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

Latest revision as of 13:29, 19 January 2024

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.

[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, 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.

[edit] Standard-layout

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

• 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.

[edit] 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

[edit] 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

[edit] Notes

[edit] Defect reports

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

[edit] See also