Expressions
An expression is a sequence of operators and their operands, that specifies a computation.
Expression evaluation may produce a result (e.g., evaluation of 2 + 2 produces the result 4), may generate side-effects (e.g. evaluation of printf ("%d", 4) sends the character '4' to the standard output stream), and may designate objects or functions.
Contents
[edit] General
- value categories (lvalue, non-lvalue object, function designator) classify expressions by their values
- order of evaluation of arguments and subexpressions specifies the order in which intermediate results are obtained
[edit] Operators
| Common operators | ||||||
|---|---|---|---|---|---|---|
| assignment | increment decrement |
arithmetic | logical | comparison | member access |
other |
|
a = b |
++a |
+a |
!a |
a == b |
a[b] |
a(...) |
- operator precedence defines the order in which operators are bound to their arguments
- alternative representations are alternative spellings for some operators
[edit] Conversions
- Implicit conversions take place when types of operands do not match the expectations of operators
- Casts may be used to explicitly convert values from one type to another.
[edit] Other
- constant expressions can be evaluated at compile time and used in compile-time context (non-VLA(since C99)array sizes, static initializers, etc)
- generic selections can execute different expressions depending on the types of the arguments
- Floating-point arithmetic may raise exceptions and report errors as specified in math_errhandling
- The standard #pragmas
FENV_ACCESS,FP_CONTRACT, andCX_LIMITED_RANGEas well as the floating-point evaluation precision and rounding direction control the way floating-point arithmetic are executed.
[edit] Primary expressions
The operands of any operator may be other expressions or they may be primary expressions (e.g. in 1 + 2 * 3, the operands of operator+ are the subexpression 2 * 3 and the primary expression 1).
Primary expressions are any of the following:
Any expression in parentheses is also classified as a primary expression: this guarantees that the parentheses have higher precedence than any operator.
[edit] Constants and literals
Constant values of certain types may be embedded in the source code of a C program using specialized expressions known as literals (for lvalue expressions) and constants (for non-lvalue expressions)
- integer constants are decimal, octal, or hexadecimal numbers of integer type.
- character constants are individual characters of type int suitable for conversion to a character type or of type char8_t,(since C23) char16_t, char32_t, or(since C11) wchar_t
- floating constants are values of type float, double, or long double
- predefined constants true/false are values of type bool
- predefined constant
nullptris a value of type nullptr_t
- string literals are sequences of characters of type char[], char8_t[](since C23), char16_t[], char32_t[],(since C11) or wchar_t[] that represent null-terminated strings
- compound literals are values of struct, union, or array type directly embedded in program code
[edit] Unevaluated expressions
The operands of the sizeof operator are expressions that are not evaluated (unless they are VLAs)(since C99). Thus, size_t n = sizeof(printf ("%d", 4)); does not perform console output.
The operands of the _Alignof (until C23)alignof (since C23) operator, the controlling expression of a generic selection, and size expressions of VLAs that are operands of _Alignof(until C23)alignof(since C23) are also expressions that are not evaluated.
[edit] References
- C23 standard (ISO/IEC 9899:2024):
- 6.5 Expressions (p: TBD)
- 6.6 Constant expressions (p: TBD)
- C17 standard (ISO/IEC 9899:2018):
- 6.5 Expressions (p: 55-75)
- 6.6 Constant expressions (p: 76-77)
- C11 standard (ISO/IEC 9899:2011):
- 6.5 Expressions (p: 76-105)
- 6.6 Constant expressions (p: 106-107)
- C99 standard (ISO/IEC 9899:1999):
- 6.5 Expressions (p: 67-94)
- 6.6 Constant expressions (p: 95-96)
- C89/C90 standard (ISO/IEC 9899:1990):
- 3.3 EXPRESSIONS
- 3.4 CONSTANT EXPRESSIONS