Constructors and member initializer lists (original) (raw)

Constructors are non-static member functions declared with a special declarator syntax, they are used to initialize objects of their class types.

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[edit] Syntax

Constructors are declared using member function declarators of the following form:

| | | | | ---------------------------------------------------------------------------------- | | | | class-name ( parameter-list (optional) ) except (optional) attr (optional) | | | | | | |

The only specifiers allowed in the declaration specifiers of a constructor declaration are friend, inline, constexpr(since C++11), consteval(since C++20), and explicit (in particular, no return type is allowed). Note that cv- and ref-qualifiers are not allowed either: const and volatile semantics of an object under construction only kick in after the most-derived constructor completes.

The identifier expression of class-name must have one of the following forms:

[edit] Member initializer list

The body of a function definition of any constructor of class T, before the opening brace of the compound statement, may include the member initializer list , whose syntax is the colon character **:**, followed by the comma-separated list of one or more member-initializer s, each of which has the following syntax:
member initializer (1)
class initializer (2)
class-pack initializer ... (3) (since C++11)

  1. Direct-initializes the data member named by member with initializer . member can only name non-static data members.

  2. Initializes a class object with initializer . class can only name the following classes:

  1. Initializes multiple base class subobjects using a pack expansion.
member - an identifier that names a data member
class - a class name
class-pack - a pack that expands to zero or more classes
initializer - an initializer that does not begin with =

struct S { int n;   S(int); // constructor declaration   S() : n(7) {} // constructor definition: // ": n(7)" is the initializer list // ": n(7) {}" is the function body };   S::S(int x) : n{x} {} // constructor definition: ": n{x}" is the initializer list   int main() { S s; // calls S::S() S s2(10); // calls S::S(int) }

[edit] Explanation

Constructors have no names and cannot be called directly. They are invoked when initialization takes place, and they are selected according to the rules of initialization. The constructors without explicit specifier are converting constructors. The constructors with a constexpr specifier make their type a literal type. Constructors that may be called without any argument are default constructors. Constructors that take another object of the same type as the argument are copy constructors and move constructors.

Before the compound statement that forms the function body of the constructor begins executing, initialization of all direct bases, virtual bases, and non-static data members is finished. The member initializer list is the place where non-default initialization of these subobjects can be specified. For bases that cannot be default-initialized and for non-static data members that cannot be initialized by default-initialization or by their default member initializer, if any(since C++11), such as members of reference and const-qualified types, member initializers must be specified. (Note that default member initializers for non-static data members of class template instantiations may be invalid if the member type or initializer is dependent.)(since C++11) No initialization is performed for anonymous unions or variant members that do not have a member initializer or default member initializer(since C++11).

The initializers where class names a virtual base class are ignored during construction of any class that is not the most derived class of the object that is being constructed.

Names that appear in initializer are evaluated in scope of the constructor:

class X { int a, b, i, j; public: const int& r; X(int i) : r(a) // initializes X::r to refer to X::a , b{i} // initializes X::b to the value of the parameter i , i(i) // initializes X::i to the value of the parameter i , j(this->i) // initializes X::j to the value of X::i {} };

Exceptions that are thrown from member initializers may be handled by a function try block.

If a non-static data member has a default member initializer and also appears in a member initializer list, then the member initializer is used and the default member initializer is ignored: struct S { int n = 42; // default member initializer S() : n(7) {} // will set n to 7, not 42 }; (since C++11)

Reference members cannot be bound to temporaries in a member initializer list:

struct A { A() : v(42) {} // Error const int& v; };

Note: same applies to default member initializer.

[edit] Operations during construction and destruction

Member functions (including virtual member functions) can be called for an object under construction or destruction. Similarly, an object under construction or destruction can be the operand of typeid or dynamic_cast.

However, if these operations are performed during any of the following evaluations, the behavior is undefined:

Delegating constructor If the name of the class itself appears as class-or-identifier in the member initializer list, then the list must consist of that one member initializer only; such a constructor is known as the delegating constructor, and the constructor selected by the only member of the initializer list is the target constructor.In this case, the target constructor is selected by overload resolution and executed first, then the control returns to the delegating constructor and its body is executed.Delegating constructors cannot be recursive. class Foo { public: Foo(char x, int y) {} Foo(int y) : Foo('a', y) {} // Foo(int) delegates to Foo(char, int) }; Inheriting constructors See using declaration. (since C++11)

[edit] Initialization order

The order of member initializers in the list is irrelevant, the actual order of initialization is as follows:

  1. If the constructor is for the most-derived class, virtual bases are initialized in the order in which they appear in depth-first left-to-right traversal of the base class declarations (left-to-right refers to the appearance in base-specifier lists).

  2. Then, direct bases are initialized in left-to-right order as they appear in this class's base-specifier list.

  3. Then, non-static data member are initialized in order of declaration in the class definition.

  4. Finally, the body of the constructor is executed.

(Note: if initialization order was controlled by the appearance in the member initializer lists of different constructors, then the destructor wouldn't be able to ensure that the order of destruction is the reverse of the order of construction.)

[edit] Notes

Feature-test macro Value Std Feature
__cpp_delegating_constructors 200604L (C++11) Delegating constructors

[edit] Example

#include #include #include   struct Base { int n; };
  struct Class : public Base { unsigned char x; unsigned char y; std::mutex m; std::lock_guard<std::mutex> lg; std::fstream f; std::string s;   Class(int x) : Base{123}, // initialize base class x(x), // x (member) is initialized with x (parameter) y{0}, // y initialized to 0 f{"test.cc", std::ios::app}, // this takes place after m and lg are initialized s(func), // func is available because init-list is a part of constructor lg(m), // lg uses m, which is already initialized m{} // m is initialized before lg even though it appears last here {} // empty compound statement   Class(double a) : y(a + 1), x(y), // x will be initialized before y, its value here is indeterminate lg(m) {} // base class initializer does not appear in the list, it is // default-initialized (not the same as if Base() were used, which is value-init)   Class() try // function try block begins before the function body, which includes init list : Class(0.0) // delegate constructor { // ... } catch (...) { // exception occurred on initialization } };   int main() { Class c; Class c1(1); Class c2(0.1); }

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
CWG 194 C++98 the declarator syntax of constructor only allowedat most one function specifier (e.g. a constructorcannot be declared inline explicit) multiple functionspecifiers allowed
CWG 257 C++98 it was unspecified whether an abstract class shouldprovide member initializers for its virtual base classes specified as not requiredand such member initializersare ignored during execution
CWG 263 C++98 the declarator syntax of constructorprohibited constructors from being friends allowed constructorsto be friends
CWG 1345 C++98 anonymous union members without defaultmember initializers were default-initialized they are not initialized
CWG 1435 C++98 the meaning of “class name” in thedeclarator syntax of constructor was unclear changed the syntax to a specializedfunction declarator syntax
CWG 1696 C++98 reference members could be initialized to temporaries(whose lifetime would end at the end of constructor) such initializationis ill-formed

[edit] References

[edit] See also