Editing Operators in C and C++ (section)

==Expression evaluation order==
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 (lower order is higher precedence).<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>

Operator precedence is not affected by overloading. 

{| class="wikitable"
|-
! style="text-align: left" | Order
! style="text-align: left" | Operator
! style="text-align: left" | Description
! style="text-align: left" | Associativity
|-
! 1
<small>highest</small>
| <code>::</code>
| [[Scope resolution operator#C.2B.2B|Scope resolution]] (C++ only)
| None
|-
! rowspan=11| 2
| style="border-bottom-style: none" | <code>++</code>
| style="border-bottom-style: none" | Postfix increment
| style="vertical-align: top" rowspan="11" | Left-to-right
|-
| style="border-bottom-style: none; border-top-style: none" | <code>--</code>
| style="border-bottom-style: none; border-top-style: none" | Postfix decrement
|-
| style="border-bottom-style: none; border-top-style: none" | <code>()</code>
| style="border-bottom-style: none; border-top-style: none" | Function call
|-
| style="border-bottom-style: none; border-top-style: none" | <code>[]</code>
| style="border-bottom-style: none; border-top-style: none" | Array subscripting
|-
| style="border-bottom-style: none; border-top-style: none" | <code>.</code>
| style="border-bottom-style: none; border-top-style: none" | Element selection by reference
|-
| style="border-bottom-style: none; border-top-style: none" | <code>-&gt;</code>
| style="border-bottom-style: none; border-top-style: none" | Element selection through pointer
|-
| style="border-bottom-style: none; border-top-style: none" | <code>typeid()</code>
| style="border-bottom-style: none; border-top-style: none" | [[Run-time type information]] (C++ only) (see [[typeid]])
|-
| style="border-bottom-style: none; border-top-style: none" | <code>const_cast</code>
| style="border-bottom-style: none; border-top-style: none" | Type cast (C++ only) (see [[const_cast]])
|-
| style="border-bottom-style: none; border-top-style: none" | <code>dynamic_cast</code>
| style="border-bottom-style: none; border-top-style: none" | Type cast (C++ only) (see [[dynamic cast]])
|-
| style="border-bottom-style: none; border-top-style: none" | <code>reinterpret_cast</code>
| style="border-bottom-style: none; border-top-style: none" | Type cast (C++ only) (see [[reinterpret_cast]])
|-
| style="border-top-style: none" | <code>static_cast</code>
| style="border-top-style: none" | Type cast (C++ only) (see [[static_cast]])
|-
! rowspan="13" | 3
| style="border-bottom-style: none" | <code>++</code>
| style="border-bottom-style: none" | Prefix increment
| rowspan="13" style="vertical-align: top" | Right-to-left
|-
| style="border-bottom-style: none; border-top-style: none" | <code>--</code>
| style="border-bottom-style: none; border-top-style: none" | Prefix decrement
|-
| style="border-bottom-style: none; border-top-style: none" | <code>+</code>
| style="border-bottom-style: none; border-top-style: none" | Unary plus
|-
| style="border-bottom-style: none; border-top-style: none" | <code>-</code>
| style="border-bottom-style: none; border-top-style: none" | Unary minus
|-
| style="border-bottom-style: none; border-top-style: none" | <code>!</code>
| style="border-bottom-style: none; border-top-style: none" | Logical NOT
|-
| style="border-bottom-style: none; border-top-style: none" | <code>~</code>
| style="border-bottom-style: none; border-top-style: none" | Bitwise NOT ([[ones' complement]])
|-
| style="border-bottom-style: none; border-top-style: none" | <code>(''type'')</code>
| style="border-bottom-style: none; border-top-style: none" | Type cast
|-
| style="border-bottom-style: none; border-top-style: none" | <code>*</code>
| style="border-bottom-style: none; border-top-style: none" | Indirection (dereference)
|-
| style="border-bottom-style: none; border-top-style: none" | <code>&</code>
| style="border-bottom-style: none; border-top-style: none" | Address-of 
|-
| style="border-bottom-style: none; border-top-style: none" | <code>sizeof</code>
| style="border-bottom-style: none; border-top-style: none" | [[Sizeof]]
|-
| style="border-bottom-style: none; border-top-style: none" | <code>_Alignof</code>
| style="border-bottom-style: none; border-top-style: none" | Alignment requirement (since C11)
|-
| style="border-bottom-style: none; border-top-style: none" | <code>new</code>, <code>new[]</code>
| style="border-bottom-style: none; border-top-style: none" | Dynamic memory allocation (C++ only)
|-
| style="border-top-style: none" | <code>delete</code>, <code>delete[]</code>
| style="border-top-style: none" | Dynamic memory deallocation (C++ only)
|-
! rowspan=2| 4
| style="border-bottom-style: none" | <code>.*</code>
| style="border-bottom-style: none" | Pointer to member (C++ only)
| style="vertical-align: top" rowspan="2" | Left-to-right
|-
| style="border-bottom-style: none; border-top-style: none" | <code>->*</code>
| style="border-bottom-style: none; border-top-style: none" | Pointer to member (C++ only)
|-
! rowspan=3| 5
| style="border-bottom-style: none" | <code>*</code>
| style="border-bottom-style: none" | Multiplication
| style="vertical-align: top" rowspan="3" | Left-to-right
|-
| style="border-bottom-style: none; border-top-style: none" | <code>/</code>
| style="border-bottom-style: none; border-top-style: none" | Division
|-
| style="border-bottom-style: none; border-top-style: none" | <code>%</code>
| style="border-bottom-style: none; border-top-style: none" | [[Modulo operation|Modulo]] (remainder)
|-
! rowspan=2| 6
| style="border-bottom-style: none" | <code>+</code>
| style="border-bottom-style: none" | Addition
| style="vertical-align: top" rowspan="2" | Left-to-right
|-
| style="border-bottom-style: none; border-top-style: none" | <code>-</code>
| style="border-bottom-style: none; border-top-style: none" | Subtraction
|-
! rowspan=2| 7
| style="border-bottom-style: none" | <code>&lt;&lt;</code>
| style="border-bottom-style: none" | [[Bitwise operation|Bitwise]] left shift
| style="vertical-align: top" rowspan="2" | Left-to-right
|-
| style="border-bottom-style: none; border-top-style: none" | <code>&gt;&gt;</code>
| style="border-bottom-style: none; border-top-style: none" | [[Bitwise operation|Bitwise]] right shift
|-
! rowspan=1| 8
| style="border-bottom-style:none;" | <code>&lt;=&gt;</code>
| style="border-bottom-style:none;" | [[Three-way comparison]] (Introduced in [[C++20]] - C++ only)
| style="vertical-align: top" rowspan="1" | Left-to-right
|-
! rowspan=4| 9
| style="border-bottom-style: none" | <code>&lt;</code>
| style="border-bottom-style: none" | Less than
| style="vertical-align: top" rowspan="4" | Left-to-right
|-
| style="border-bottom-style: none; border-top-style: none" | <code>&lt;=</code>
| style="border-bottom-style: none; border-top-style: none" | Less than or equal to
|-
| style="border-bottom-style: none; border-top-style: none" | <code>&gt;</code>
| style="border-bottom-style: none; border-top-style: none" | Greater than
|-
| style="border-bottom-style: none; border-top-style: none" | <code>&gt;=</code>
| style="border-bottom-style: none; border-top-style: none" | Greater than or equal to
|-
! rowspan=2| 10
| style="border-bottom-style: none" | <code>==</code>
| style="border-bottom-style: none" | Equal to
| style="vertical-align: top" rowspan="2" | Left-to-right
|-
| style="border-bottom-style: none; border-top-style: none" | <code>!=</code>
| style="border-bottom-style: none; border-top-style: none" | Not equal to
|-
! 11
| <code>&</code>
| Bitwise AND
| Left-to-right
|-
! 12
| <code>^</code>
| Bitwise XOR (exclusive or)
| Left-to-right
|-
! 13
| <code><nowiki>|</nowiki></code>
| Bitwise OR (inclusive or)
| Left-to-right
|-
! 14
| <code>&&</code>
| Logical AND
| Left-to-right
|-
! 15
| <code><nowiki>||</nowiki></code>
| Logical OR
| Left-to-right
|-
! rowspan="2" | 16
| style="border-bottom-style: none" | <code>co_await</code>
| rowspan="2" | Coroutine processing (C++ only)
| rowspan="2" | Right-to-left
|-
| style="border-top-style: none" | <code>co_yield</code>
|-
! rowspan="13" | 17
| style="border-bottom-style: none" | <code>?:</code>
| style="border-bottom-style: none" | [[Ternary conditional operator]]
| rowspan="13" | Right-to-left
|-
| style="border-bottom-style: none; border-top-style: none" | <code>=</code>
| style="border-bottom-style: none; border-top-style: none" | Direct assignment
|-
| style="border-bottom-style: none; border-top-style: none" | <code>+=</code>
| style="border-bottom-style: none; border-top-style: none" | Assignment by sum
|-
| style="border-bottom-style: none; border-top-style: none" | <code>-=</code>
| style="border-bottom-style: none; border-top-style: none" | Assignment by difference
|-
| style="border-bottom-style: none; border-top-style: none" | <code>*=</code>
| style="border-bottom-style: none; border-top-style: none" | Assignment by product
|-
| style="border-bottom-style: none; border-top-style: none" | <code>/=</code>
| style="border-bottom-style: none; border-top-style: none" | Assignment by quotient
|-
| style="border-bottom-style: none; border-top-style: none" | <code>%=</code>
| style="border-bottom-style: none; border-top-style: none" | Assignment by remainder
|-
| style="border-bottom-style: none; border-top-style: none" | <code>&lt;&lt;=</code>
| style="border-bottom-style: none; border-top-style: none" | Assignment by bitwise left shift
|-
| style="border-bottom-style: none; border-top-style: none" | <code>&gt;&gt;=</code>
| style="border-bottom-style: none; border-top-style: none" | Assignment by bitwise right shift
|-
| style="border-bottom-style: none; border-top-style: none" | <code>&=</code>
| style="border-bottom-style: none; border-top-style: none" | Assignment by bitwise AND
|-
| style="border-bottom-style: none; border-top-style: none" | <code>^=</code>
| style="border-bottom-style: none; border-top-style: none" | Assignment by bitwise XOR
|-
| style="border-bottom-style: none; border-top-style: none" | <code><nowiki>|</nowiki>=</code>
| style="border-bottom-style: none; border-top-style: none" | Assignment by bitwise OR
|-
| style="border-top-style: none" | <code>throw</code>
| style="border-top-style: none" | Throw operator (exceptions throwing, C++ only)
|-
! 18
<small>lowest</small>
| <code>,</code>
| [[Comma operator|Comma]]
| Left-to-right
|}

===Details===
Although this table is adequate for describing most evaluation order, it does not describe a few details. 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, the immediate, un-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>.

===Chained expressions===
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>).
* Similarly, with <code>3*x++</code>, where though the post-fix {{mono|++}} is designed to act AFTER the entire expression is evaluated, the precedence table makes it clear that ONLY {{mono|x}} gets incremented (and NOT <code>3*x</code>). In fact, the expression (<code>tmp=x++</code>, <code>3*tmp</code>) is evaluated with {{mono|tmp}} being a temporary value. It is functionally equivalent to something like (<code>tmp=3*x</code>, <code>++x</code>, <code>tmp</code>).
[[Image:Precedence 2.png|center|thumb|Precedence and bindings]]
* Abstracting the issue of precedence or binding, consider the diagram above for the expression 3+2*y[i]++. The compiler's job is to resolve the diagram into an expression, one in which several unary operators (call them 3+( . ), 2*( . ), ( . )++ and ( . )[ i ]) are competing to bind to y. The order of precedence table resolves the final sub-expression they each act upon: ( . )[ i ] acts only on y, ( . )++ acts only on y[i], 2*( . ) acts only on y[i]++ and 3+( . ) acts 'only' on 2*((y[i])++). It is important to note that WHAT sub-expression gets acted on by each operator is clear from the precedence table but WHEN each operator acts is not resolved by the precedence table; in this example, the ( . )++ operator acts only on y[i] by the precedence rules but binding levels alone do not indicate the timing of the postfix ++ (the ( . )++ operator acts only after y[i] is evaluated in the expression).

===Binding===
The binding of operators in C and C++ is specified by a factored language grammar, rather than a precedence table. This creates some subtle conflicts. For example, in C, the syntax for a conditional expression is:
<syntaxhighlight lang="c">logical-OR-expression ? expression : conditional-expression</syntaxhighlight>
while in C++ it is:
<syntaxhighlight lang="cpp">logical-OR-expression ? expression : assignment-expression</syntaxhighlight>
Hence, the expression:
<syntaxhighlight lang="text">e = a < d ? a++ : a = d</syntaxhighlight>
is parsed differently in the two languages. In C, this expression is a syntax error, because the syntax for an assignment expression in C is:
<syntaxhighlight lang="c">unary-expression '=' assignment-expression</syntaxhighlight>
In C++, it is parsed as:
<syntaxhighlight lang="cpp">e = (a < d ? a++ : (a = d))</syntaxhighlight>
which is a valid expression.<ref>{{cite web |title=C Operator Precedence - cppreference.com |url=https://en.cppreference.com/w/c/language/operator_precedence |website=en.cppreference.com |access-date=10 April 2020}}</ref><ref>{{Cite web|url=https://stackoverflow.com/questions/13515434/does-the-c-c-ternary-operator-actually-have-the-same-precedence-as-assignment/13515505|title=Does the C/C++ ternary operator actually have the same precedence as assignment operators?|website=Stack Overflow|access-date=2019-09-22}}</ref>

To use the comma operator in a function call argument expression, variable assignment, or a comma-separated list, use of parentheses is required.<ref>{{cite web |title=Other operators - cppreference.com |url=https://en.cppreference.com/w/c/language/operator_other |website=en.cppreference.com |access-date=10 April 2020}}</ref><ref>{{cite web |title=c++ - How does the Comma Operator work |url=https://stackoverflow.com/questions/54142/how-does-the-comma-operator-work/ |website=Stack Overflow |access-date=1 April 2020}}</ref> For example, 

<syntaxhighlight lang="cpp">
int a = 1, b = 2, weirdVariable = (++a, b), d = 4;
</syntaxhighlight>

===Criticism of bitwise and equality operators precedence===
The precedence of the bitwise logical operators has been criticized.<ref name="Bell">{{Citation | url = https://www.bell-labs.com/usr/dmr/www/chist.html | title = C history § Neonatal C| publisher = Bell labs}}.</ref> Conceptually, & and | are arithmetic operators like * and +.

The expression {{cpp|1=a & b == 7}} is syntactically parsed as {{cpp|1=a & (b == 7)}} whereas the expression {{cpp |1=a + b == 7}} is parsed as {{cpp|1=(a + b) == 7}}. This requires parentheses to be used more often than they otherwise would.

Historically, there was no syntactic distinction between the bitwise and logical operators. In [[BCPL]], [[B (programming language)|B]] and early C, the operators {{cpp|&& {{!!}}}} didn't exist. Instead {{cpp|& {{!}}}} had different meaning depending on whether they are used in a 'truth-value context' (i.e. when a Boolean value was expected, for example in {{cpp|1= if (a==b & c) {...} }} it behaved as a logical operator, but in {{cpp|1= c = a & b}} it behaved as a bitwise one). It was retained so as to keep [[backward compatibility]] with existing installations.<ref>{{cite web|url=https://www.perlmonks.org/?node_id=1159769|title=Re^10: next unless condition|website=www.perlmonks.org|access-date=23 March 2018}}</ref>

Moreover, in C++ (and later versions of C) equality operations, with the exception of the three-way comparison operator, yield [[Boolean data type|bool]] type values which are conceptually a single bit (1 or 0) and as such do not properly belong in "bitwise" operations.