Doc. no.
WG21/N2102=06-0172
Date:
2006-10-31
Project: Programming Language C++
Reply to: Beman Dawes <bdawes@acm.org>
Introduction
Summary of proposed changes
To Do
Features and benefits of POD types
Motivating examples
std::pair example
Endian example
Two structs example
Atomic example
Coupling between POD's and aggregates
Rationale for changes
Proposed changes to the Working Paper
POD in the Standard, with changes
Impact on existing code
Impact on existing ABI's
Interactions with other proposals
Revision history
Acknowledgements
References
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:
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 | Benefits |
Byte copyable guarantees [3.9 ¶2-3, basic.types] |
|
C layout-compatibility guarantees, byte copyable guarantees [9.2 ¶14-17, class.mem], initialization rules. |
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Static initialization guarantees [3.6.2, basic.start.init] |
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Are aggregates |
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Various rules for non-POD's |
|
std::pair
exampleMatt 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 example pair becomes a POD, solving the issue.
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 thevalue_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.
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 becomes a POD, solving all the issues.
Lawrence Crowl provided this example.
Consider a class providing atomic operations. Among other requirements, it should:
For best C++ coding practice, the data should be private and the usual copy constructor and copy assignment idioms used to make the class non-copyable. But that would make the class a non-POD under current rules.
With the proposed resolution, the class becomes a standard-layout class, solving both issues.
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.
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.
This table summaries the new decomposition of requirements:
Requirement | Classes and types requirement applies to |
Trivial default constructor or default constructor with no effects, trivial copy constructor, trivial copy assignment, trivial destructor; ditto members and bases. | trivial, POD |
No virtual functions, no virtual bases | trivial, standard-layout, POD |
All non-static members have same access control; no base classes with non-static data members; no non-static members are references; non-static member arrays also meet requirements. | standard-layout, POD |
Added text is shown in
green and underlined. Deleted text is shown
in red with strikethrough.
Commentary is shown in boxed italics. |
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 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, a trivial copy constructor (12.8), 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:
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 class is [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; ~LD(); }; struct POD { // both trivial and standard-layout int i; int j; }; -- end example] Similarly, a
POD-union is |
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1.8 ¶5 [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 |
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3.6.2 ¶1 Initialization of non-local objects
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
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3.8 ¶2 Object Lifetime [ Note: the lifetime of an array object or of an object of
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3.8 ¶5 Object Lifetime 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 — 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). |
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3.8 ¶6 Object Lifetime 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
— 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. |
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3.9 ¶2 Types For any object (other than a base-class subobject) of
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3.9 ¶3 Types For any |
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3.9 ¶4 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 |
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3.9 ¶10 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. |
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3.9 ¶11 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
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5.2 ¶7 Postfix expressions 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
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5.3.4 ¶16 New A new-expression
that creates an object of type T initializes that object as follows: |
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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:
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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
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5.19 ¶5 Constant expressions 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 |
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6.7 ¶3 Declaration statement 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 |
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6.8 ¶4 Ambiguity resolution 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 |
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8.5 ¶5 Initializers
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8.5 ¶9 Initializers If no initializer is
specified for an object, and the object is of (possibly cv-qualified)
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8.5 ¶14 Initializers 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 |
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8.5.1 ¶1 Aggregates
An
aggregate
is an array or a class (clause 9) with no user-declared constructors (12.1),
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).
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9.2 ¶12 Class members [class.mem]
Nonstatic data members of a (non-union) class
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9.2 ¶15-18 Class members [class.mem]
15 Two 16 Two 17 If a 18 A pointer to a |
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9.5 ¶1 Unions 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
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11.1 ¶3 Access Specifiers The
order of allocation of data members with
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12.6.2 ¶4 Initializing bases and members 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 |
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12.7 ¶1 Construction and destruction
For an object of 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 |
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17.1.3 character container type 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.
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18.1 ¶4 Types The macro offsetof(type,
member-designator) accepts a restricted set of type arguments in this
International Standard. If type is not a |
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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:
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21 ¶1 Strings library 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.”
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25.4 ¶4 C library algorithms The function
signature: |
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 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.
Allowing standard-layout classes to have base classes, even restricted to base classes without non-static data members, 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.
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 1 - N2102
Initial version - N2062
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.
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.