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]).

(2.1)
overload resolution for direct-initializing an object of type C from an xvalue of type C selects a constructor that is a direct member of C and is neither user-provided nor deleted,
(2.2)
overload resolution for assigning to an lvalue of type C from an xvalue of type C selects an assignment operator function that is a direct member of C and is neither user-provided nor deleted, and
(2.3)
C has a destructor that is neither user-provided nor deleted.

A class is eligible for trivial relocation unless it

(3.1)
has any virtual base classes,
(3.2)
has a base class that is not a trivially relocatable class,
(3.3)
has a non-static data member of an object type that is not of a trivially relocatable type, or
(3.4)
has a deleted destructor,

except that it is implementation-defined whether an otherwise-eligible union having one or more subobjects of polymorphic class type is eligible for trivial relocation.

A class C is a trivially relocatable class if it is eligible for trivial relocation and

(4.1)
has the trivially_relocatable_if_eligible class-property-specifier,
(4.2)
is a union with no user-declared special member functions, or
(4.3)
is default-movable.

[Note 1 :

A class with const-qualified or reference non-static data members can be trivially relocatable.

— end note]

A class C is eligible for replacement unless

(6.1)
it has a base class that is not a replaceable class,
(6.2)
it has a non-static data member that is not of a replaceable type,
(6.3)
overload resolution fails or selects a deleted constructor when direct-initializing an object of type C from an xvalue of type C ([dcl.init.general]),
(6.4)
overload resolution fails or selects a deleted assignment operator function when assigning to an lvalue of type C from an xvalue of type C ([expr.assign], [over.assign]), or
(6.5)
it has a deleted destructor.

A class C is a replaceable class if it is eligible for replacement and

(7.1)
has the replaceable_if_eligible class-property-specifier,
(7.2)
is a union with no user-declared special member functions, or
(7.3)
is default-movable.

[Note 2 :

Accessibility of the special member functions is not considered when establishing trivial relocatability or replaceability.

— end note]

[Note 3 :

Not all trivially copyable classes are trivially relocatable or replaceable.

— end note]

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

(10.1)
has no non-static data members of type non-standard-layout class (or array of such types) or reference,
(10.2)
has no virtual functions and no virtual base classes,
(10.3)
has the same access control for all non-static data members,
(10.4)
has no non-standard-layout base classes,
(10.5)
has at most one base class subobject of any given type,
(10.6)
has all non-static data members and bit-fields in the class and its base classes first declared in the same class, and
(10.7)
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. 88
[Note 4 :
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]
- (10.7.1)
  If X is a non-union class type with no non-static data members, the set M(X) is empty.
- (10.7.2)
  If X is a non-union class type with a non-static data member of type 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 and the elements of .
- (10.7.3)
  If X is a union type, the set M(X) is the union of all and the set containing all , where each is the type of the non-static data member of X.
- (10.7.4)
  If X is an array type with element type , the set M(X) consists of and the elements of .
- (10.7.5)
  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 5 :

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

Their layout is specified in [class.mem.general] and [expr.rel] .

— end note]

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

[Note 6 :

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

— end note]

A class S is an implicit-lifetime class if

(16.1)
it is an aggregate whose destructor is not user-provided or
(16.2)
it has at least one trivial eligible constructor and a trivial, non-deleted destructor.

88) 88)

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]).