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 user-defined. Note that a constructor with formal parameters can still be called without arguments if default arguments were provided in the constructor's definition.

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:

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:

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:

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:

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 an explicitly defined one with an empty body. For example:

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.

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

On the other hand in C++11 a default constructor can be explicitly created:

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