original x
+		new_y = -x;
+		x = new_x;
+		y = new_y;
+	}
+}
+```
+
+## Parameters
+
+- Three different kinds of **parameters**:
+	- **Value Parameters**
+	- **Reference Parameters**
+	- **Const Reference Parameters**
+
+### Value Parameters
+
+- Example:
+	- The function's parameter (`p` in this case) is referred to as the **formal parameter** to distinguish it from the value that is passed in during the function call
+
+```
+int rotations_needed(point p){
+	//Postcondition: The value returned is the number of 90-degree clockwise rotations needed to move p into the upper-right quadrant (where x>=0 and y>=0)
+
+	int answer;
+	answer = 0;
+	while((p.get_x() < 0) || (p.get_y() < 0)){
+		p.rotate90();
+		++answer;
+	}
+	return answer;
+}
+
+//...
+point sample(6, -4); //Constructor places the point at x=6, y=-4
+cout << "x coordinate is " << sample.get_x() << " y coordinate is " << sample.get_y() << endl;
+cout << "Rotations: " << rotations_needed(sample) << endl;
+//The passed value (sample in this case) is the argument (sometimes called the actual argument or the actual parameter)
+cout << "x coordinate is " << sample.get_x() << "y coordinate is " << sample.get_y() << endl;
+
+//output:
+x coordinate is 6 y coordinate is -4
+Rotations: 3
+x coordinate is 6 y coordinate is -4
+```
+
+- A **value parameter** is declared by writing the type name followed by the parameter name
+- With a value parameter, the **argument** provides the initial value for the **formal parameter**
+- The value parameter is implemented as a **local variable** of the function
+	- Therefore: Any changes made to the parameter in the body of the function **will leave the argument unaltered**
+
+### Reference Parameters
+
+- Example:
+
+```
+void rotate_to_upper_right(point& p){
+	//Postcondition: The point p has been rotated in 90-degree increments until p has been moved into the upper right quadrant (where x>=0 and y>=0)
+
+	while((p.get_x() < 0) || (p.get_y() < 0)){
+		p.rotate90();
+	}
+}
+
+//...
+point(6, -4); // Constructor places point at x = 6, y = -4
+cout << "x coordinate is " << sample.get_x() << " y coordinate is " << sample.get_y() << endl;
+rotate_to_upper_right(sample);
+cout << "x coordinate is " << sample.get_x() << "y coordinate is " << sample.get_y() << endl;
+
+//output:
+x coordinate is 6 y coordinate is -4
+Rotations: 3
+x coordinate is 4 y coordinate is 6
+```
+
+- A **reference parameter** is declared by writing the type name followed by the character `&` and the parameter name
+- With a reference parameter, any use of the parameter within the body of the functino will access the argument from the calling program
+- Changes made to the formal parameter in the body of the function **will alter the argument**
+
+### Pitfall
+
+- In order for a reference parameter to work correctly, **the data type of an argument must match exactly with the data type of the formal parameter**
+
+```
+void make_int_2(int& 1){
+	//Postcondition: i has been set to 2
+
+	i = 2;
+}
+```
+
+```
+double d;
+d = 0;
+make_int_2(d);
+cout << d;
+
+//Output: 0
+```
+
+- No change in `d` because `d` is the wrong data type, a separate *integer* copy of d is created to use as the argument
+
+- If the argument's data type does not exactly match the data type of the formal parameter:
+	- The compiler will try to **convert** the argument to the correct type
+	- If the conversion is possible, then **the compiler treats the argument like a value parameter**, passing a **copy** of the argument to the function
+
+### Const Reference Parameters
+
+- When we don't want a programmer to worry about whether the function changes the actual argument
+- A solution that provides **the efficiency of a reference parameter** along with the **security of a value parameter**
+
+- Example:
+	- A function that computes the distance between two points
+	- The function has two point parameters, and **neither parameter is changed by the function**
+	- `double distance(const point& p1, const point& p2);`
+
+## When the Type of a Function's Return Valeu is a Class
+
+- The type of a function's return value may be a class
+
+```
+point middle(const point& p1, const point& p2){
+	//Postcondition: The value returned is the point that is halfway between p1 and p2
+
+	double x_midpoint, y_midpoint;
+
+	//Compute the x and y midpoints
+	x_midpoint = (p1.get_x() + p2.get_x())/2;
+	y_midpoint = (p1.get_y() + p2.get_y())/2;
+
+	//Construct a new point and return it
+	point midpoint(x_midpoint, y_midpoint);
+
+	return midpoint;
+}
+```
+
+- C++ return statement uses a copy constructor to copy the function's return value to a temporary location before returning the value to the calling program
+
+---
+
+# Operator Overloading
+
+- **Header file** of the Point class
+
+```
+namespace scu_coen79_2A{
+
+    class point{
+
+        public:
+            //CONSTRUCTOR
+            //with default arguments (discussed later)
+            point(double initial_x = 0.0, double initial_y = 0.0);
+
+            //MODIFICATION MEMBER FUNCTIONS
+            void shift(double x_amount, double y_amount);
+            void rotate90();
+
+            //CONSTANT MEMBER FUNCTIONS
+            double get_x() const { return x; };
+            double get_y() const { return y; };
+
+        private:
+            double x; //x coordinate of this point
+            double y; //y coordinate of this point
+    };
+}
+```
+
+- A **binary function** is a function with two arguments
+- When we develop a new data structure, we usually need to overload the binary operators
+- For example: For the point class, **we cannot use the regular == operator to decide if two points are equal**
+	- We need to **overload** the `==` operator to compare particular member variables of the two objects
+
+```
+point p1, p2;
+if(p1 == p2) cout << "Those points are equal." << endl;
+```
+
+- Defining a new meaning for an operator is called **overloading the operator**
+
+## Overloading Binary Comparison Operators
+
+- The name of the new function is `operator ==`
+
+```
+bool operator ==(const point& p1, const point& p2){
+	//Postcondition: The value returned is true if p1 and p2 are identifcal; otherwise false is returned
+
+	return (p1.get_x() == p2.get_x()) && (p1.get_y() == p2.get_y());
+}
+```
+
+- Note:
+	- When overloading an operator, **the common usages of that operator are still available**
+	- For example, we can use `==` to test the equality of two integers or two doubles
+	- For each use of `==`, the compiler determines the data type of the objects being compared and uses the appropriate comparison function
+
+## Overloading Binary Arithmetic Operators
+
+- Example: If we could add two of our points, then we might write this program:
+
+```
+point speed1(5, 7);
+point speed2(1, 2);
+point total;
+total = speed1 + speed2;
+cout << total.get_x() << endl;
+cout << total.get_y() << endl;
+
+//Output:
+//6
+//9
+```
+
+```
+point operator +(const point& p1, const point& p2){
+	//Postcondition: The sum of p1 and p2 is returned
+
+	double x_sum, y_sum;
+
+	//Compute the x and y of the sum
+	x_sum = (p1.get_x() + p2.get_x());
+	y_sum = (p1.get_y() + p2.get_y());
+
+	point sum(x_sum, y_sum);
+
+	return sum;
+}
+```
+
+## Overloading Output and Input Operators
+
+- The standard C++ data types can be written and read using the **output operator <<** and **the input operator <<**
+
+```
+int i;
+cin >> i;
+cout << i;
+```
+
+- We would like to support the following input and output operations for the point class
+
+```
+point p;
+cin << p;
+cout << p;
+```
+
+- Overloading the output operator for the `point` class
+
+```
+ostream& operator <<(ostream& outs, const point& source);
+```
+
+- The data type of `cout` is `ostream`, which means "output stream"
+- `ostream` class is part of the `iostream` library facility
+- The `outs` parameter is a reference parameter
+- The function can change the output stream (by writing to it), and the change will affect the actual argument (such as the standard output stream, `cout`)
+
+- Example:
+
+```
+cout << p;
+```
+
+- The first argument, `cout`, is an `ostream`
+- The second argument, `p`, is a `point`
+
+- Also note that:
+	- The return type of the function is `ostream&`
+	- This return type means that the functions return as `ostream`
+	- Called a **reference return type**
+	- C++ permits the "**chaining**" of output statements such as the following:
+		- `cout << "The points are " << p << " and " << q << endl;`
+
+```
+ostream& operator <<(ostream& outs, const point& source){
+	//Postcondition: The x and y coordinates of source have been written to outs
+	//Postcondition: The return value is the ostream outs
+	//Library facilities used: iostream
+
+	outs << source.get_x() << " " << source.get_y();
+	return outs;
+}
+
+istream& operator >>(istream& ins, point& target){
+	//Postcondition: The x and y coordinates of target have been read from ins
+	//Postcondition: The return value is the istream ins
+	//Library facilities used: iostream
+	//Friend of point class
+
+	ins >> target.x >> target.y;
+	return ins;
+}
+```
+
+- In the input function, the input is sent to the private member variables! However, only member functions can access private member variables
+- Two solutions:
+	- To write new member functions to set a point's coordinates and use these member functions within the input function's implementation
+	- You can grant special permission for the input functino to access the private members of the point class - Called a **Friend Function**