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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. == 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).〔C++ standard, ISO/IEC 14882:1998, 12.1.5 C++ standard, ISO/IEC 14882:2003, 12.1.5〕 For example: class MyClass ; MyClass :: MyClass() : x(100) // constructor defined int main() 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() 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 ; MyClass :: MyClass(int i, std::string s) // constructor defined 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();; or allocated dynamically, e.g. new MyClass (). 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 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 an explicitly defined one with an empty body. For example:〔Computer Science A Structured Approach Using C++ by Behrouz A. Forouzan and Richard F. Gilberg〕 class MyClass ; int main() 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 ; MyClass :: MyClass (int y) int main() 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.〔Computer Science A Structured Approach Using C++ by Behrouz A. Forouzan and Richard F. Gilberg〕On the other hand in C++11 a default constructor can be explicitly created: class MyClass ; Or explicitly inhibited: class MyClass ; 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「default constructor」の詳細全文を読む スポンサード リンク
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