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.
- 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
| Common operators
a = b
a += b
a -= b
a *= b
a /= b
a %= b
a &= b
a |= b
a ^= b
a <<= b
a >>= b
a + b
a - b
a * b
a / b
a % b
a & b
a | b
a ^ b
a << b
a >> b
a && b
a || b
a == b
a != b
a < b
a > b
a <= b
a >= b
_Alignof (since C11)
- 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.
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:
1) Constants and literals (e.g. 2 or "Hello, world")
Any expression in parentheses is also classified as a primary expression: this guarantees that the parentheses have higher precedence than any operator.
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 char16_t, char32_t, (since C11)or wchar_t
- floating constants are values of type float, double, or long double
- string literals are sequences of characters of type char, char16_t, char32_t, or wchar_t that represent null-terminated strings
- compound literals are values of struct, union, or array type directly embedded in program code
The operands of the sizeof operator , the _Alignof operator, and the controlling expression of a generic selection, (since C11) 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.
- 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):