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

argument

no tokens replaced by

placemarker token

placemarker preprocessor

EXAMPLE

# ## #

footnote 148

#define hash_hash # ## #

#define mkstr(a) # a

#define in_between(a) mkstr(a)

#define join(c, d) in_between(c hash_hash d)

char p[] = join(x, y); // equivalent to

// char p[] = "x ## y"

The expansion produces, at various stages:

join(x, y)

in_between(x hash_hash y)

in_between(x ## y)

mkstr(x ## y)

"x ## y"

In other words, expanding hash_hash produces a new token, consisting of two adjacent sharp signs, but this new token is not the ## operator.

C++

This example is the response to a DR against C90. While there has been no such DR in C++, it is to be expected that WG21 would provide the same response.

148) Placemarker preprocessing tokens do not appear in the syntax because they are temporary entities that 1947 exist only within translation phase 4.

C90

Support for the concept of placemarker preprocessing tokens is new in C99.

C++

Support for the concept of placemarker preprocessing tokens is new in C99 and they are not described in the C++ Standard.

6.10.3.4 Rescanning and further replacement

1948 After all parameters in the replacement list have been substituted and # and ## processing has taken place, all placemarker preprocessing tokens are removed.

C90

Support for the concept of placemarker preprocessing tokens is new in C99.

C++

Support for the concept of placemarker preprocessing tokens is new in C99 and does not exist in C++.

1953 The resulting completely macro-replaced preprocessing token sequence is not processed as a preprocessing directive even if it resembles one, but all pragma unary operator expressions within it are then processed as specified in 6.10.9 below.

C90

Support for _Pragma unary operator expressions is new in C99.

C++

Support for _Pragma unary operator expressions is new in C99 and is not available in C++.

expanded token sequence

not treated as a directive

6.10.3.5 Scope of macro definitions

1955 Macro definitions have no significance after translation phase 4.

C90

This observation is new in C99.

C++

This observation is not made in the C++ document.

1962 EXAMPLE 5 To illustrate the rules for placemarker preprocessing tokens, the sequence

#define t(x,y,z) x ## y ## z

int j[] = { t(1,2,3), t(,4,5), t(6,,7), t(8,9,), t(10,,), t(,11,), t(,,12), t(,,) };

macro definition no signifi-

cance after

EXAMPLE placemarker

EXAMPLE variable macro arguments

#define debug(...) fprintf(stderr, __VA_ARGS__) #define showlist(...) puts(#__VA_ARGS__) #define report(test, ...) ((test)?puts(#test):\

printf(__VA_ARGS__))

debug("Flag"); debug("X = %d\n", x);

showlist(The first, second, and third items.); report(x>y, "x is %d but y is %d", x, y);

results in

fprintf(stderr, "Flag" ); fprintf(stderr, "X = %d\n", x );

puts( "The first, second, and third items." ); ((x>y)?puts("x>y"):

printf("x is %d but y is %d", x, y));

. . . , and renders the program ill-formed.

Both language standards require that a diagnostic be issued. But the C Standard does not specify that the construct alters the conformance status of the translation unit. However, given that the occurrence of this directive causes translation to terminate, this is a moot point.

16.5p1

89 #error terminate translation

1977 If the preprocessing token STDC does immediately follow pragma in the directive (prior to any macro replacement), then no macro replacement is performed on the directive, and the directive shall have one of the following forms150) whose meanings are described elsewhere:

#pragma STDC FP_CONTRACT on-off-switch #pragma STDC FENV_ACCESS on-off-switch #pragma STDC CX_LIMITED_RANGE on-off-switch

C90

Support for the preprocessing token STDC in pragma directives is new in C99.

C++

Support for the preprocessing token STDC in pragma directives is new in C99 and is not specified in the C++ Standard.

1979 149) An implementation is not required to perform macro replacement in pragmas, but it is permitted except for in standard pragmas (where STDC immediately follows pragma).

footnote 149

#if identifier replaced by 0

Support for the _ _STDC_VERSION_ _ macro was first introduced in Amendment 1 to C90, where it was specified to have the value 199409L. In a C90 implementation (with no support for Amendment 1) occurrences of this macro are likely to be replaced by 0 (because it will not be defined as a macro).

C++

Support for the _ _STDC_VERSION_ _ macro is not available in C++.

C90

Support for conditionally defined macros is new in C99.

C90

This observation is new in the C99 Standard.

C++

Like C90, the C++ Standard does not make this observation.

The integer constant 1, intended to indicate adherence to the specifications in informative annex G (IEC 60559 compatible complex arithmetic).

C90

Support for the _ _STDC_IEC_559_COMPLEX_ _ macro is new in C99.

__STDC__IEC_55 macro

2015 Restriction of the significance of an external name to fewer than 255 characters (considering each universal character name or extended source character as a single character) is an obsolescent feature that is a concession to existing implementations.

C90

Part of the future language direction specified in C90 was implemented in C99.

significant

characters future language directions

6.11.4Character escape sequences

6.11.5Storage-class specifiers

6.11.6Function declarators

6.11.7Function definitions

6.11.8Pragma directives

2021 Pragmas whose first preprocessing token is STDC are reserved for future standardization.

C90

Support for this form of pragma directive is new in C99.

6.11.9 Predefined macro names

2022 Macro names beginning with _ _STDC_ are reserved for future standardization.

C90

The specification of this reserved set of macro name spellings is new in C99.

Pragma directives

future language directions

Predefined macro names

future language directions

References

Citations added in version 1.0b start at 1449.

1.M. Celce-Murcia and D. Larsen-Freeman. The Grammar Book: An ESL/EFL Teacher’s Course. Heinle & Heinle, second edition, 1999.

2.T. H. Gibbs. The design and implementation of a parser and frontend for the ISO C++ language and validation of the parser. PhD thesis, Clemson University, May 2003.

3.ISO. ISO/IEC 9945-1:1990 Information technology —Portable Operating System Interface (POSIX). ISO, 1990.

4.D. M. Ritchie. The development of the C language. Second History of Programming Languages conference, 1993.

5.B. Stroustrup. The Design and Evolution of C++. Addison–Wesley, 1999.

6.R. L. Velduizen. C++ templates as partial evaluation. Technical Report TR519, Indiana University, July 2000.

7.E. D. Willink. Meta-Compilation for C++. PhD thesis, University of Surrey, June 2001.