Default constructor

In computer programming languages the term “default constructor” can refer to a constructor that is automatically generated by the compiler in the absence of any programmer-defined constructors (e.g. in Java), and is usually a nullary constructor. In other languages (e.g. in C++) it is a constructor that can be called without having to provide any arguments, irrespective of whether the constructor is auto-generated or used-defined. Note that a constructor with formal parameters can still be called without arguments if default arguments were provided in the constructor's definition.

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). For example:

class MyClass
{
public:
    MyClass();  // constructor declared

private:
    int x;
};

MyClass :: MyClass()  // constructor defined
{
    x = 100;
}

int main()
{
    MyClass m;  // at runtime, object m is created, and the default constructor is called
}

When allocating memory dynamically, the constructor may be called by adding parenthesis after the class name. In a sense, this is an explicit call to the constructor:

int main()
{
    MyClass * pointer = new MyClass();  // at runtime, an object is created, and the
                                        // default constructor is called
}

If the constructor does have one or more parameters, but they all have default values, then it is still a default constructor. Remember that each class can have at most one default constructor, either one without parameters, or one whose all parameters have default values, such as in this case:

class MyClass
{
public:
    MyClass (int i = 0, std::string s = "");  // constructor declared

private:
    int x;
    int y;
    std::string z;
};

MyClass :: MyClass(int i, std::string s)     // constructor defined
{
    x = 100;
    y = i;
    z = s;
}

In C++, default constructors are significant because they are automatically invoked in certain circumstances; and therefore, in these circumstances, it is an error for a class to not have a default constructor:

• 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 or new MyClass(). The default constructor of MyClass 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 of MyClass 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 ten elements, which are filled with a default-constructed MyClass> object.

If a class has no explicitly defined constructors, the compiler will implicitly declare and define a default constructor for it. This implicitly defined default constructor is equivalent to a an explicitly defined one with an empty body. For example:

class MyClass
{
    int x;  // no constructor, so the compiler produces an (implicit) default constructor
};

int main()
{
    MyClass m;   // no error at runtime: the (implicit) default constructor is called
}

If constructors are explicitly defined for a class, but they are all non-default, the compiler will not implicitly define a default constructor, leading to a situation where the class does not have a default constructor. This is the reason for a typical error, demonstrated by the following example.

class MyClass
{
public:
    MyClass (int y);  // declaration a non-default constructor

private:
    int x;
};

MyClass :: MyClass (int y)
{
    x = y;
}

int main()
{
    MyClass m(100);     // the non-default constructor is called
    MyClass * p;        // for pointer declarations, the compiler does not need to know about constructors
    p = new MyClass();  // error at runtime: no default constructor
    return 0;
}

Since neither the writer nor the compiler has defined a default constructor, the creation of the objected pointed to by p leads to an error.

Java and C#

In both Java and C#, 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 it does nothing. A programmer-defined constructor that takes no parameters is also called a default constructor.