Difference between revisions of "cpp/language/data members"

Revision as of 21:44, 8 December 2021

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

• extern and register storage class specifiers are not allowed;

• 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;

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.

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

Standard layout

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:

struct S
{
    int n;
    std::string s;
    S() : n(7) {} // direct-initializes n, default-initializes s
};

struct S
{
    int n = 7;
    std::string s{'a', 'b', 'c'};
    S() {} // default member initializer will copy-initialize n, list-initialize s
};

#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
}

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

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

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

struct A;
extern A a;
 
struct A
{
    const A& a1{A{a, a}}; // OK
    const A& a2{A{}};     // error
};
 
A a{a, a};                // OK

Usage

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

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();
};

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

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

Defect reports

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

See also