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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.

Contents

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, members not separated by an access specifier(until C++11)with the same member access(since C++11) and non-zero size(since C++20) are always allocated so that the members declared later have higher addresses within a class object. Members seperated 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). 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 PODType.

(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 type (see StandardLayoutType for the list of requirements).

(since C++11)

Two standard-layout non-union class types may have a common initial sequence of non-static data members and bit-fields, for a sequence of one or more initial members (in order of declaration), if the members have layout-compatible types either both declared with [[no_unique_address]] attribute or declared without the attribute(since C++20), and if they are bit-fields, they have the same width. 

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)(since C++11), 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;
    std::cout << x << '\n'; // prints 1 (default initializer ran)
    S s2(7);
    std::cout << x << '\n'; // prints 1 (default initializer did not run)
}

Output: 

0
1
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() 
};
 (since C++11)

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

It is an error if a default member initializer has a subexpression that would execute aggregate initialization which would use the same initializer: 

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++14)

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).

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 613 C++98 unevaluated uses of non-static data members not allowed such uses are allowed
CWG 1397 C++11 class was regarded as complete
in the default member initializers 		default member init cannot trigger
definition of default ctor
CWG 1696 C++98 reference members could be initialized to
temporaries (whose lifetime would end at the end of ctor) 		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

See also

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