Object-Oriented Data Structures # WEEK03 : Constructors - default, copy & copy assignment operator

보다 구조적으로, c++ 하단에서 어떤일이 일어나는지 보여준다.

무심코 썼던 함수라거나, 전역변수를 초기화하지 않았을 때 디폴트값이 설정되는 것등을 해주는 보이지 않는 녀석들에 관한 이야기.

클래스 생성자, 자동 복사 생성자 등

 

생성자는 기본 생성자가 있고 복사해서 생성하는 방법이 있다. 

그리고 복사해서 생성할 때 변수 공간에 할당하는 연산자를 알아볼 것.

 

 

3.1 Class Constructor

 

When an instance pf a class is created, the class constructor sets up the initial state of the object.

ex. Default : Unit Cube(d=1)

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Automatic Default Constructor

If we do not provide any custom constructors, the C++ compiler provides an automatic default constructor for our class for free!

 

The automatic default constructor will only initialize all member variables to their default values.

 

* because no constructors present, automatic default constructor 

생성자가 없으면 자동 디폴트로 생성자가 생긴다.

 

Custom Default  Constructor

 

The simplest constructor we can provide is a custom default constructor that specifies the state of the object when the object is constructed. We define one by creating:

세 가지 요건을 모두 만족해야 사용자 지정 기본 생성자 정의 가능

• A member function with the same name of the class
• The function takes zero parameters
• The function does not have a return type.

Cube::Cube() // custom default constructor

 

Custom  Constructor

 

We can also specify custom, non-default constructors that require client code to supply argument:

Cube::Cube(double length) // one-argument ctor specifying initial length

 

If any custom constructor is defined, an automatic defualt constructor is not defined.

동시에 존재할 수 없음. 어떤 커스텀 생성자가 정의되면 자동 기본 생성자가 정의되는 것을 방지할 수 있다.

만약 둘 다 없다면 컴파일러가 에러를 발생시킬 것

 

C++ Constructor, set up the initial value of the class

In almost class we build, we going to define constructors so that we can control state of the class.

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3.2 Copy Constructor

 

In C++, a copy constructor is a special constructor that exists to make a copy of an existing object.

 

Automatic Copy Constructor

If we do not provide a custom copy constructor, the C++ compiler provides an automatic copy constructor for our class for free!

 

The automatic copy constructor will copy the contents of all member variables.

 

Custom Copy Constructor

A custom copy constructor is:

• A class constructor
• Has exactly one argument
  • The argument must be const reference of the same type as the class.

Cube::Cube(const Cube & obj)

 

복사 생성자는 거의 모든 경우에 자동으로 호출된다.

 

Copy Constructor Invocation

Often, copy constructors are invoked automatically:

• Passing an object as a parameter(by value)
• Returning an object from a function (by value)
• Initializing a new object

1	main에서 foo()호출
2	foo()에서 Cube c 생성 시, default Constructor 호출
3	foo()에서 main으로 c return할 때 copy Constructor 호출
4	main에서 c2라는 변수에 c를 대입할 때 copu Constructor 호출

 

1	Cube c; Cube myCube; -> default constructor로 cube 생성
2	myCube = c; 여기는 copy Constructor가 호출될 것으로 예상되지만 이미 다른 default constructor로 생성된 상태로 copy Constructor가 호출되지 않는다.
	대신 assignment operator가 호출되는 것! 이미 생성된 것을 대입하는 것이니까

 

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3.3 Copy Assignment Operator

 

코딩에서의 연산자 = 는 대입연산자! 복사해서 넣어줌.

 

In C++, a copy assignment operator defines the behavior when an object is copied using the assignment operator =

 

Copy Constructor vs Assignment

 

복사 생성자는 새로운 것을 만들고, 할당 연산자는 기존에 만들어진 것을 대입.

 

A copy constructor creates a new object (constructor).

 

A assignment operator assigns a value to an existing object.

• An assignment operator is always called on an object that has already been constructed

 

Automatic Assignment Operator

If an assignment operator is not provided, the C++ compiler provides an automatic assignment operator.

 

The automatic assignment operator will copy the contents of all member variables.

 

A custom assignment operator is:

• Is a public member function of the class.
• Has the function name operator =
• Has a return value of a reference of the class' type.
• Has exactly one argument
  • The argument must be const reference of the class' type

Cube & Cube:: operator = (const Cube & obj)