###### tags: `程設筆記`
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# Chaper 9 Struct, Class
## Class
Reference:
http://ettsengnote.blogspot.com/2016/03/cclasses-and-objects-part-ii.html
http://ettsengnote.blogspot.com/2016/03/cclasses-and-objects-part-ii.html

* What is class?
* Class is a template with Data members and Methods. A instance generated by a template is called an "object".
* public, private and protected
Reference:https://ithelp.ithome.com.tw/articles/10225135
* Public => Accessible to everone.
* Private => Only accessible to the "Class".
* Protected => Only accessible to the "class" and its "subclass".

* ".", "->" and "::"
Reference: https://www.alexleo.click/cc-%E7%AE%AD%E9%A0%AD-%E3%80%81%E9%BB%9E-%E3%80%81%E9%9B%99%E5%86%92%E8%99%9F-%E7%94%A8%E6%B3%95/

Ex1:

#include <iostream>
#include <string>
using namespace std;
class Say_sth{
    public:
    void input(string name, string words);
    void output();
    private:
    string name;
    string words;
};    
void swap(Say_sth &a, Say_sth &b){
    Say_sth temp=a;
    a=b;
    b=temp;
}
void Say_sth::input(string name, string words){
    this->name=name;
    this->words=words;
}
void Say_sth::output(){
    cout<<name<<" said: "<<words<<"\n";
}
int main(){
    Say_sth say[2];
    string name, words;
    for(int i=0; i<2; i++){
        getline(cin, name);
        getline(cin, words);
        say[i].input(name, words);
    }
    swap(say[0], say[1]);
    for(int i=0; i<2; i++){
        say[i].output();
    }
}

Ex2:

#include <iostream>
using namespace std;
class Count{
    public:
    inline void set(int value){
        this->value=value;
    }
    inline void print(){
        cout<<value<<"\n";
    }
    private:
    int value;
};
int main(){
    Count counter;
    Count *counterPtr=&counter;
    Count counterRef=counter;

    counter.set(1);
    counter.print();

    counterRef.set(2);
    counterRef.print();

    counterPtr->set(3);
    counterPtr->print();
}

* Writing Header file(".h")
Ex:

"Say_sth.h":

#ifndef TIME_H
#define TIME_H
#include <iostream>
#include <string>
using namespace std;
class Say_sth{
    public:
    Say_sth();
    void input(string name, string words);
    void output();
    private:
    string name;
    string words;
};
#endif

Say_sth.cpp:

#include <iostream>
#include <string>
#include "Say_sth.h"
using namespace std;
Say_sth(){
    name=words="";
}
void swap(Say_sth &a, Say_sth &b){
    Say_sth temp=a;
    a=b;
    b=temp;
}
void Say_sth::input(string name, string words){
    this->name=name;
    this->words=words;
}
void Say_sth::output(){
    cout<<name<<" said: "<<words<<"\n";
}

Say_sth_main.cpp:

#include <iostream>
#include "Say_sth.h"
using namespace std;
int main(){
    Say_sth say[2];
    string name, words;
    for(int i=0; i<2; i++){
        getline(cin, name);
        getline(cin, words);
        say[i].input(name, words);
    }
    swap(say[0], say[1]);
    for(int i=0; i<2; i++){
        say[i].output();
    }
}

Result:

* Constructor and Destructor
Reference: https://www.codementor.io/@supernerdd7/constructor-and-destructor-in-c-1r8kkogm6j
* Constant and Class
* To specify an object is not modifiable.
* A const member function is specified as const bothin its prototype and in its definition.
* Composition: Objects as Members of Classes
Reference:https://medium.com/@pumbaawithmask/%E4%B8%80%E4%BA%9B%E7%AD%86%E8%A8%98-c-98cad9874dd3
* The constructor of one class will automatically create a default constructor(default copy constructor) to copy an object when needed.
Ex:

    #include <iostream>
    using namespace std;
    class Name{
        public:
        Name(string fn="F_Name", string ln="L_name");
        void setName(string f_name, string l_name);
        void const output();
        private:
        string f_name;
        string l_name;
    };
    Name::Name(string fn, string ln)
        :f_name(fn),l_name(ln)
    {

    }
    void Name::setName(string f_name, string l_name){
        this->f_name=f_name;
        this->l_name=l_name;
    }
    void const Name::output(){
        cout<<f_name<<" "<<l_name;
    }
    class Say_sth{
        public:
        Say_sth(Name &name, string w="Helloworld");
        void input(Name &name, string words);
        void const output();
        private:
        Name test;
        string words;
    };
    Say_sth::Say_sth(Name &name, string w)
        :test(name),words(w)
    {

    }
    void Say_sth::input(Name &name, string words){
        this->test=name;
        this->words=words;
    }
    void const Say_sth::output(){
        test.output();
        cout<<" said "<<words<<"\n";
    }
    int main(){
        Name name, name2;
        name.setName("Ryan", "Lin");
        name2.setName("Bryan", "Lin");
        Say_sth say(name);
        say.output();
        say.input(name2,"HaHaPiyan");
        say.output();
    }

* Initializer(Constructor)
Ex:
* Friend Function
* Defined outside public or private.
* Have the access to public and private members.
Ex:

    #include <iostream>
    using namespace std;
    class Say_sth{
        friend void input(Say_sth &say_sth, string name, string words);
        public:
        Say_sth(string n="Name", string w="Helloworld");
        void const output();
        private:
        string name;
        string words;
    };
    Say_sth::Say_sth(string n, string w)
        :name(n), 
            words(w)
    {

    }
    void input(Say_sth &say_sth, string n, string w){
        say_sth.name=n;
        say_sth.words=w;
    }
    void const Say_sth::output(){
        cout<<name<<" said: "<<words<<"\n";
    }
    int main(){
        Say_sth test;
        input(test, "Ryan", "HaHaPiyan");
        test.output();
    }

* Static Member Function/Data
* A static can only access to a static data.

#include <iostream>
using namespace std;
class Say_sth{
    public:
    Say_sth(string n="Name", string w="Helloworld");
    static int getCount();
    void input(string name, string words);
    void const output();
    private:
    string name;
    string words;
    static int count;
};
int Say_sth::count=0; //Only declaring the value at global scope is allowed.
int Say_sth::getCount(){
	return count;
}
Say_sth::Say_sth(string n, string w)
    :name(n), 
        words(w)
{
    
}
void Say_sth::input(string name, string words){
	this->name=name;
	this->words=words;
	count++;
}
void const Say_sth::output(){
	cout<<name<<" said: "<<words<<"\n";
}
int main(){
	Say_sth test;
	string name, words;
	while(getline(cin,name)){
		getline(cin,words);
		test.input(name, words);
		test.output();
		cout<<Say_sth::getCount()<<"\n";
	}
}

Result:

* this->
* Usually used when one variable is the same as the member of a class.
* About return *thih and return this.

Ex1(same name):

Ex2(cascaded member-function called):

    #include <iostream>
    using namespace std;
    class Say_sth{
        public:
        Say_sth(string n="Name", string w="Helloworld");
        Say_sth &setName(string name);
        Say_sth &setWords(string words);
        void const output();
        private:
        string name;
        string words;
        static int count;
    };
    Say_sth::Say_sth(string n, string w)
        :name(n), 
            words(w)
    {

    }
    Say_sth &Say_sth::setName(string name){
        this->name=name;
        return *this;
    }
    Say_sth &Say_sth::setWords(string words){
        this->words=words;
        return *this;
    }
    void const Say_sth::output(){
        cout<<name<<" said: "<<words<<"\n";
    }
    int main(){
        Say_sth test;
        string name, words;
        while(getline(cin,name)){
            getline(cin,words);
            test.setName(name).setWords(words);
            test.output();
        }
    }

Ex3:

    #include <iostream>
    using namespace std;
    class Say_sth{
        public:
        Say_sth(string n="Name", string w="Helloworld");
        void input(string name, string words);
        void const output();
        private:
        string name;
        string words;
    };
    Say_sth::Say_sth(string n, string w)
        :name(n), 
            words(w)
    {

    }
    void Say_sth::input(string name, string words){
        this->name=name;
        this->words=words;
    }
    void const Say_sth::output(){
        cout<<name<<" said: "<<words<<"\n";
        cout<<this->name<<" said: "<<this->words<<"\n";
        cout<<(*this).name<<" said: "<<(*this).words<<"\n";
        //The code above are the same.
    }
    int main(){
        Say_sth test;
        test.input("Ryan", "HaHaPiyan");
        test.output();
    }

Result:
* Operator Overloading
* Operators that can be overloaded
* Operators that cannot be overloaded

Ex:

    #include <iostream>
    using namespace std;
    class Complex{
        public:
        Complex(double real=0, double img=0);
        Complex operator+(const Complex &a);
        Complex operator-(const Complex &a);
        void Display();
        private:
        double Real;
        double Img;
    };
    Complex::Complex(double real, double img)
    :Real(real),Img(img)
    {

    }
    Complex Complex::operator+(const Complex &a){
        return Complex(Real+a.Real, Img+a.Img);
    }
    Complex Complex::operator-(const Complex &a){
        return Complex(Real-a.Real, Img-a.Img);
    }
    void Complex::Display(){
        cout<<"Real: "<<Real<<"\n";
        cout<<"Img: "<<Img<<"\n";
    }
    int main(){
        Complex a(1, 2), b(3, 4), c;
        c=a+b;
        c.Display();
        c=a-b;
        c.Display();
    }

## Struct
* Basic Application

Ex:

#include <iostream>
#include <string>
using namespace std;
struct  Client_data{
    string name;
    int account_id, balance;
};
int balance_addition(Client_data a, Client_data b){
    return a.balance+b.balance;
}

int main(){
    Client_data Client, Client_2;
    Client.name="Ryan";
    Client.account_id=1;
    Client.balance=100;
    Client_2=Client; //copy all data from Client to Client_2
    cout<<"Client: "<<Client.account_id<<" "<<Client.name<<" "<<Client.balance<<"\n";
    cout<<"Client_2: "<<Client_2.account_id<<" "<<Client_2.name<<" "<<Client_2.balance<<"\n";
    cout<<"Addition of Client.balance and Client_2.balance:"<<balance_addition(Client, Client_2)<<"\n";
}

* Pointers for a Structure

Ex:

#include <iostream>
#include <string>
using namespace std;
struct Complex{
    double real;
    double img;
};
int main(){
    Complex a, b;
    Complex *p, *q;
    a.real=1;
    a.img=2;
    p=&a;
    cout<<p->img<<"\n";
    cout<<p->real<<"\n";
    cout<<(*p).img<<"\n";
    cout<<(*p).real<<"\n";
    q=&b;
    q->real=p->img+p->real;
    cout<<q->real<<"\n";
    cout<<b.real<<"\n";
}