Lecture 13
Constructors
A constructor is a member function with the same name as its class. For example:
- class X {
- public:
- X(); // constructor for class X
- };
Constructors are used to create, and can initialize, objects of their class type.
You cannot declare a constructor as virtual or static, nor can you declare a constructor as const, volatile, or const volatile.
You do not specify a return type for a constructor. A return statement in the body of a constructor cannot have a return value.
- An introduction to C++ constructors with examples
- Calling other constructors in a constructor
Default constructors
A default constructor is a constructor that either has no parameters, or if it has parameters, all the parameters have default values.
If no user-defined constructor exists for a class A and one is needed, the compiler implicitly declares a default parameterless constructor A::A(). This constructor is an inline public member of its class. The compiler will implicitly define A::A() when the compiler uses this constructor to create an object of type A.
The compiler first implicitly defines the implicitly declared constructors of the base classes and nonstatic data members of a class A before defining the implicitly declared constructor of A.
A constructor of a class A is trivial if all the following are true:
- It is implicitly defined
- A has no virtual functions and no virtual base classes
- All the direct base classes of A have trivial constructors
- The classes of all the nonstatic data members of A have trivial constructors
If any of the above are false, then the constructor is nontrivial.
Destructors
Destructors are usually used to deallocate memory and do other cleanup for a class object and its class members when the object is destroyed. A destructor is called for a class object when that object passes out of scope or is explicitly deleted.
A destructor is a member function with the same name as its class prefixed by a ~ (tilde). For example:
- class X {
- public:
- // Constructor for class X
- X();
- // Destructor for class X
- ~X();
- };
A destructor takes no arguments and has no return type. Its address cannot be taken. Destructors cannot be declared const, volatile, const volatile or static. A destructor can be declared virtual or pure virtual.
If no user-defined destructor exists for a class and one is needed, the compiler implicitly declares a destructor. This implicitly declared destructor is an inline public member of its class.
The compiler will implicitly define an implicitly declared destructor when the compiler uses the destructor to destroy an object of the destructor's class type. Suppose a class A has an implicitly declared destructor. The following is equivalent to the function the compiler would implicitly define for A:
A::~A() { }
The compiler first implicitly defines the implicitly declared destructors of the base classes and nonstatic data members of a class A before defining the implicitly declared destructor of A
A destructor of a class A is trivial if all the following are true:
- It is implicitly defined
- All the direct base classes of A have trivial destructors
- The classes of all the nonstatic data members of A have trivial destructors
- If any of the above are false, then the destructor is nontrivial.
A union member cannot be of a class type that has a nontrivial destructor.
Class members that are class types can have their own destructors. Both base and derived classes can have destructors, although destructors are not inherited. If a base class A or a member of A has a destructor, and a class derived from A does not declare a destructor, a default destructor is generated.
The default destructor calls the destructors of the base class and members of the derived class.
The destructors of base classes and members are called in the reverse order of the completion of their constructor:
- The destructor for a class object is called before destructors for members and bases are called.
- Destructors for nonstatic members are called before destructors for base classes are called.
- Destructors for nonvirtual base classes are called before destructors for virtual base classes are called.