Default constructors

 

C++

Language

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

 

C++ language

General topics
Preprocessor
Comments
Keywords
ASCII chart
Escape sequences
History of C++
Flow control
Conditional execution statements
if statement	switch statement
Iteration statements
for loop range-for loop (C++11)	while loop do-while loop
Jump statements
continue statement break statement	goto statement return statement
Functions
function declaration
lambda function declaration
function template
inline specifier
exception specifications (deprecated)
noexcept specifier (C++11)
Exceptions
throw expression	try-catch block
Namespaces
namespace declaration	namespace aliases
Types
fundamental types compound types enumeration types	union types function types
decltype specifier (C++11)
Specifiers
cv specifiers
storage duration specifiers
constexpr specifier (C++11)
auto specifier (C++11)
alignas specifier (C++11)
Literals
integer literals character literals floating-point literals string literals	boolean literals nullptr (C++11) user-defined (C++11)
Expressions
value categories order of evaluation	operators operator precedence
alternative representations
Utilities
Types
typedef declaration
type alias declaration (C++11)
attributes (C++11)
Casts
implicit conversions
const_cast conversion
static_cast conversion
dynamic_cast conversion
reinterpret_cast conversion
C-style and functional cast
Memory allocation
new expression	delete expression
Classes
class declaration this pointer access specifiers	friend specifier initializer lists
Class-specific function properties
virtual function override specifier (C++11) final specifier (C++11)	explicit (C++11) static
Special member functions
default constructor copy constructor move constructor (C++11)	copy assignment move assignment (C++11) destructor
Templates
class template
function template
template specialization
parameter packs (C++11)
Miscellaneous
Inline assembly

 

A default constructor is a constructor which can be called with no arguments (either defined with an empty parameter list, or with default arguments provided for every parameter). A type with a public default constructor is DefaultConstructible.

Syntax

ClassName ( ) ;	(1)
ClassName :: ClassName ( ) body	(2)
ClassName() = delete ;	(3)	(since C++11)
ClassName() = default ;	(4)	(since C++11)

Explanation

1) Declaration of a default constructor

2) Definition of the constructor outside the class body

3) Inhibiting the automatic generation of a default constructor

4) Explicitly forcing the automatic generation of a default constructor

ClassName is the identifier of the enclosing class

The default constructors are called when:

• creating objects or arrays of static, thread-local, and automatic storage duration that are declared without an initializer, (T obj; or T arr[10];)
• creating objects of dynamic storage duration when the new-expression is written without an initializer (new T;)
• creating arrays of dynamic storage duration with the expression new T[n]
• creating value-initialized temporary objects with the cast expression T().

Implicitly-declared default constructor

If no user-defined constructors of any kind are provided for a class type (struct, class, or union), the compiler will always declare a default constructor as an inline public member of its class. If some user-defined constructors are present, the user may still force the generation of the implicitly declared constructor with the keyword default (since C++11).

Deleted implicitly-declared default constructor

The implicitly-declared or defaulted default constructor for class T is undefined (until C++11) / defined as deleted (since C++11) in any of the following is true:

• T has a member of reference type (without a brace-or-equal initializer(since C++11)).
• T has a const member (without a brace-or-equal initializer(since C++11)) or a user-defined default constructor.
• T has a member (without a brace-or-equal initializer(since C++11)), which has a deleted default constructor, or its default constructor is ambiguous or inaccessible from this constructor.
• T has a direct or virtual base which has a deleted default constructor, or it is ambiguous or inaccessible from this constructor.
• T has a direct or virtual base which has a deleted destructor, or a destructor that is inaccessible from this constructor.
• T is a union with at least one variant member with non-trivial default constructor(since C++11).
• T is a union and all of its variant members are const.

Trivial default constructor

The implicitly-declared default constructor for class T is trivial if all of the following is true:

• T has no virtual member functions
• T has no virtual base classes
• T has no non-static members with brace-or-equal initializers (since C++11)
• Every direct base of T has a trivial default constructor
• Every non-static member of class type has a trivial default constructor

A trivial default constructor is a constructor that performs no action. Objects with trivial default constructors can be created by using reinterpret_cast on any storage, e.g. on memory allocated with std::malloc. All data types compatible with the C language (POD types) are trivially default-constructible.

Implicitly-defined default constructor

If the implicitly-declared default constructor is not deleted or trivial, it is defined (that is, a function body is generated and compiled) by the compiler, and it has exactly the same effect as a user-defined constructor with empty body and empty initializer list. That is, it calls the default constructors of the bases and of the non-static members of this class.

Example

struct A {
    int x;
    A(int x = 1) : x(x) {} // user-defined default ctor
};
struct B : A {
    // B::B() is implicitly-defined, calls A::A()
};
struct C {
    A obj;
    // C::C() is implicitly-defined, calls A::A()
};
struct D : A {
    D(int y) : A(y) {}
    // D::D() is not declared because another constructor exists
};
struct E : A
{
    E(int y) : A(y) {}
    E() = default; // explicitly defaulted, calls A::A()
};
 
struct F {
    int& ref; // reference member
    const int c; // const member
    // Bad::Bad() is deleted
};
 
int main()
{
    A a;
    B b;
//  D d; // compile error
    E e;
//  F f; // compile error
}