11 Classes [class]

11.2 Properties of classes [class.prop]

A trivially copyable class is a class:

(1.1)
that has at least one eligible copy constructor, move constructor, copy assignment operator, or move assignment operator ([special], [class.copy.ctor], [class.copy.assign]),
(1.2)
where each eligible copy constructor, move constructor, copy assignment operator, and move assignment operator is trivial, and
(1.3)
that has a trivial, non-deleted destructor ([class.dtor]).

A trivial class is a class that is trivially copyable and has one or more eligible default constructors ([class.default.ctor]), all of which are trivial.

[Note 1:

In particular, a trivially copyable or trivial class does not have virtual functions or virtual base classes.

— end note]

A class S is a standard-layout class if it:

(3.1)
has no non-static data members of type non-standard-layout class (or array of such types) or reference,
(3.2)
has no virtual functions and no virtual base classes,
(3.3)
has the same access control for all non-static data members,
(3.4)
has no non-standard-layout base classes,
(3.5)
has at most one base class subobject of any given type,
(3.6)
has all non-static data members and bit-fields in the class and its base classes first declared in the same class, and
(3.7)
has no element of the set M(S) of types as a base class, where for any type X, M(X) is defined as follows.89
[Note 2:
M(X) is the set of the types of all non-base-class subobjects that can be at a zero offset in X.
— end note]
- (3.7.1)
  If X is a non-union class type with no non-static data members, the set M(X) is empty.
- (3.7.2)
  If X is a non-union class type with a non-static data member of type $X_{0}$ that is either of zero size or is the first non-static data member of X (where said member may be an anonymous union), the set M(X) consists of $X_{0}$ and the elements of $M (X_{0})$ .
- (3.7.3)
  If X is a union type, the set M(X) is the union of all $M (U_{i})$ and the set containing all $U_{i}$ , where each $U_{i}$ is the type of the $i^{th}$ non-static data member of X.
- (3.7.4)
  If X is an array type with element type $X_{e}$ , the set M(X) consists of $X_{e}$ and the elements of $M (X_{e})$ .
- (3.7.5)
  If X is a non-class, non-array type, the set M(X) is empty.

[Example 1: struct B { int i; }; // standard-layout class struct C : B { }; // standard-layout class struct D : C { }; // standard-layout class struct E : D { char : 4; }; // not a standard-layout class struct Q {}; struct S : Q { }; struct T : Q { }; struct U : S, T { }; // not a standard-layout class — end example]

A standard-layout struct is a standard-layout class defined with the class-key struct or the class-key class.

A standard-layout union is a standard-layout class defined with the class-key union.

[Note 3:

Standard-layout classes are useful for communicating with code written in other programming languages.

Their layout is specified in [class.mem].

— end note]

[Example 2: struct N { // neither trivial nor standard-layout int i; int j; virtual ~N(); }; struct T { // trivial but not standard-layout int i; private: int j; }; struct SL { // standard-layout but not trivial int i; int j; ~SL(); }; struct POD { // both trivial and standard-layout int i; int j; }; — end example]

[Note 4:

Aggregates of class type are described in [dcl.init.aggr].

— end note]

A class S is an implicit-lifetime class if

(9.1)
it is an aggregate whose destructor is not user-provided or
(9.2)
it has at least one trivial eligible constructor and a trivial, non-deleted destructor.

89)89)

This ensures that two subobjects that have the same class type and that belong to the same most derived object are not allocated at the same address ([expr.eq]).