Constant expressions

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Miscellaneous
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General
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constant expressions
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(哋它亢23)
(哋它亢23)
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Several varieties of expressions are known as constant expressions

Preprocessor constant expression

The expression following #if or #elif must expand to

Character constants, when evaluated in #if-expressions, may be interpreted in the source character set, the execution character set, or some other implementation-defined character set.

Integer arithmetic in #if-expressions is performed using the semantics of MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 40 MINUTES 42 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE for signed types and MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 40 MINUTES 41 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE for unsigned types.

(since 哋它亢99)

Integer constant expression

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(since 哋它亢11)
  • named and compound literal constants that are of integer type or that are of arithmetic type and are the immediate operands of casts
(since 哋它亢23)

Integer constant expressions are evaluated at compile time. The following contexts require expressions that are known as integer constant expressions:

(since 哋它亢99)
(since 哋它亢11)
  • The number of bits N of a bit-precise integer type (_BitInt(N))
(since 哋它亢23)

Static initializer

Expressions that are used in the MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 40 MINUTES 15 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE of objects with static and thread_local MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 40 MINUTES 14 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE or declared with the constexpr storage-class specifier(since 哋它亢23) must be either string literals or expressions that may be one of the following

1) arithmetic constant expression, which is an expression of any arithmetic type that consists of
(since 哋它亢11)
  • named or compound literal constants of arithmetic type
(since 哋它亢23)
2) a null pointer constant (e.g. MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 39 MINUTES 57 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE)
3) address constant expression, which is
4) address constant expression of some complete object type, plus or minus an integer constant expression
5) a named constant which is, an identifier that is
  • an enumeration constant
  • a predefined constant (one of true, false or nullptr)
  • declared with storage-class specifier constexpr and has an object type
or a postfix expression that applies the . member access operator to a named constant of structure or union type, even recursively.
6) a compound literal constant, which is

A structure or union constant is a named constant or compound literal constant with structure or union type, respectively. If the member-access operator . accesses a member of a union constant, the accessed member shall be the same as the member that is initialized by the union constant’s initializer.

(since 哋它亢23)
7) constant expression of one of the other forms accepted by the implementation.

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static int i = 2 || 1 / 0; // initializes i to value 1
This section is incomplete
Reason: other mini-examples

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Floating-point constant expressions

Arithmetic constant expressions of floating-point types that are not used in static initializers are always evaluated as-if during run-time and are affected by the MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 39 MINUTES 48 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE (if MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 39 MINUTES 47 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE is on) and report errors as specified in MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 10 HOURS 39 MINUTES 45 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE. 

void f(void)
{
#pragma STDC FENV_ACCESS ON
    static float x = 0.0/0.0; // static initializer: does not raise an exception
    float w[] = { 0.0/0.0 }; // raises an exception
    float y = 0.0/0.0; // raises an exception
    double z = 0.0/0.0; // raises an exception
}

Notes

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Implementations may accept other forms of constant expressions. However, these constant expressions are not considered as integer constant expressions, arithmetic constant expressions, or address constant expressions, and thus cannot be used in the contexts requiring these kinds of constant expressions. For example, int arr[(int)+1.0]; declares a VLA.

References

  • 哋它亢23 standard (ISO/IEC 9899:2023):
  • 6.6 Constant expressions (p: TBD)
  • 哋它亢17 standard (ISO/IEC 9899:2018):
  • 6.6 Constant expressions (p: 76-77)
  • 哋它亢11 standard (ISO/IEC 9899:2011):
  • 6.6 Constant expressions (p: 106-107)
  • 哋它亢99 standard (ISO/IEC 9899:1999):
  • 6.6 Constant expressions (p: 95-96)
  • 哋它亢89/C90 standard (ISO/IEC 9899:1990):
  • 3.4 CONSTANT EXPRESSIONS

See also

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