[フレーム]

C++ Tutorial

C++ HOME C++ Intro C++ Get Started C++ Syntax C++ Output C++ Comments C++ Variables C++ User Input C++ Data Types C++ Operators C++ Strings C++ Math C++ Booleans C++ If...Else C++ Switch C++ While Loop C++ For Loop C++ Break/Continue C++ Arrays C++ Structures C++ Enums C++ References C++ Pointers C++ Memory Management

C++ Functions

C++ Functions C++ Function Parameters C++ Function Overloading C++ Scope C++ Recursion C++ Lambda

C++ Classes

C++ OOP C++ Classes/Objects C++ Class Methods C++ Constructors C++ Access Specifiers C++ Encapsulation C++ Friend Functions C++ Inheritance C++ Polymorphism C++ Templates C++ Files C++ Date

C++ Errors

C++ Errors C++ Debugging C++ Exceptions C++ Input Validation

C++ Data Structures

C++ Data Structures & STL C++ Vectors C++ List C++ Stacks C++ Queues C++ Deque C++ Sets C++ Maps C++ Iterators C++ Algorithms

C++ Namespaces

C++ Namespaces

C++ Projects

C++ Projects

C++ How To

C++ Add Two Numbers C++ Random Numbers

C++ Reference

C++ Reference C++ Keywords C++ <iostream> C++ <fstream> C++ <cmath> C++ <string> C++ <cstring> C++ <ctime> C++ <vector> C++ <algorithm>

C++ Examples

C++ Examples C++ Real-Life Examples C++ Compiler C++ Exercises C++ Quiz C++ Syllabus C++ Study Plan C++ Certificate


C++ Constructors


Constructors

A constructor is a special method that is automatically called when an object of a class is created.

To create a constructor, use the same name as the class, followed by parentheses ():

Example

class MyClass { // The class
public: // Access specifier
MyClass() { // Constructor
cout << "Hello World!";
}
};

int main() {
MyClass myObj; // Create an object of MyClass (this will call the constructor)
return 0;
}
Try it Yourself »

Constructor Rules

  • The constructor has the same name as the class.
  • It has no return type (not even void).
  • It is usually declared public.
  • It is automatically called when an object is created.

Constructor with Parameters

Constructors can also take parameters (just like regular functions), which can be useful for setting initial values for attributes.

The following class have brand, model and year attributes, and a constructor with different parameters. Inside the constructor we set the attributes equal to the constructor parameters (brand=x, etc).

When we call the constructor (by creating an object of the class), we pass parameters to the constructor, which will set the value of the corresponding attributes to the same:

Example

class Car { // The class
public: // Access specifier
string brand; // Attribute
string model; // Attribute
int year; // Attribute
Car(string x, string y, int z) { // Constructor with parameters
brand = x;
model = y;
year = z;
}
};

int main() {
// Create Car objects and call the constructor with different values
Car carObj1("BMW", "X5", 1999);
Car carObj2("Ford", "Mustang", 1969);

// Print values
cout << carObj1.brand << " " << carObj1.model << " " << carObj1.year << "\n";
cout << carObj2.brand << " " << carObj2.model << " " << carObj2.year << "\n";
return 0;
}
Try it Yourself »

Constructor Defined Outside the Class

You can also define the constructor outside the class using the scope resolution operator ::.

Example

class Car { // The class
public: // Access specifier
string brand; // Attribute
string model; // Attribute
int year; // Attribute
Car(string x, string y, int z); // Constructor declaration
};

// Constructor definition outside the class
Car::Car(string x, string y, int z) {
brand = x;
model = y;
year = z;
}

int main() {
// Create Car objects and call the constructor with different values
Car carObj1("BMW", "X5", 1999);
Car carObj2("Ford", "Mustang", 1969);

// Print values
cout << carObj1.brand << " " << carObj1.model << " " << carObj1.year << "\n";
cout << carObj2.brand << " " << carObj2.model << " " << carObj2.year << "\n";
return 0;
}
Try it Yourself »

Why Constructors Are Useful

Constructors run by themselves when you create an object. They set things up so everything is ready right away.

Think of it like this: When you order a pizza (object), the constructor is the chef who adds the sauce, cheese, and toppings before it gets to you - you don't have to do it yourself!




Track your progress - it's free!
×

Contact Sales

If you want to use W3Schools services as an educational institution, team or enterprise, send us an e-mail:
sales@w3schools.com

Report Error

If you want to report an error, or if you want to make a suggestion, send us an e-mail:
help@w3schools.com

FORUM ABOUT ACADEMY
W3Schools is optimized for learning and training. Examples might be simplified to improve reading and learning.
Tutorials, references, and examples are constantly reviewed to avoid errors, but we cannot warrant full correctness
of all content. While using W3Schools, you agree to have read and accepted our terms of use, cookie and privacy policy.

Copyright 1999-2025 by Refsnes Data. All Rights Reserved. W3Schools is Powered by W3.CSS.

AltStyle によって変換されたページ (->オリジナル) /