Non-static data members

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Non-static data members are the variables that 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[10] = {1,2}; // non-static data member with initializer (C++11)
    std::string s, *ps; // two non-static data members
    struct NestedS {
        std::string s;
    } d5, *d6; // two non-static data members of nested type
    char bit : 2; // two-bit bitfield
};

Any simple declarations are allowed, except

  • extern and register storage class specifiers are not allowed
  • thread_local storage class specifier is not allowed (but it is allowed for static data members)
  • incomplete types are not allowed: in particular, a class C cannot have a non-static data member of class 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.

In addition, bit field declarations are allowed.

Non-static data members are allowed to have the same name as the name of the enclosing class unless the class has a user-declared constructor.

Contents

[edit] 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.

For non-union class types, members with the same member access are always allocated so that the members declared later have higher addresses within a class object. Members with different access control 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.

[edit] Standard layout

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).

Two standard-layout non-union class types may have a common initial sequence of non-static data members and bit-fields (since C++17), for a sequence of one or more initial members (in order of declaration), if the members have layout-compatible types and if they are bit-fields, they have the same width (since C++17). 

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 (since C++17), are layout-compatible enumerations, or if their common initial sequence consists of every non-static data member and bit field (since C++17) (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:

  • If a standard-layout union holds two (or more) standard-layout structs as members, and these structs have a common initial sequence of data members, it is well-defined to examine any member of that common initial sequence regardless of which member of the union is active.
(until C++17)
  • 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)
(since C++17)
  • A pointer to an object of standard-layout struct type can be reinterpret_cast to pointer to its first non-static data member (if it has non-static data members) or otherwise its first base class subobject (if it has any), and vice versa. (padding is not allowed before the first data member). 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 struct

[edit] 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 brace-or-equal initializer, which is simply an initializer included in the member declaration, which is used if the member is omitted in the member initializer list 
struct S {
    int n = 7;
    std::string s{'a', 'b', 'c'};
    S() // copy-initializes n, list-initializes s
    { }
};

If a member has a brace-or-equal initializer and also appears in the member initialization list in a constructor, the brace-or-equal initializer is ignored. 

#include <iostream>
int x = 0;
struct S {
    int n = ++x;
    S() {} // uses brace-or-equal initializer
    S(int arg) : n(arg) {} // uses member initializer list
};
int main()
{
    std::cout << x << '\n'; // prints 0
    S s1;
    std::cout << x << '\n'; // prints 1
    S s2(7);
    std::cout << x << '\n'; // prints 1
}
 (since C++11)

Reference members cannot be bound to temporaries in a brace-or-equal initializer 

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 (brace-or-equal initializer ignored because v appears in a constructor)
         // however a2.v is a dangling reference

Reference members cannot be initialized with brace-or-equal-initializers if it 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++17)

[edit] 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 brace-or-equal initializers) 
struct S {
   int m;
   int n;
   int x = m;  // OK: implicit this allowed in brace-or-equal initializers (C++11)
   S(int init) : m(init),
                 n(m) // OK:implicit this allowed in member initializer lists
   {
       this->f();  // explicit member access expression
       f(); // implicit this allowed in member functions
   }
   void f();
};
2) To form a pointer to 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) 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
 (since C++11)

[edit] See also

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