Jones D.M.The new C standard (C90 and C++).An economic and cultural commentary.2005

3.1p2

declaration specifiers

C++

A declaration is a definition unless . . . , or it is a typedef declaration (7.1.3), . . .

A typedef name is not considered to be a definition in C ++. However, this difference does not cause any C compatibility consequences. Enumerations constants are not explicitly excluded in the unless list. They are thus definitions.

The declaration specifiers consist of a sequence of specifiers that indicate the linkage, storage duration, and 1347 part of the type of the entities that the declarators denote.

declarator list of

C++

The C++ Standard does not make this observation.

The init-declarator-list is a comma-separated sequence of declarators, each of which may have additional 1348 type information, or an initializer, or both.

object

type complete by end

The C++ Standard does not make this observation.

If an identifier for an object is declared with no linkage, the type for the object shall be complete by the end 1351 of its declarator, or by the end of its init-declarator if it has an initializer;

1354

storage-class-specifier: typedef

extern static auto register

C++

The C++ Standard classifies typedef (7.1p1) as a decl-specifier, not a storage-class-specifier

(which also includes mutable, a C++ specific keyword).

Constraints

1355 At most, one storage-class specifier may be given in the declaration specifiers in a declaration. 100)

C++

storageclass specifier syntax

C++

It is called a decl-specifier in the C++ Standard (7.1p1).

1359 A declaration of an identifier for an object with storage-class specifier register suggests that access to the object be as fast as possible.

C++

A register specifier has the same semantics as an auto specifier together with a hint to the implementation that the object so declared will be heavily used.

Translator implementors are likely to assume that the reason a developer provides this hint is that they are expecting the translator to make use of it to improve the performance of the generated machine code. The

register storage-class

7.1.1p3

unary & 1078 operand

constraints

C++

This requirement does not apply in C++.

Thus, the only operator that can be applied to an array declared with storage-class specifier register is 1365 sizeof.

unary & 1078 operand

constraints

C++

This observation is not true in C++.

If an aggregate or union object is declared with a storage-class specifier other than typedef, the properties 1366 resulting from the storage-class specifier, except with respect to linkage, also apply to the members of the

object, and so on recursively for any aggregate or union member objects.

C90

This wording did not appear in the C90 Standard and was added by the response to DR #017q6.

C++

The nonterminal for these terminals is called simple-type-specifier in C++ (7.1.5.2p1). The C++ Standard does contain a nonterminal called type-specifier. It is used in a higher-level production (7.1.5p1) that includes cv-qualifier.

The C++ Standard includes wchar_t and bool (the identifier bool is defined as a macro in the header stdbool.h in C) as type-specifiers (they are keywords in C++). The C++ Standard does not include _Bool, _Complex and _Imaginary, either as keywords or type specifiers.

Constraints

1369 At least one type specifier shall be given in the declaration specifiers in each declaration, and in the specifierqualifier list in each struct declaration and type name.

C90

This requirement is new in C99.

In C90 an omitted type-specifier implied the type specifier int. Translating a source file that contains such a declaration will cause a diagnostic to be issued and are no longer considered conforming programs.

declaration at least one type specifier

1372

--~ void --~ char

--~ signed char --~ unsigned char

--~ short, signed short, short int, or signed short int --~ unsigned short, or unsigned short int

--~ int, signed, or signed int --~ unsigned, or unsigned int

--~ long, signed long, long int, or signed long int --~ unsigned long, or unsigned long int

--~ long long, signed long long, long long int, or signed long long int --~ unsigned long long, or unsigned long long int

--~ float --~ double

--~ long double --~ _Bool

--~ float _Complex --~ double _Complex

--~ long double _Complex --~float _Imaginary --~double _Imaginary --~long double _Imaginary

--~ struct or union specifier --~ enum specifier

--~ typedef name

type specifiers possible sets of

bit-field int

C++

The list of combinations, given above as being new in C99 are not supported by C++.

Like C99, the C++ Standard does not require a translator to provide an implicit function declaration returning int (footnote 80) being supplied for a missing type specifier.

The type specifier s _Complex and _Imaginary shall not be used if the implementation does not provide those 1373 complex types.102)

C90

Support for this type specifier is new in C99.

C++

Support for these type specifiers is new in C99 and are not specified as such in the C ++ Standard. The header <complex> defines template classes and associated operations whose behavior provides the same functionality as that provided, in C, for objects declared to have type _Complex. There are no equivalent definitions for _Imaginary.

Semantics

Each of the comma-separated sets designates the same type, except that for bit-fields, it is implementation- 1377 defined whether the specifier int designates the same type as signed int or the same type as unsigned

int.

C90

Each of the above comma-separated sets designates the same type, except that for bit-fields, the type signed int (or signed) may differ from int (or no type specifiers).

C++

Rather than giving a set of possibilities, the C++ Standard lists each combination of specifiers and its associated type (Table 7).

6.7.2.1 Structure and union specifiers

1380

struct-or-union-specifier:

struct-or-union identifieropt { struct-declaration-list } struct-or-union identifier

struct-or-union:

struct union

struct-declaration-list: struct-declaration

struct-declaration-list struct-declaration struct-declaration:

specifier-qualifier-list struct-declarator-list ; specifier-qualifier-list:

type-specifier specifier-qualifier-listopt type-qualifier specifier-qualifier-listopt

struct-declarator-list: struct-declarator

struct-declarator-list , struct-declarator struct-declarator:

declarator

declaratoropt : constant-expression

C++

The C++ Standard uses the general term class to refer to these constructs. This usage is also reflected in naming of the nonterminals in the C++ syntax. The production struct-or-union is known as class-key in C++ and also includes the keyword class. The form that omits the brace enclosed list of members is known as an elaborated-type-specifier (7.1.5.3) in C++.

Constraints

1381 A structure or union shall not contain a member with incomplete or function type (hence, a structure shall not contain an instance of itself, but may contain a pointer to an instance of itself), except that the last member of a structure with more than one named member may have incomplete array type;

C90

Support for the exception on the last named member is new in C99.

C++

It is a design feature of C++ that class types can contain incomplete and function types. Source containing instances of such constructs is making use of significant features of C ++ and there is unlikely to be any expectation of being able to successfully process it using a C translator.

The exception on the last named member is new in C99 and this usage is not supported in the C++ Standard.

The following describes a restriction in C++ that does not apply in C.

struct/union syntax

member not types

bit-field maximum width

Change: In C++, a typedef name may not be redefined in a class declaration after being used in the declaration

Example:

typedef int I; struct S {

I i;

int I; // valid C, invalid C++ };

Rationale: When classes become complicated, allowing such a redefinition after the type has been used can create confusion for C++ programmers as to what the meaning of ’I’ really is.

The expression that specifies the width of a bit-field shall be an integer constant expression that has a 1383 nonnegative value that shall not exceed the numberwidth of bits in an object of the type that iswould be specified ifwere the colon and expression are omitted.

bit-field packed into

The C++ Standard does not specify (9.6p1) that the specified number of bits is used for the value representation.

If the value 0 or 1 is stored into a nonzero-width bit-field of type _Bool, the value of the bit-field shall compare 1398 equal to the value stored.

C90

Support for the type _Bool is new in C99.

If enough space remains, a bit-field that immediately follows another bit-field in a structure shall be packed 1400 into adjacent bits of the same unit.

C++

footnote 103

footnote 105

member alignment

The wording in the C++ Standard refers to the bit-field, not the addressable allocation unit in which it resides. Does this wording refer to the alignment within the addressable allocation unit?

9.6p1

Alignment of bit-fields is implementation-defined. Bit-fields are packed into some addressable allocation unit.

C++

The C++ Standard specifies (3.9p5) that the alignment of all object types is implementation-defined.

There may therefore be unnamed padding within a structure object, but not at its beginning, as necessary to achieve the appropriate alignment.

C++

This commentary applies to POD-struct types (9.2p17) in C++. Such types correspond to the structure types available in C.

1417 A pointer to a union object, suitably converted, points to each of its members (or if a member is a bit-field, then to the unit in which it resides), and vice versa.

C++

union members start same address