LESSON 51/53Objects

Classes and Objects

GOALBuild a structure with rules that protects its own data.

A class is a struct with rules. Anyone can put anything in a struct — including a score of -50. Hiding the data in a class means it only changes through the routes you allow. For contest problems a struct is usually enough; this lesson is here for the Cambridge syllabus and for the two lessons that follow.

ON THIS PAGE

The example
Line by line
Watch it run
struct or class?
When NOT to make one
Common mistakes
Key words
Quick check
Now you try

SHOW EXAMPLE IN

//THE EXAMPLE

main.cpp

#include <iostream>
#include <string>
using namespace std;

class Student {
private:
    string name;
    int score;

public:
    void set(string n, int s) {
        name = n;
        if (s < 0) s = 0;
        if (s > 100) s = 100;
        score = s;
    }
    int get() {
        return score;
    }
};

int main() {
    Student a;
    a.set("Bat", 150);
    cout << a.get() << endl;

    a.set("Bat", 72);
    cout << a.get() << endl;
    return 0;
}

> what you see

100
72

Try this code

//LINE BY LINE

private:

Everything below this is off limits from OUTSIDE. Writing a.score is refused by the compiler.

public:

Everything below this can be called from outside.

if (s > 100) s = 100;

This is the rule. A struct has no such guard, so 150 would go straight in.

class Student {

A class starts private, a struct starts public. That is the entire difference.

Watch it run

Student a; is made. Neither field can be seen from outside.

1 / 6

struct or class?

In C++ there is exactly one difference: a struct starts public and a class starts private. Everything else is the same — a struct can have methods and constructors too.

What you need	Use
Хэдэн талбарыг хамт барих / Hold a few fields together	`struct`
Утгыг хамгаалах дүрэмтэй / Rules that protect the value	`class`
Тэмцээний код / Contest code	Бараг үргэлж `struct` / almost always `struct`

In a contest a struct is shorter and can be built with {a, b} directly. Writing a class where you need no protection is just more typing.

When NOT to make one

A class is for fields that belong TOGETHER. Bundling unrelated things can feel tidy and makes the code harder to read.

Situation	What to do
Оюутны нэр, оноо, анги / A student's name, score, class	Нэг бүтэц / one type — they travel together
Ганц утга / A single value	Энгийн хувьсагч / just a variable
Нэг л удаа хэрэглэх хоёр тоо / Two numbers used once	Хос / a pair or tuple
«Бүх юмаа хийх» бүтэц / A "holds everything" type	Хэсэг болгон салга / split it up

A quick test: if you change one field, must another change with it? If so, they belong together. If not, they probably do not.

//COMMON MISTAKES

✗ wrong

class Student { string name; }; ... s.name = "Bat";

✓ right

public: гэж бичих / Add public:, or use a struct

A class is private by default, so this fails to compile. Code that worked on a struct will not work here.

✗ wrong

Бүх зүйлийг класс болгох / Making everything a class

✓ right

Ихэнхдээ struct хангалттай / A struct is usually enough

In a contest speed matters. Where you do not need the guard, a struct is shorter and clearer.

//KEY WORDS

private

Reachable only from inside the class.

public

Reachable from outside.

encapsulation

Hiding data so it can only change through routes you approve.

//QUICK CHECK

What is the basic difference between class and struct?

//NOW YOU TRY

Write a class holding a bank balance. Withdrawing more than the balance must be refused, leaving the balance unchanged.

Open editor

Shortest Path with Weights Methods and Constructors

//THE EXAMPLE

main.cpp

#include <iostream>
#include <string>
using namespace std;

class Student {
private:
    string name;
    int score;

public:
    void set(string n, int s) {
        name = n;
        if (s < 0) s = 0;
        if (s > 100) s = 100;
        score = s;
    }
    int get() {
        return score;
    }
};

int main() {
    Student a;
    a.set("Bat", 150);
    cout << a.get() << endl;

    a.set("Bat", 72);
    cout << a.get() << endl;
    return 0;
}

> what you see

100
72

Try this code

//LINE BY LINE

private:

Everything below this is off limits from OUTSIDE. Writing a.score is refused by the compiler.

public:

Everything below this can be called from outside.

if (s > 100) s = 100;

This is the rule. A struct has no such guard, so 150 would go straight in.

class Student {

A class starts private, a struct starts public. That is the entire difference.

struct or class?

In C++ there is exactly one difference: a struct starts public and a class starts private. Everything else is the same — a struct can have methods and constructors too.

What you need	Use
Хэдэн талбарыг хамт барих / Hold a few fields together	`struct`
Утгыг хамгаалах дүрэмтэй / Rules that protect the value	`class`
Тэмцээний код / Contest code	Бараг үргэлж `struct` / almost always `struct`

In a contest a struct is shorter and can be built with {a, b} directly. Writing a class where you need no protection is just more typing.

When NOT to make one

A class is for fields that belong TOGETHER. Bundling unrelated things can feel tidy and makes the code harder to read.

Situation	What to do
Оюутны нэр, оноо, анги / A student's name, score, class	Нэг бүтэц / one type — they travel together
Ганц утга / A single value	Энгийн хувьсагч / just a variable
Нэг л удаа хэрэглэх хоёр тоо / Two numbers used once	Хос / a pair or tuple
«Бүх юмаа хийх» бүтэц / A "holds everything" type	Хэсэг болгон салга / split it up

A quick test: if you change one field, must another change with it? If so, they belong together. If not, they probably do not.

//COMMON MISTAKES

✗ wrong

class Student { string name; }; ... s.name = "Bat";

✓ right

public: гэж бичих / Add public:, or use a struct

A class is private by default, so this fails to compile. Code that worked on a struct will not work here.

✗ wrong

Бүх зүйлийг класс болгох / Making everything a class

✓ right

Ихэнхдээ struct хангалттай / A struct is usually enough

In a contest speed matters. Where you do not need the guard, a struct is shorter and clearer.

//QUICK CHECK

What is the basic difference between class and struct?