What is the order of constructor call when the object of derived class B is declared?
I must add to the previous answers because everyone seems to be ignoring it Show
When you have a derived class instance being created, it is true that the code inside the constructor of the base will be called before the code inside the constructor of the derived, but keep in mind that the derived is still technically "created" before the base. And when you have the derived class destructor being called, it is true that the code inside the derived destructor is called before the code inside the base destructor, but also keep in mind that the base is destroyed before the derived. When I am saying created/destroyed I am actually referring to allocated/deallocated. If you look at the memory layout of these instances, you will see that the derived instance composes the base instance. For example: Memory of derived: 0x00001110 to 0x00001120 Memory of base : 0x00001114 to 0x00001118 Therefore, the derived class must be allocated BEFORE the base in the construction. And the derived class must be deallocated AFTER the base in the destruction. If you have the following code:
So if you created Construction:
Destruction:
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In this articleTo customize how a class initializes its members, or to invoke functions when an
object of your class is created, define a constructor. A constructor has the same name as the class and no return value. You can define as many overloaded constructors as needed to customize initialization in various ways. Typically, constructors have public accessibility so that code outside the class definition or inheritance hierarchy can create objects of the class. But you can also declare a constructor as Constructors
can optionally take a member initializer list. It's a more efficient way to initialize class members than assigning values in the constructor body. The following example shows a class
When you declare an instance of a class, the compiler chooses which constructor to invoke based on the rules of overload resolution:
Member initializer listsA constructor can optionally have a member initializer list, which initializes class members before the constructor
body runs. (A member initializer list isn't the same thing as an initializer list of type Prefer member initializer lists over assigning values in the body of the constructor. A member initializer list directly initializes the members. The following example shows the member initializer list, which consists of all the
The identifier must refer to a class member; it's initialized with the value of the argument. The argument can be one of the constructor parameters, a function call or a
To ensure base classes are fully initialized before the derived constructor runs, call any parameterized base class constructors in the initializer list. Default constructorsDefault constructors typically have no parameters, but they can have parameters with default values.
Default constructors are one of the
special member functions. If no constructors are declared in a class, the compiler provides an implicit
If you rely on an implicit default constructor, be sure to initialize members in the class definition, as shown in the previous example. Without those initializers, the members would be uninitialized and the Volume() call would produce a garbage value. In general, it's good practice to initialize members in this way even when not relying on an implicit default constructor. You can prevent the compiler from generating an implicit default constructor by defining it as deleted:
A compiler-generated
default constructor will be defined as deleted if any class members aren't default-constructible. For example, all members of class type, and their class-type members, must have a default constructor and destructors that are accessible. All data members of reference type and all When you call a compiler-generated default constructor and try to use parentheses, a warning is issued:
This statement is an
example of the "Most Vexing Parse" problem. You could interpret If any non-default constructors are declared, the compiler doesn't provide a default constructor:
If a class has no default constructor, an array of objects of that class can't be constructed by using square-bracket syntax alone. For example, given the previous code block, an array of Boxes can't be declared like this:
However, you can use a set of initializer lists to initialize an array of Box objects:
For more information, see Initializers. Copy constructorsA copy constructor initializes an object by copying the member values from an object of the same type. If your class members are all simple types such as scalar values, the compiler-generated copy constructor is sufficient and you don't need to define your own. If your class requires more complex initialization, then you need to implement a custom copy constructor. For example, if a class member is a pointer then you need to define a copy constructor to allocate new memory and copy the values from the other's pointed-to object. The compiler-generated copy constructor simply copies the pointer, so that the new pointer still points to the other's memory location. A copy constructor may have one of these signatures:
When you define a copy constructor, you should also define a copy assignment operator (=). For more information, see Assignment and Copy constructors and copy assignment operators. You can prevent your object from being copied by defining the copy constructor as deleted:
Attempting to copy the object produces error C2280: attempting to reference a deleted function. Move constructorsA move constructor is a special member function that moves ownership of an existing object's data to a new variable without copying the original data. It takes an rvalue reference as its first parameter, and any later parameters must have default values. Move constructors can significantly increase your program's efficiency when passing around large objects.
The compiler chooses a move constructor when the object is initialized by another object of the same type, if the other object is about to be destroyed and no longer needs its resources. The following example shows one case when a move constructor is selected by overload resolution. In the constructor that calls
If a class doesn't define a move constructor, the compiler generates an implicit one if there's no user-declared copy constructor, copy assignment operator, move assignment operator, or destructor. If no explicit or implicit move constructor is defined, operations that would otherwise use a move constructor use the copy constructor instead. If a class declares a move constructor or move assignment operator, the implicitly declared copy constructor is defined as deleted. An implicitly declared move constructor is defined as deleted if any members that are class types lack a destructor or if the compiler can't determine which constructor to use for the move operation. For more information about how to write a non-trivial move constructor, see Move Constructors and Move Assignment Operators (C++). Explicitly defaulted and deleted constructorsYou can explicitly default copy constructors, default constructors, move constructors, copy assignment operators, move assignment operators, and destructors. You can explicitly delete all of the special member functions.
For more information, see Explicitly Defaulted and Deleted Functions. constexpr constructorsA constructor may be declared as constexpr if
Initializer list constructorsIf a constructor takes a
And then create Box objects like this:
Explicit constructorsIf a class has a constructor with a single parameter, or if all parameters except one have a default value, the parameter type can be implicitly converted to the class type. For example, if the
It's possible to initialize a Box like this:
Or pass an int to a function that takes a Box:
Such conversions can be useful in some cases, but more often they can lead to subtle but serious errors in your code. As a general rule, you should use the
When the constructor is explicit, this line causes a compiler error: Order of constructionA constructor performs its work in this order:
The following example shows the order in which base class and member constructors are called in the constructor for a derived class. First, the base constructor is called. Then, the base-class members are initialized in the order in which they appear in the class declaration. Finally, the derived constructor is called.
Here's the output:
A derived class constructor always calls a base class constructor, so that it can rely on completely constructed base classes before any extra work is done. The base class constructors are called in order of derivation—for example, if If a base class doesn't have a default constructor, you must supply the base class constructor parameters in the derived class constructor:
If a constructor throws an exception, the order of destruction is the reverse of the order of construction:
Derived constructors and extended aggregate initializationIf the constructor of a base class is non-public, but accessible to a derived class, then you can't use empty braces to initialize an
object of the derived type under The following example shows C++14 conformant behavior:
In C++17, The following example shows C++17 behavior in Visual Studio 2017 and later in
Constructors for classes that have multiple inheritanceIf a class is derived from multiple base classes, the base class constructors are invoked in the order in which they're listed in the declaration of the derived class:
You should expect the following output:
Delegating constructorsA delegating constructor calls a different constructor in the same class to do some of the work of initialization. This feature is useful when you have multiple constructors that all have to perform similar work. You can write the main logic in one constructor and invoke it from others. In the following trivial example, Box(int) delegates its work to Box(int,int,int):
The object created by the constructors is fully initialized as soon as any constructor is finished. For more information, see Delegating Constructors. Inheriting constructors (C++11)A derived class can inherit the constructors from a direct base class by using a
Visual Studio 2017 and later: The A class template can inherit all the constructors from a type argument if that type specifies a base class:
A deriving class can't inherit from multiple base classes if those base classes have constructors that have an identical signature. Constructors and composite classesClasses that contain class-type members are known as composite classes. When a class-type member of a composite class is created, the constructor is called before the class's own
constructor. When a contained class lacks a default constructor, you must use an initialization list in the constructor of the composite class. In the earlier
In this section
See alsoClasses and structs FeedbackSubmit and view feedback for What is the order in which constructors are called when an object of a derived class is created?The base class constructors are called in order of derivation—for example, if ClassA is derived from ClassB , which is derived from ClassC , the ClassC constructor is called first, then the ClassB constructor, then the ClassA constructor.
In what order are the class constructors and destructors called when a derived class object is created explain with the help of suitable program?As constructor is the first function called by the compiler when an object is created and the destructor is the last class member called by the compiler for an object. If the constructor and destructor are not declared by the user, the compiler defines the default constructor and destructor of a class object.
In what order are constructors called?When a class object is created using constructors, the execution order of constructors is: Constructors of Virtual base classes are executed, in the order that they appear in the base list. Constructors of nonvirtual base classes are executed, in the declaration order.
When a derived class object is created the constructor of the derived class is called first and then the base class constructor is called?1 Answer. Best explanation: First the base class constructor is invoked. When we create a derived class object, the system tries to invoke its constructor but the class is derived so first the base class must be initialized, hence in turn the base class constructor is invoked before the derived class constructor.
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