Non-static data members

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Non-static data members are declared in a member specification of a class. 

class S
{
    int n;              // non-static data member
    int& r;             // non-static data member of reference type
    int a[2] = {1, 2};  // non-static data member with default member initializer (C++11)
    std::string s, *ps; // two non-static data members
 
    struct NestedS
    {
        std::string s;
    } d5;               // non-static data member of nested type
 
    char bit : 2;       // two-bit bitfield
};

Any simple declarations are allowed, except

  • thread_local storage class specifier is not allowed (but it is allowed for static data members);
(since C++11)
  • incomplete types, abstract class types, and arrays thereof are not allowed: in particular, a class C cannot have a non-static data member of type C, although it can have a non-static data member of type C& (reference to C) or C* (pointer to C);
  • a non-static data member cannot have the same name as the name of the class if at least one user-declared constructor is present;
(since C++11)

In addition, bit-field declarations are allowed.

Layout

When an object of some class C is created, each non-static data member of non-reference type is allocated in some part of the object representation of C. Whether reference members occupy any storage is implementation-defined, but their storage duration is the same as that of the object in which they are members.

For non-union class types, non-zero-sized (since C++20) members not separated by an access specifier (until C++11)with the same member access (since C++11) are always allocated so that the members declared later have higher addresses within a class object. Members separated by an access specifier (until C++11)with different access control (since C++11) are allocated in unspecified order (the compiler may group them together).

(until C++23)

For non-union class types, non-zero-sized members are always allocated so that the members declared later have higher addresses within a class object. Note that access control of member still affects the standard-layout property (see below).

(since C++23)

Alignment requirements may necessitate padding between members, or after the last member of a class.

Standard-layout

A class is considered to be standard-layout and to have properties described below if and only if it is a POD class.

(until C++11)

A class where all non-static data members have the same access control and certain other conditions are satisfied is known as standard-layout class (see standard-layout class for the list of requirements).

(since C++11)

The common initial sequence of two standard-layout non-union class types is the longest sequence of non-static data members and bit-fields in declaration order, starting with the first such entity in each of the classes, such that

  • either both entities are declared with [[no_unique_address]] attribute or neither is,
(since C++20)
  • corresponding entities have layout-compatible types,
  • corresponding entities have the same alignment requirements, and
  • either both entities are bit-fields with the same width or neither is a bit-field. 
struct A { int a; char b; };
struct B { const int b1; volatile char b2; }; 
// A and B's common initial sequence is A.a, A.b and B.b1, B.b2
 
struct C { int c; unsigned : 0; char b; };
// A and C's common initial sequence is A.a and C.c
 
struct D { int d; char b : 4; };
// A and D's common initial sequence is A.a and D.d
 
struct E { unsigned int e; char b; };
// A and E's common initial sequence is empty

Two standard-layout non-union class types are called layout-compatible if they are the same type ignoring cv-qualifiers, if any, are layout-compatible enumerations (i.e. enumerations with the same underlying type), or if their common initial sequence consists of every non-static data member and bit-field (in the example above, A and B are layout-compatible).

Two standard-layout unions are called layout-compatible if they have the same number of non-static data members and corresponding non-static data members (in any order) have layout-compatible types.

Standard-layout types have the following special properties:

  • In a standard-layout union with an active member of non-union class type T1, it is permitted to read a non-static data member m of another union member of non-union class type T2 provided m is part of the common initial sequence of T1 and T2 (except that reading a volatile member through non-volatile glvalue is undefined).
  • A pointer to an object of standard-layout class type can be reinterpret_cast to pointer to its first non-static non-bitfield data member (if it has non-static data members) or otherwise any of its base class subobjects (if it has any), and vice versa. In other words, padding is not allowed before the first data member of a standard-layout type. Note that strict aliasing rules still apply to the result of such cast.
  • The macro offsetof may be used to determine the offset of any member from the beginning of a standard-layout class.

Member initialization

Non-static data members may be initialized in one of two ways:

1) In the member initializer list of the constructor. 
struct S
{
    int n;
    std::string s;
    S() : n(7) {} // direct-initializes n, default-initializes s
};
2) Through a default member initializer, which is a brace or equals initializer included in the member declaration and is used if the member is omitted from the member initializer list of a constructor. 
struct S
{
    int n = 7;
    std::string s{'a', 'b', 'c'};
    S() {} // default member initializer will copy-initialize n, list-initialize s
};

If a member has a default member initializer and also appears in the member initialization list in a constructor, the default member initializer is ignored for that constructor.

Run this code
#include <iostream>
 
int x = 0;
struct S
{
    int n = ++x;
    S() {}                 // uses default member initializer
    S(int arg) : n(arg) {} // uses member initializer 
};
 
int main()
{
    std::cout << x << '\n'; // prints 0
    S s1;                   // default initializer ran
    std::cout << x << '\n'; // prints 1
    S s2(7);                // default initializer did not run
    std::cout << x << '\n'; // prints 1
}

Default member initializers are not allowed for bit-field members.

(until C++20)

Members of array type cannot deduce their size from member initializers: 

struct X
{
    int a[] = {1, 2, 3};  // error
    int b[3] = {1, 2, 3}; // OK
};

Default member initializers are not allowed to cause the implicit definition of a defaulted default constructor for the enclosing class or the exception specification of that constructor: 

struct node
{
    node* p = new node; // error: use of implicit or defaulted node::node() 
};

Reference members cannot be bound to temporaries in a default member initializer (note; same rule exists for member initializer lists): 

struct A
{
    A() = default;     // OK
    A(int v) : v(v) {} // OK
    const int& v = 42; // OK
};
 
A a1;    // error: ill-formed binding of temporary to reference
A a2(1); // OK (default member initializer ignored because v appears in a constructor)
         // however a2.v is a dangling reference
 (since C++11)


If a reference member is initialized from its default member initializer (until C++20)a member has a default member initializer (since C++20) and a potentially-evaluated subexpression thereof is an aggregate initialization that would use that default member initializer, the program is ill-formed: 

struct A;
extern A a;
 
struct A
{
    const A& a1{A{a, a}}; // OK
    const A& a2{A{}};     // error
};
 
A a{a, a};                // OK
 (since C++17)

Usage

The name of a non-static data member or a non-static member function can only appear in the following three situations:

1) As a part of class member access expression, in which the class either has this member or is derived from a class that has this member, including the implicit this-> member access expressions that appear when a non-static member name is used in any of the contexts where this is allowed (inside member function bodies, in member initializer lists, in the in-class default member initializers). 
struct S
{
    int m;
    int n;
    int x = m;            // OK: implicit this-> allowed in default initializers (C++11)
 
    S(int i) : m(i), n(m) // OK: implicit this-> allowed in member initializer lists
    {
        this->f();        // explicit member access expression
        f();              // implicit this-> allowed in member function bodies
    }
 
    void f();
};
2) To form a pointer to non-static member. 
struct S
{
    int m;
    void f();
};
 
int S::*p = &S::m;       // OK: use of m to make a pointer to member
void (S::*fp)() = &S::f; // OK: use of f to make a pointer to member
3) (for data members only, not member functions) When used in unevaluated operands. 
struct S
{
    int m;
    static const std::size_t sz = sizeof m; // OK: m in unevaluated operand
};
 
std::size_t j = sizeof(S::m + 42); // OK: even though there is no "this" object for m
Notes: such uses are allowed via the resolution of CWG issue 613 in N2253, which is treated as a change in C++11 by some compilers (e.g. clang).

Notes

Feature-test macro Value Std Comment
__cpp_nsdmi 200809L (C++11) Non-static data member initializers
__cpp_aggregate_nsdmi 201304L (C++14) Aggregate classes with default member initializers

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 80 C++98 all data members cannot have the same name
as the name of the class (breaks C compatibility) 		allow non-static data members
share the class name if there is
no user-declared constructor
CWG 190 C++98 when determining layout compatibility,
all members were considered 		only consider non-
static data members
CWG 613 C++98 unevaluated uses of non-static data members not allowed such uses are allowed
CWG 645 C++98 it was unspecified whether bit-field and
non-bit-field members are layout compatible 		not layout compatible
CWG 1397 C++11 class was regarded as complete
in the default member initializers 		default member init cannot trigger
definition of default constructor
CWG 1425 C++98 it was unclear whether a standard-layout object
shares the same address with the first non-static
data member or the first base class subobject 		non-static data member
if present, otherwise base
class subobject if present
CWG 1696 C++98 reference members could be initialized to temporaries
(whose lifetime would end at the end of constructor) 		such init is ill-formed
CWG 1719 C++98 differently cv-qualified types weren't layout-compatible cv-quals ignored, spec improved
CWG 2254 C++11 pointer to standard-layout class with no data
members can be reinterpret_cast to its first base class 		can be reinterpret_cast
to any of its base classes
CWG 2583 C++11 common initial sequence did not consider alignment requirements considered

See also

classes
static members
non-static member functions
(C++11)
 checks if a type is a standard-layout type
(class template)
byte offset from the beginning of a standard-layout type to specified member
(function macro)
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