Operators in C and C++: Difference between revisions

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==Operator precedence==

During expression evaluation, the order in which sub-expressions are evaluated is determined by [[order of operations|precedence]] and [[operator associativity|associativity]]. An operator with higher precedence is evaluated before a operator of lower precedence and the operands of an operator are evaluated based on associativity. The following table describes the precedence and associativity of the C and C++ operators. Operators are shown in groups of equal precedence with groups ordered in descending precedence from top to bottom.<ref>{{cite book | title = ISO/IEC 9899:201x Programming Languages - C | date = 19 December 2011 | publisher = open-std.org – The C Standards Committee | pages = 465}}</ref><ref>{{cite tech report |title=the ISO C 1999 standard, section 6.5.6 note 71 |institution=ISO |year=1999 }}</ref><ref>{{cite web |title=C++ Built-in Operators, Precedence and Associativity |url=https://docs.microsoft.com/en-US/cpp/cpp/cpp-built-in-operators-precedence-and-associativity |website=docs.microsoft.com |access-date=11 May 2020 |language=en-us}}</ref>

The syntax of expressions in C and C++ is specified by a [[phrase structure grammar]].<ref>{{cite book | title = ISO/IEC 9899:201x Programming Languages - C | date = 19 December 2011 | publisher = open-std.org – The C Standards Committee | pages = 465}}</ref> The table given here has been inferred from the grammar.{{Citation needed| date = January 2009}} For the ISO C 1999 standard, section 6.5.6 note 71 states that the C grammar provided by the specification defines the precedence of the C operators, and also states that the operator precedence resulting from the grammar closely follows the specification's section ordering:

"[[C syntax|The [C] syntax]] [i.e., grammar] specifies the precedence of operators in the evaluation of an expression, which is the same as the order of the major subclauses of this subclause, highest precedence first."<ref>{{cite tech report |title=the ISO C 1999 standard, section 6.5.6 note 71 |institution=ISO |year=1999 }}</ref>

A precedence table, while mostly adequate, cannot resolve a few details. In particular, note that the [[ternary operator]] allows any arbitrary expression as its middle operand, despite being listed as having higher precedence than the assignment and comma operators. Thus <code>a ? b, c : d</code> is interpreted as <code>a ? (b, c) : d</code>, and not as the meaningless <code>(a ? b), (c : d)</code>. So, the expression in the middle of the conditional operator (between <code>'''?'''</code> and <code>''':'''</code>) is parsed as if parenthesized. Also, note that the immediate, unparenthesized result of a C cast expression cannot be the operand of <code>sizeof</code>. Therefore, <code>sizeof (int) * x</code> is interpreted as <code>(sizeof(int)) * x</code> and not <code>sizeof ((int) * x)</code>.

Operator precedence is not affected by overloading.

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<ref>{{cite web |title=C++ Built-in Operators, Precedence and Associativity |url=https://docs.microsoft.com/en-US/cpp/cpp/cpp-built-in-operators-precedence-and-associativity |website=docs.microsoft.com |access-date=11 May 2020 |language=en-us}}</ref>

===Notes===

AAlthough precedencethis table, while mostlyis adequate for describing most evaluation order, cannotit resolvedoes not describe a few details. In particular, note that theThe [[ternary operator]] allows any arbitrary expression as its middle operand, despite being listed as having higher precedence than the assignment and comma operators. Thus <code>a ? b, c : d</code> is interpreted as <code>a ? (b, c) : d</code>, and not as the meaningless <code>(a ? b), (c : d)</code>. So, the expression in the middle of the conditional operator (between <code>'''?'''</code> and <code>''':'''</code>) is parsed as if parenthesized. Also, note that the immediate, unparenthesizedun-parenthesized result of a C cast expression cannot be the operand of <code>sizeof</code>. Therefore, <code>sizeof (int) * x</code> is interpreted as <code>(sizeof(int)) * x</code> and not <code>sizeof ((int) * x)</code>.

The precedence table determines the order of binding in chained expressions, when it is not expressly specified by parentheses.

* For example, <code>++x*3</code> is ambiguous without some precedence rule(s). The precedence table tells us that: {{mono|x}} is 'bound' more tightly to {{mono|++}} than to {{mono|*}}, so that whatever {{mono|++}} does (now or later—see below), it does it ONLY to {{mono|x}} (and not to <code>x*3</code>); it is equivalent to (<code>++x</code>, <code>x*3</code>).