Doc. no.   N2172=07-0032
Date:        2007-03-09
Project:     Programming Language C++
Reply to:   Beman Dawes <bdawes@acm.org>

POD's Revisited; Resolving Core Issue 568 (Revision 2)

Introduction
Summary of proposed changes
To Do
Features and benefits of POD types
Motivating examples
    std::pair example
    Endian example
    Two structs example
Coupling between POD's and aggregates
Rationale for changes
Proposed changes to the Working Paper
Open issues
Impact on existing code
Impact on existing ABI's
Interactions with other proposals
Revision history
Acknowledgements
References

Introduction

This paper proposes a resolution for Core Issue 568, Definition of POD is too strict, submitted by Matt Austern.

POD's as defined in the current working paper has several problems:

Summary of proposed changes

To Do

The current language rules for initialization (8.5) sometimes require POD types. In this proposal, such requirements have been relaxed to only require trivial types. It is anticipated that these rules can be further refined and relaxed, but that work is deferred pending proposed wording to implement the N2100 Initializer lists proposal.

Features and benefits of POD types

Features Benefits
Byte copyable guarantees [3.9 2-3, basic.types]
  • Programs can safely apply coding optimizations, particularly std::memcpy.
C layout-compatibility guarantees, byte copyable guarantees [9.2 14-17, class.mem], initialization rules.
  • C++ programs can interoperate with functions written in C and other languages.
  • C++ programs can, after considering compiler, alignment, and data type constraints, perform binary I/O such that files to interoperate with other languages and platforms.
  • C language compatibility.
Static initialization guarantees [3.6.2, basic.start.init]
  • Programs can avoid order-of-initialization issues.
  • Multi-threaded programs can avoid data races during initialization.
Are aggregates
  • Brace-enclosed initializer lists allowed.
Various rules for non-POD's
  • Compilers apply data layout optimizations to non-POD's.
  • Compilers assume non-aliasing, allowing code generation optimizations for non-POD's.

Motivating examples

std::pair example

Matt Austern provided this example:

If a program has two arrays of type std::pair<int,int>, then it is natural to expect that memcpy(A2,A1,sizeof(A2)) would be safe. Programmers have trouble imagining any implementation in which a byte-for-byte copy of std::pair<int,int> wouldn't do the right thing. Unfortunately, that's not what the language standard says. It says that byte-for-byte copies are guaranteed to work only for PODs. std::pair<T,U> isn't a class aggregate, since it has a user-defined constructor, and that means it also isn't a POD.

std::pair has a user-defined constructor essentially for syntactic reasons: because in some cases it looks nicer to write "std::pair<int,int> p(1,2);" than to write "std::pair<int,int> p = {1,2};". It seems a shame that this syntactic change caused the loss of the important semantic property of PODness. It's especially a shame because it means something formally doesn't work when on all real-world implementations it actually does work. It also encourages programmers to rely on undefined behavior, which is something the standard should not encourage.

With the proposed resolution, the std::pair<int,int> example is still not a POD because its default constructor has effects. With the proposal in place, however, it becomes possible to turn std::pair into a POD by removing the default constructor's effects. To avoid breaking existing code, that can be done under control of an additional template parameter. The intent is to propose such an addition to the LWG as the core language POD proposal moves forward.

Endian example

Beman Dawes provided this example:

Here is an example of something in development for Boost, based on classes used in industrial applications for many years. The fact that it is a template partial specialization isn't material to this discussion and can be ignored.

template <typename T, std::size_t n_bits>
class endian< big, T, n_bits, unaligned > : cover_operators< endian< big, T, n_bits >, T >
{
  BOOST_STATIC_ASSERT( (n_bits/8)*8 == n_bits );
public:
  typedef T value_type;
  endian() {}
  endian(T i) { detail::store_big_endian<T, n_bits/8>(bytes, i); }
  operator T() const { return detail::load_big_endian<T, n_bits/8>(bytes); }
private:
  char bytes[n_bits/8];
};

But it isn't a POD, so it won't work at all in unions and uses such as binary I/O rely on undefined behavior. Since the primary rationale for the existence of endian is to do binary I/O, forcing the user to rely on undefined behavior is unfortunate to say the least.

Here is what would have to be done to make it a POD:

Remove the constructors. But that makes initialization painful, so boosters are proposing to add an ugly and unintuitive static init function, and an operator= from the value_type. Those are partial workarounds, but not really what the designers, Beman Dawes and Darin Adler, wanted.

Make the data member public. But this encourages a poor design practice.

Eliminate the base class. But the only way to do that without the highly error-prone duplication of the functions provided by the base class is to introduce a lengthy macro. Enough said.

In other words, making this class a POD under current language rules would do serious damage to interface ease-of-use and to code quality, and would encourage poor design practices. Yet the only data member in the class is an array of char, so programmers intuitively expect the class to be memcpyable and binary I/O-able.

With the proposed resolution, the class becomes a POD, solving all the issues.

Two structs example

Matt Austern provided this example in Core DR 568:

It’s silly for the standard to make layout and memcpy guarantees for this class:

struct A {
  int n;
};

but not for this one:

struct B {
  int n;
  B(n_) : n(n_) { }
};

With either A or B, it ought to be possible to save an array of those objects to disk with a single call to Unix’s write(2) system call or the equivalent. At present the standard says that it’s legal for A but not B, and there isn’t any good reason for that distinction.

With the proposed resolution, the class can be easily changed (by adding a no effects default constructor) to become a POD, solving all the issues.

Coupling between POD's and aggregates

POD's provide object representation guarantees, layout-compatibility guarantees, memory contiguity guarantees, and memory copy-ability guarantees for fairly simple types, yet leave compilers much latitude in such matters for more complicated types.

Aggregates provide well-defined initialization from initializer-clauses.

The two concepts are at most tangential, if not completely orthogonal. Thus to define POD in terms of aggregates creates an unnecessary and confusing dependency. It makes otherwise straightforward changes to the Standard POD and aggregate sections much more difficult because of the need to analyze a potential change for impact on both POD's and aggregates. The coupling is confusing to users, causing them to make mistaken assumptions about POD's. The coupling may be part of the reason even committee members cannot accurately remember the full rules for POD-ness.

Rationale for changes

The proposed changes decompose the current POD requirements into trivial type requirements and standard-layout type requirements, and remove the dependency on the definition of aggregates. Because these decomposed requirements are somewhat less restrictive than the requirements for aggregates, the effect is to make POD's more broadly useful and solve the problems identified in the Introduction and Motivating examples. It also opens up the possibility of designing useful classes that meet one or the other, but not both, of the new trivial and standard-layout requirements.

As a consequence of  allowing members of any access control in standard-layout types, the current requirement that POD data members have no intervening access-specifiers is changed to require only that such data members have the same access control. This change is believed to also be more in line with programmer expectations than the current requirements.

Changes are not proposed that would allow POD's to have base classes with non-static data members. There was no apparent way to allow these cases without putting undue restrictions on how compilers allocate base class data in relation to derived class data.

Proposed changes to the Working Paper

Added text is shown in green and underlined. Deleted text is shown in red with strikethrough.

 Commentary is shown in boxed italics. Commentary is not part of the proposed WP changes.

Commentary is high-lighted if it requires some action by the committee, such as alterations required if some other pending proposal is accepted.

Since issue 538 is currently in review status,  changes to clause 9 paragraph 4 are shown relative to 538's proposed wording.

The following table lists all uses of POD, and related topics, in the current working paper, with proposed changes. Because the change to clause 9, paragraph 4,is critical to understanding the other changes, it is presented first.

Working Paper Text

9 4 Classes [class]

A structure is a class defined with the class-key struct; its members and base classes (clause 10) are public by default
(clause 11). A union is a class defined with the class-key union; it holds only one data member at a time (9.5). [Note: aggregates of class type are described in 8.5.1. —end note]

A trivial-class is a class that:

— has a trivial default constructor (12.1) or a default constructor defined in the class definition and having no effects, and

If N2210 or equivalent is accepted, strike the words "or a default constructor defined in the class definition and having no effects". These words are a workaround for a lack of expressiveness in the current language that is eliminated by N2210 .

— a trivial copy constructor (12.8), and
— a trivial copy assignment operator (13.5.3, 12.8), and
— a trivial destructor (12.4).

[Note: That precludes virtual functions, virtual bases, and members or bases with non-trivial default constructors having effects, non-trivial copy constructors, non-trivial copy assignments, or non-trivial destructors. --end note]

A standard-layout-class is a class that:

— has no non-static data members of type non-standard-layout-class (or array of such types) or reference, and
— has no virtual functions (10.3) and no virtual base classes (10.1), and
— has the same access control (clause 11) for all non-static data members, and
— has no non-standard-layout base classes, and
— has either no non-static data members in the most-derived class and at most one base class with non-static data members or has no base classes with non-static data members, and
— has no base classes of the same type as the first non-static data  member. [footnote]

[footnote] 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]).

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: Standard-layout classes are useful for communicating with code written in other programming languages. The layout is specified in 9.2. -- end note]

A POD-struct is an aggregate a class that is both a trivial class and standard-layout class, and has no non-static data members of type non-POD-struct, non-POD-union (or array of such types) or reference, and has no user-declared copy assignment operator and no user-declared destructor. Similarly, a POD-union is an aggregate a union that has no non-static data members of type non-POD-struct, non-POD-union (or array of such types) or reference, and has no user-declared copy assignment operator and no user-declared destructor. A POD class is a class that is either a POD-struct or a POD-union. [Note: virtual functions and base classes are prohibited in unions (9.5). -- end note.]

[Example:

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]
1.8 5 The C++ object  model [intro.object]

Unless it is a bit-field (9.6), a most derived object shall have a non-zero size and shall occupy one or more bytes of storage. Base class subobjects may have zero size. An object of POD type (3.9) trivial or standard-layout (clause 3.9) type shall occupy contiguous bytes of storage.
3.6.2 ¶1 Initialization of non-local objects [basic.start.init]

Objects with static storage duration (3.7.1) shall be zero-initialized (8.5) before any other initialization takes place. A reference with static storage duration and an object of POD trivial type with static storage duration can be initialized with a constant expression (5.19); this is called constant initialization. Together, zero-initialization and constant initialization are called static initialization; all other initialization is dynamic initialization. Static initialization shall be performed before any dynamic initialization takes place. Dynamic initialization of an object is either ordered or unordered. Definitions of explicitly specialized class template static data members have ordered initialization. Other class template static data members (i.e., implicitly or explicitly instantiated specializations) have unordered initialization. Other objects defined in namespace scope have ordered initialization. Objects defined within a single translation unit and with ordered initialization shall be initialized in the order of their definitions in the translation unit. The order of initialization is unspecified for objects with unordered initialization and for objects defined in different translation units. [ Note: 8.5.1 describes the order in which aggregate members are initialized. The initialization of local static objects is described in 6.7. —end note ]
3.8 ¶2 Object Lifetime [basic.life]

 [ Note: the lifetime of an array object or of an object of POD trivial type (3.9) starts as soon as storage with proper size and alignment is obtained, and its lifetime ends when the storage which the array or object occupies is reused or released. 12.6.2 describes the lifetime of base and member subobjects. —end note ]
3.8 ¶5 Object Lifetime [basic.life]

Before the lifetime of an object has started but after the storage which the object will occupy has been allocated39) or, after the lifetime of an object has ended and before the storage which the object occupied is reused or released, any pointer that refers to the storage location where the object will be or was located may be used but only in limited ways. Such a pointer refers to allocated storage (3.7.3.2), and using the pointer as if the pointer were of type void*, is well-defined. Such a pointer may be dereferenced but the resulting lvalue may only be used in limited ways, as described below. If the object will be or was of a class type with a non-trivial destructor, and the pointer is used as the operand of a delete-expression, the program has undefined behavior. If the object will be or was of a non-POD non-trivial class type, the program has undefined behavior if:

— the pointer is used to access a non-static data member or call a non-static member function of the object, or

— the pointer is implicitly converted (4.10) to a pointer to a base class type, or

— the pointer is used as the operand of a static_cast (5.2.9) (except when the conversion is to void*, or to void* and subsequently to char*, or unsigned char* )

— the pointer is used as the operand of a dynamic_cast (5.2.7).
3.8 ¶6 Object Lifetime [basic.life]

Similarly, before the lifetime of an object has started but after the storage which the object will occupy has been allocated or, after the lifetime of an object has ended and before the storage which the object occupied is reused or released, any lvalue which refers to the original object may be used but only in limited ways. Such an lvalue refers to allocated storage (3.7.3.2), and using the properties of the lvalue which do not depend on its value is well-defined. If an lvalue-to-rvalue conversion (4.1) is applied to such an lvalue, the program has undefined behavior; if the original object will be or was of a non-POD non-trivial class type, the program has undefined behavior if:

— the lvalue is used to access a non-static data member or call a non-static member function of the object, or

— the lvalue is implicitly converted (4.10) to a reference to a base class type, or

— the lvalue is used as the operand of a static_cast (5.2.9) except when the conversion is ultimately to cv char& or cv unsigned char& ), or

— the lvalue is used as the operand of a dynamic_cast (5.2.7) or as the operand of typeid.
3.9 ¶2 Types [basic.types]

For any object (other than a base-class subobject) of POD trivial type T, whether or not the object holds a valid value of type T, the underlying bytes (1.7) making up the object can be copied into an array of char or unsigned char.41) If the content of the array of char or unsigned char is copied back into the object, the object shall subsequently hold its original value.
3.9 ¶3 Types [basic.types]

For any POD trivial type T, if two pointers to T point to distinct T objects obj1 and obj2, where neither obj1 nor obj2 is a base-class subobject, if the value of obj1 is copied into obj2, using the std::memcpy library function, obj2 shall subsequently hold the same value as obj1.
3.9 ¶4 Types [basic.types]

The object representation of an object of type T is the sequence of N unsigned char objects taken up by the object of type T, where N equals sizeof(T). The value representation of an object is the set of bits that hold the value of type T. For POD trivial types, the value representation is a set of bits in the object representation that determines a value, which is one discrete element of an implementation-defined set of values.42)
3.9 ¶10 Types [basic.types]

Arithmetic types (3.9.1), enumeration types, pointer types, and pointer to member types (3.9.2), and cv-qualified versions of these types (3.9.3) are collectively called scalar types.

Scalar types, POD-struct types, POD-union types (clause 9), arrays of such types and cv-qualified versions of these types (3.9.3) are collectively called POD types.

Scalar types, trivial-class types (clause 9), arrays of such types and cv-qualified versions of these types (3.9.3) are collectively called trivial types.

Scalar types, standard-layout-class types (clause 9), arrays of such types and cv-qualified versions of these types (3.9.3) are collectively called standard-layout types.
3.9 ¶11 Types [basic.types]

If two types T1 and T2 are the same type, then T1 and T2 are layout-compatible types. [ Note: Layout-compatible enumerations are described in 7.2. Layout-compatible POD-structs standard-layout-structs and POD-unions standard-layout-unions are described in 9.2. —end note ]
5.2 ¶7 Postfix expressions [expr.post]

When there is no parameter for a given argument, the argument is passed in such a way that the receiving function can obtain the value of the argument by invoking va_arg (18.8). The lvalue-to-rvalue (4.1), array-to-pointer (4.2), and function-to-pointer (4.3) standard conversions are performed on the argument expression. After these conversions, if the argument does not have arithmetic, enumeration, pointer, pointer to member, or class type, the program is ill-formed. If the argument has a non-POD non-trivial class type (clause 9), the behavior is undefined. If the argument has integral or enumeration type that is subject to the integral promotions (4.5), or a floating point type that is subject to the floating point promotion (4.6), the value of the argument is converted to the promoted type before the call. These promotions are referred to as the default argument promotions.
5.3.4 ¶16 New [expr.new]

A new-expression that creates an object of type T initializes that object as follows:
— If the new-initializer is omitted:
    — If T is a (possibly cv-qualified) non-POD non-trivial class type (or array thereof), the object is default-initialized (8.5). If T is a const-qualified type, the underlying class type shall have a user-declared default constructor.
    — Otherwise, the object created has indeterminate value. If T is a const-qualified type, or a (possibly cv-qualified) POD trivial class type (or array thereof) containing (directly or indirectly) a member of const-qualified type, the program is ill-formed;
— If the new-initializer is of the form (), the item is value-initialized (8.5);
— If the new-initializer is of the form (expression-list) and T is a class type, the appropriate constructor is called, using expression-list as the arguments (8.5);
— If the new-initializer is of the form (expression-list) and T is an arithmetic, enumeration, pointer, or pointer-to-member type and expression-list comprises exactly one expression, then the object is initialized to the (possibly converted) value of the expression (8.5);
— Otherwise the new-expression is ill-formed.
5.9 ¶7 Relational operators [expr.rel]

Pointers to objects or functions of the same type (after pointer conversions) can be compared, with a result defined as follows:
...
— If two pointers point to non-static data members of the same object, or to subobjects or array elements of such
members, recursively, the pointer to the later declared member compares greater provided the two members are
not separated by an access-specifier label (11.1) have the same access control (clause 11) and provided their class is not a union.

— If two pointers point to non-static data members of the same object separated by an access-specifier label (11.1) with different access control (clause 11) the result is unspecified.

See rationale
5.19 ¶4 Constant expressions

An address constant expression is a pointer to an lvalue designating an object of static storage duration, a string literal (2.13.4), or a function. The pointer shall be created explicitly, using the unary & operator, or implicitly using a non-type template parameter of pointer type, or using an expression of array (4.2) or function (4.3) type. The subscripting operator [] and the class member access . and -> operators, the & and * unary operators, and pointer casts (except dynamic_casts, 5.2.7) can be used in the creation of an address constant expression, but the value of an object shall not be accessed by the use of these operators. If the subscripting operator is used, one of its operands shall be an integral constant expression. An expression that designates the address of a subobject of a non-POD non-trivial class object (clause 9) is not an address constant expression (12.7). Function calls shall not be used in an address constant expression, even if the function is inline and has a reference return type.
5.19 ¶5 Constant expressions [expr.const]

A reference constant expression is an lvalue designating an object of static storage duration, a non-type template parameter of reference type, or a function. The subscripting operator [], the class member access . and -> operators, the & and * unary operators, and reference casts (except those invoking user-defined conversion functions (12.3.2) and except dynamic_casts (5.2.7)) can be used in the creation of a reference constant expression, but the value of an object shall not be accessed by the use of these operators. If the subscripting operator is used, one of its operands shall be an integral constant expression. An lvalue expression that designates a member or base class of a non-POD non-trivial class object (clause 9) is not a reference constant expression (12.7). Function calls shall not be used in a reference constant expression, even if the function is inline and has a reference return type.
6.7 ¶3 Declaration statement [stmt.dcl]

It is possible to transfer into a block, but not in a way that bypasses declarations with initialization. A program that jumps82) from a point where a local variable with automatic storage duration is not in scope to a point where it is in scope is ill-formed unless the variable has POD trivial type (3.9) and is declared without an initializer (8.5).
6.8 ¶4 Ambiguity resolution [stmt.ambig]

The zero-initialization (8.5) of all local objects with static storage duration (3.7.1) is performed before any other initialization takes place. A local object of POD trivial type (3.9) with static storage duration initialized with constant-expressions is initialized before its block is first entered. An implementation is permitted to perform early initialization of other local objects with static storage duration under the same conditions that an implementation is permitted to statically initialize an object with static storage duration in namespace scope (3.6.2). Otherwise such an object is initialized the first time control passes through its declaration; such an object is considered initialized upon the completion of its initialization. If the initialization exits by throwing an exception, the initialization is not complete, so it will be tried again the next time control enters the declaration. If control re-enters the declaration (recursively) while the object is being initialized, the behavior is undefined.
8.5 ¶5 Initializers [dcl.init]

To zero-initialize an object of type T means:

— if T is a scalar type (3.9), the object is set to the value 0 (zero), taken as an integral constant expression, converted to T;93)
— if T is a non-union class type, each non-static data member and each base-class subobject is zero-initialized;
— if T is a union type, the object’s first named data member94) is zero-initialized;
— if T is an array type, each element is zero-initialized;
— if T is a reference type, no initialization is performed.

To default-initialize an object of type T means:

— if T is a non-POD non-trivial class type (clause 9), the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor);
— if T is a trivial class type ([class]) with a default constructor, there is no effect (and the initialization is ill-formed if T has an inaccessible default constructor);
 
If N2210 or equivalent is accepted, strike the words "if T is a trivial class type ([class]) with a default constructor, there is no effect (and the initialization is ill-formed if T has an inaccessible default constructor);". These words are a workaround for a lack of expressiveness in the current language that is eliminated by N2210 .

— if T is an array type, each element is default-initialized;
— otherwise, the object is zero-initialized.

To value-initialize an object of type T means:

— if T is a class type (clause 9) with a user-declared constructor (12.1), then the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor);
— if T is a non-union class type without a user-declared constructor, then every non-static data member and base-class component of T is value-initialized;95)
— if T is an array type, then each element is value-initialized;
— otherwise, the object is zero-initialized
8.5 ¶9 Initializers [dcl.init]

If no initializer is specified for an object, and the object is of (possibly cv-qualified) non-POD non-trivial class type (or array thereof), the object shall be default-initialized; if the object is of const-qualified type, the underlying class type shall have a user-declared default constructor. Otherwise, if no initializer is specified for a non-static object, the object and its subobjects, if any, have an indeterminate initial value97); if the object or any of its subobjects are of const-qualified type, the program is ill-formed.
8.5 ¶14 Initializers [dcl.init]

When an aggregate with static storage duration is initialized with a brace-enclosed initializer-list, if all the member initializer expressions are constant expressions, and the aggregate is a POD trivial type, the initialization shall be done during the static phase of initialization (3.6.2); otherwise, it is unspecified whether the initialization of members with constant expressions takes place during the static phase or during the dynamic phase of initialization.[footnote]

[footnote] trivial types with user-declared constructors are not aggregates ([dcl.init.aggr]) and so are not required to be statically initialized.

8.5.1 ¶1 Aggregates [dcl.init.aggr]

An aggregate is an array or a class (clause 9) with no user-declared constructors (12.1) other than a default constructor defined in the class definition and having no effects, no private or protected non-static data members (clause 11), no base classes with non-static data members (clause 10), and no virtual functions (10.3).
 
If N2210 or equivalent is accepted, strike the words "other than a default constructor defined in the class definition and having no effects". These words are a workaround for a lack of expressiveness in the current language that is eliminated by N2210 .

 
Portland: "no user-declared constructors" wording unchanged at request of CWG.

9.2 ¶12 Class members [class.mem]

Nonstatic data members of a (non-union) class declared without an intervening access-specifier with the same access control (clause 11) are allocated so that later members have higher addresses within a class object. The order of allocation of non-static data members separated by an access-specifier with different access control is unspecified (11.1). Implementation alignment requirements might cause two adjacent members not to be allocated immediately after each other; so might requirements for space for managing virtual functions (10.3) and virtual base classes (10.1).
 
See rationale.

9.2 ¶15-18 Class members [class.mem]

15 Two POD-struct standard-layout-struct (clause 9) types are layout-compatible if they have the same number of non-static data members, and corresponding non-static data members (in order) have layout-compatible types (3.9).

16 Two POD-union standard-layout-union (clause 9) types are 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 (3.9).

17 If a POD-union standard-layout-union contains two or more POD-structs standard-layout-structs that share a common initial sequence, and if the POD-union standard-layout-union object currently contains one of these POD-structs standard-layout-structs, it is permitted to inspect the common initial part of any of them. Two POD-structs standard-layout-structs share a common initial sequence if corresponding members have layout-compatible types (and, for bit-fields, the same widths) for a sequence of one or more initial members.

18 A pointer to a POD-struct standard-layout-struct object, suitably converted using a reinterpret_cast, points to its initial member (or if that member is a bit-field, then to the unit in which it resides) and vice versa. [ Note: There might therefore be unnamed padding within a POD-struct standard-layout-struct object, but not at its beginning, as necessary to achieve appropriate alignment. —end note ]
9.5 ¶1 Unions [class.union]

In a union, at most one of the data members can be active at any time, that is, the value of at most one of the data members can be stored in a union at any time. [ Note: one special guarantee is made in order to simplify the use of unions: If a POD-union standard-layout-union contains several POD-structs standard-layout-structs that share a common initial sequence (9.2), and if an object of this POD-union standard-layout-union type contains one of the POD-structs standard-layout-structs, it is permitted to inspect the common initial sequence of any of POD-struct standard-layout-struct members; see 9.2. —end note ] The size of a union is sufficient to contain the largest of its data members. Each data member is allocated as if it were the sole member of a struct. A union can have member functions (including constructors and destructors), but not virtual (10.3) functions. A union shall not have base classes. A union shall not be used as a base class. An object of a class with a non-trivial default constructor (12.1), a non-trivial copy constructor (12.8), a non-trivial destructor (12.4), or a non-trivial copy assignment operator (13.5.3, 12.8) cannot be a member of a union, nor can an array of such objects. If a union contains a static data member, or a member of reference type, the program is ill-formed.
11.1 ¶3 Access Specifiers [class.access.spec]

The order of allocation of data members with separate access-specifier labels different access control is unspecified (9.2), except as described in [class.mem].
 
The "except as described in [class.mem]" wording is a clarification to make it clearer that [class.mem] applies.

12.6.2 ¶4 Initializing bases and members [class.base.init]

If a given non-static data member or base class is not named by a mem-initializer-id (including the case where there is no mem-initializer-list because the constructor has no ctor-initializer), then

— If the entity is a non-static data member of (possibly cv-qualified) class type (or array thereof) or a base class, and the entity class is a non-POD non-trivial class the entity is default-initialized (8.5). If the entity is a non-static data member of a const-qualified type, the entity class shall have a user-declared default constructor.

— Otherwise, the entity is not initialized. If the entity is of const-qualified type or reference type, or of a (possibly cv-qualified) POD trivial class type (or array thereof) containing (directly or indirectly) a member of a const-qualified type, the program is ill-formed.

After the call to a constructor for class X has completed, if a member of X is neither specified in the constructor’s mem-initializers, nor default-initialized, nor value-initialized, nor given a value during execution of the body of the constructor, the member has indeterminate value.
12.7 ¶1 Construction and destruction [class.cdtor]

For an object of non-POD non-trivial class type (clause 9) before the constructor begins execution and after the destructor finishes execution, referring to any non-static member or base class of the object results in undefined behavior. [ Example:

struct X { int i; };                 
struct Y : X { X(); }; // non-trivial                   
struct A { int a; };                 
struct B : public A { int j; Y y; B(); }; // non-trivial

extern B bobj;
B* pb = &bobj;         // OK
int* p1 = &bobj.a;     // undefined, refers to base class member
int* p2 = &bobj.y.i;   // undefined, refers to member’s member

A* pa = &bobj;         // undefined, upcast to a base class type
B bobj;                // definition of bobj

extern X xobj;
int* p3 = &xobj.i;     //OK, X is a POD trivial class
X xobj;
17.1.3 character container type [defns.character.container]

a class or a type used to represent a character (17.1.2). It is used for one of the template parameters of the string and iostream class templates. A character container class shall be a POD (3.9) type.
 
No change proposed; there is no known motivation for making a change.

18.1 ¶4 Types [support.types]

The macro offsetof(type, member-designator) accepts a restricted set of type arguments in this International Standard. If type is not a POD structure or a POD union standard-layout-struct or a standard-layout-union (clause 9), the results are undefined.189) The expression offsetof(type, member-designator) is never type-dependent (14.6.2.2) and it is value-dependent (14.6.2.3) if and only if type is dependent. The result of applying the offsetof macro to a field that is a static data member or a function member is undefined.
20.4 type traits

To 20.4.2, Header <type_traits> synopsis [lib.meta.type.synop], type properties, add:

template <class T> struct is_trivial;
template <class T> struct is_standard_layout;

To 20.4.5.3 Type properties [lib.meta.unary.prop], Type Property Predicates table, add:
 
Template Condition Preconditions
template <class T>
struct is_trivial;		 T is a trivial type ([basic.types]) T shall be a complete type.
template <class T>
struct is_standard_layout;		 T is a standard-layout type ([basic.types]) T shall be a complete type.

21 ¶1 Strings library [strings]

This clause describes components for manipulating sequences of “characters,” where characters may be of any POD (3.9) type. In this clause such types are called char-like types, and objects of char-like types are called char-like objects or simply “characters.”
 
No change. Users expect c_str() and data() to return pointers to POD types.

25.4 ¶4 C library algorithms [alg.c.library]

The function signature:

qsort(void *, size_t, size_t, int (*)(const void *, const void *));

is replaced by the two declarations:

extern "C" void qsort(void* base , size_t nmemb , size_t size, int (*compar )(const void*, const void*));

extern "C++" void qsort(void* base , size_t nmemb , size_t size, int (*compar )(const void*, const void*));

both of which have the same behavior as the original declaration. The behavior is undefined unless the objects in the array pointed to by base are of POD trivial type.

Open issues

Avoiding the "default constructor with no effects" workaround

For trivial classes, Lawrence Crowl has pointed out that having to provide a default constructor in the class definition if there is another constructor present is something the user shouldn't have to do - there should be explicit syntax.

In N2210, Lawrence proposes such explicit syntax. This would have a considerable advantage:

It eliminates language counter-intuitiveness in that programs would be able to express intent directly rather than relying on an apparently useless definition to tell the compiler of their intent that a class be trivial.

Explicit trivial, standard-layout, and POD attributes

There has been some private discussion among committee members of introducing a syntax that opens up a new namespace reserved for implementers of the language, similar to the declspec and attribute extensions provided by several compilers. If that comes to pass, it might be worthwhile to introduce trivial, standard-layout, and pod attribute tags for classes to indicate that the class must meet those respective requirements. This would improve the understandability of programs, and would be more accessible to novice programmers than use of static_assert on type traits.

If the committee is concerned about possible negative impact on some compiler optimizations (and possibly ABI's) of the silent change for some existing classes from being non-POD to POD,  certain of the proposal's POD relaxations could be specified to only occur for classes specifically tagged as trivial, standard-layout, or POD.

Impact on existing code

The proposed changes will cause some existing non-POD's to become POD's. This may result in less optimization being performed. The problem can be eliminated by making the class non-POD again, for example, by adding a user-defined do-nothing destructor.

Adding a user-defined do-nothing destructor to existing code to leave POD-ness unchanged is simple enough that it could be done programmatically. If a compiler vendor felt this was a serious concern for their user-base, they might wish to provide such a program. Alternately, compilers may wish to issue warnings during a transition period if the new rules change a non-POD into a POD.

Impact on existing ABI's

Allowing standard-layout classes to have base classes forces compilers to implement the empty base optimization for standard-layout classes, and this could break a compiler's application binary interface (ABI). See 9.2/18 above.

Although this issue is still being investigated, it is believed not to be a concern for modern compilers, except in the case of multiple inheritance. Since multiple inheritance is not central to this proposal, allowing standard-layout classes or their bases to use multiple inheritance will be eliminated from the proposal if it proves contentious.

Interaction with other proposals

See N1824, Extending Aggregate Initialization. Whichever proposal is accepted first, the other will have to be reviewed, and possibly revised, accordingly.

See N2100, Initializer lists (Rev. 2). The authors of the Initializer lists proposal and the POD proposal are committed to working together to ensure the two proposals stay in sync.

See Core issue 538, Definition and usage of structure, POD-struct, POD-union, and POD class. This issue, currently in review status, clarifies POD related terminology throughout the working paper. Since it makes changes to the same text modified by this proposal, care must be taken to ensure the two proposals do not diverge.

Revision history

Revision 2 - N2172

struct S {};
struct X: S {
  S s;
}; 

Revision 1 - N2102

Initial version - N2062

Acknowledgements

Initial Version - Matt Austern, Greg Colvin, Alisdair Meredith, and Clark Nelson provided helpful comments during preparation of this proposal. Our cat Jane woke me up in the middle of the night, provoking this proposal as an alternative to counting sheep (or cats).

Revision 1 - Greg Colvin and Lawrence Crowl provided legitimately non-copyable use cases. Alberto Ganesh Barbati pointed out that the proposed resolution should be relative to the 538 proposed resolution. Martin Sebor pointed out the need for clarification of 11.1, p3. The EWG and CWG in Portland reviewed a draft of revision 1 and made many helpful comments and suggestions. Clark Nelson is facilitating progress through Core. A suggestion was made that trivial types be renamed inert POD's, or IPOD's. Mike Miller suggested that a pod_cast operation be provided to ensure interoperability between POD's and IPOD's.

Revision 2 - Lawrence Crowl identified overly restrictive requirements in standard-layout classes. Daveed Vandevoorde provided the nasty subobject example noted in the revision history and Lawrence Crowl pointed out that the subobject problem could cross several levels of inheritance. Alisdair Meredith pointed out that default initialization now has an additional special case. Alisdair also originated the idea of tagging a class as trivial, standard-layout, or POD to allow diagnostics. Daniel Krügler asked questions resolved by adding a footnote to 8.5 paragraph 14.

References

N2210 Defaulted and Deleted Functions, Lawrence Crowl, www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2210.html

N1824 Extending Aggregate Initialization, Alisdair Meredith,  www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1824.htm

Core issue 538. www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#538, Definition and usage of structure, POD-struct, POD-union, and POD class.

Core issue 568. www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#568, Definition of POD is too strict.