Default constructor

The term “default constructor” refers to a constructor that is automatically generated in the absence of explicit constructors (and perhaps under other circumstances); this automatically provided constructor is usually a nullary constructor. In specification or discussion of some languages, “default constructor” may additionally refer to any constructor that may be called without arguments, either because it is a nullary constructor or because all of its parameters have default values.

C++

In C++, the standard describes the default constructor for a class as a constructor that can be called with no arguments (this includes a constructor whose parameters all have default arguments).

In C++, default constructors are significant because they are automatically invoked in certain circumstances:

• When an object value is declared with no argument list, e.g. MyClass x;; or allocated dynamically with no argument list, e.g. new MyClass; the default constructor is used to initialize the object
• When an array of objects is declared, e.g. MyClass x[10];; or allocated dynamically, e.g. new MyClass [10]; the default constructor is used to initialize all the elements
• When a derived class constructor does not explicitly call the base class constructor in its initializer list, the default constructor for the base class is called
• When a class constructor does not explicitly call the constructor of one of its object-valued fields in its initializer list, the default constructor for the field's class is called
• In the standard library, certain containers "fill in" values using the default constructor when the value is not given explicitly, e.g. vector<MyClass>(10); initializes the vector with 10 elements, which are filled with the default-constructed value of our type.

In the above circumstances, it is an error if the class does not have a default constructor.

The compiler will implicitly define a default constructor if no constructors are explicitly defined for a class. This implicitly-declared default constructor is equivalent to a default constructor defined with a blank body. (Note: if some constructors are defined, but they are all non-default, the compiler will not implicitly define a default constructor. This means that a default constructor may not exist for a class.)

Java

In Java, a "default constructor" refers to a nullary constructor that is automatically generated by the compiler if no constructors have been defined for the class.The default constructor is also empty, meaning that is does nothing.