Operators in C and C++ explained

This is a list of operators in the C and C++ programming languages.

All listed operators are in C++ and lacking indication otherwise, in C as well. Some tables include a "In C" column that indicates whether an operator is also in C. Note that C does not support operator overloading.

When not overloaded, for the operators &&, ||, and , (the comma operator), there is a sequence point after the evaluation of the first operand.

Most of the operators available in C and C++ are also available in other C-family languages such as C#, D, Java, Perl, and PHP with the same precedence, associativity, and semantics.

Many operators specified by a sequence of symbols are commonly referred to by a name that consists of the name of each symbol. For example, += and -= are often called "plus equal(s)" and "minus equal(s)", instead of the more verbose "assignment by addition" and "assignment by subtraction".

Operators

In the following tables, lower case letters such as a and b represent literal values, object/variable names, or l-values, as appropriate. R, S and T stand for a data type, and K for a class or enumeration type. Some operators have alternative spellings using digraphs and trigraphs or operator synonyms.

Arithmetic

C and C++ have the same arithmetic operators and all can be overloaded in C++.

OperationSyntaxC++ prototype
in class Koutside class
colspan="2"
colspan="2" Subtractiona '''-''' b
colspan="2" Unary plus; integer promotion'''+'''a
colspan="2" '''-'''a
colspan="2" Multiplicationa '''*''' b
colspan="2" Divisiona '''/''' b
colspan="2" Moduloa '''%''' b
colspan="2" Prefix increment'''++'''a
colspan="2" Postfix incrementa'''++'''
colspan="2" Prefix decrement'''--'''a
colspan="2" Postfix decrementa'''--'''

Relational

All relational (comparison) operators can be overloaded in C++. Since C++20, the inequality operator is automatically generated if operator== is defined and all four relational operators are automatically generated if operator<=> is defined.[1]

Operation Syntax In CC++ prototype
in class Koutside class
colspan="2" Equal to a '''==''' b
colspan="2" Not equal to a '''= b
colspan="2" Greater than a '''>''' b
colspan="2" Less than a '''<''' b
colspan="2" Greater than or equal to a '''>=''' b
colspan="2" Less than or equal to a '''<=''' b
colspan="2" Three-way comparisona '''&lt;=&gt;''' b

Logical

C and C++ have the same logical operators and all can be overloaded in C++.

Note that overloading logical AND and OR is discouraged, because as overloaded operators they always evaluate both operands instead of providing the normal semantics of short-circuit evaluation.[2]

OperationSyntax C++ prototype
in class Koutside class
colspan="2" '''a
colspan="2" a '''&&''' b
colspan="2" a '''<nowiki>||</nowiki>''' b

Bitwise

C and C++ have the same bitwise operators and all can be overloaded in C++.

OperationSyntax C++ prototype
in class Koutside class
colspan="2" NOT'''~'''a
colspan="2" ANDa '''&''' b
colspan="2" ORa '''<nowiki>|</nowiki>''' b| | |-| colspan="2" | XOR| style="text-align:center;" | a '''^''' b| | |- | colspan="2" | Shift left| style="text-align:center;" | a '''<<''' b | | |-| colspan="2" | Shift right| style="text-align:center;" | a '''>>''' b | | |}

Assignment

C and C++ have the same assignment operators and all can be overloaded in C++.

For the combination operators, a ⊚= b (where ⊚ represents an operation) is equivalent to a = a ⊚ b, except that a is evaluated only once.

OperationSyntax C++ prototype
in class Koutside class
Addition combinationa '''+=''' b
Subtraction combination a '''-=''' b
Multiplication combination a '''*=''' b
Division combinationa '''/=''' b
Modulo combination a '''%=''' b
Bitwise AND combinationa '''&=''' b
Bitwise OR combination a '''<nowiki>|</nowiki>=''' b| | |-! | Bitwise XOR combination| style="text-align:center;" | a '''^=''' b| | |-! | Bitwise left shift combination| style="text-align:center;" | a '''<<=''' b| | |-! | Bitwise right shift combination| style="text-align:center;" | a '''>>=''' b| | |}

Member and pointer

OperationSyntaxCan overloadIn CC++ prototype
in class Koutside class
colspan="2" Subscripta'''['''b''']'''a'''<:'''b''':>'''[3]
colspan="2" Indirection
'''*'''a
colspan="2" Address-of
'''&'''a
colspan="2" Structure dereference
a'''->'''b
colspan="2" Structure reference
a'''.'''b colspan="2"
colspan="2" Member selected by pointer-to-member b of object pointed to by aa'''->*'''b
colspan="2" Member of object a selected by pointer-to-member ba'''.*'''b colspan="2"

Other

OperationSyntaxCan overloadIn CC++ prototype
in class Koutside class
colspan="2" Function calla'''('''a1, a2''')'''
colspan="2" a''',''' b
colspan="2" a '''?''' b ''':''' c colspan="2"
colspan="2" a'''::'''b colspan="2"
colspan="2" User-defined literals<nowiki>"a"_b</nowiki>
colspan="2" Sizeof'''sizeof''' a
'''sizeof''' (R) 		colspan="2"
colspan="2" Size of parameter pack'''sizeof...'''(Args) colspan="2"
colspan="2" Alignof'''alignof'''(R)
or '''_Alignof'''(R) 		colspan="2"
colspan="2" Typeof'''typeof'''(a)
'''typeof'''(R)
'''typeof_unqual'''(a)
'''typeof_unqual'''(R) 		colspan="2"
colspan="2" Decltype'''decltype'''(a)
'''decltype'''(R) 		colspan="2"
colspan="2" Type identification '''typeid'''(a)
'''typeid'''(R) 		colspan="2"
colspan="2" Conversion
(R)a [4]
colspan="2" Conversion[5] R(a)
R{a}
auto(a)
auto{a} 		colspan="2"
colspan="2" static_cast conversion'''static_cast'''<R>(a)
colspan="2" dynamic cast conversion'''dynamic_cast'''<R>(a) colspan="2"
colspan="2" const_cast conversion'''const_cast'''<R>(a) colspan="2"
colspan="2" reinterpret_cast conversion'''reinterpret_cast'''<R>(a) colspan="2"
colspan="2" '''new''' R
colspan="2" Allocate array '''new''' R'''['''n''']'''
colspan="2" Deallocate memory'''delete''' a
colspan="2" Deallocate array '''delete[]''' a
colspan="2" Exception check'''noexcept'''(a) colspan="2"
colspan="2" Reflection'''^^'''a

Synonyms

C++ defines keywords to act as aliases for a number of operators:[6]

Keyword Operator
&&
&=
&
&#x7C;
~
!
!=
&#x7C;&#x7C;
&#x7C;=
^
^=

Each keyword is a different way to specify an operator and as such can be used instead of the corresponding symbolic variation. For example, and specify the same behavior. As another example, the bitand keyword may be used to replace not only the bitwise-and operator but also the address-of operator, and it can be used to specify reference types (e.g.,).

The ISO C specification makes allowance for these keywords as preprocessor macros in the header file . For compatibility with C, C++ also provides the header, the inclusion of which has no effect. Until C++20, it also provided the corresponding header which had no effect as well.

Expression evaluation order

During expression evaluation, the order in which sub-expressions are evaluated is determined by precedence and 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).[7] [8] [9]

Operator precedence is not affected by overloading.

OrderOperatorDescriptionAssociativity
1highest::Scope resolution (C++ only)None
2++Postfix incrementLeft-to-right
--Postfix decrement
Function call
[]Array subscripting
.Element selection by reference
-&gt;Element selection through pointer
typeidRun-time type information (C++ only) (see typeid)
const_castType cast (C++ only) (see const_cast)
dynamic_castType cast (C++ only) (see dynamic cast)
reinterpret_castType cast (C++ only) (see reinterpret_cast)
static_castType cast (C++ only) (see static_cast)
3++Prefix incrementRight-to-left
--Prefix decrement
+Unary plus
-Unary minus
Logical NOT
~Bitwise NOT (ones' complement)
(''type'')Type cast
*Indirection (dereference)
&Address-of
sizeofSizeof
_AlignofAlignment requirement (since C11)
new, new[]Dynamic memory allocation (C++ only)
delete, delete[]Dynamic memory deallocation (C++ only)
4.*Pointer to member (C++ only)Left-to-right
->*Pointer to member (C++ only)
5*MultiplicationLeft-to-right
/Division
%Modulo (remainder)
6+AdditionLeft-to-right
-Subtraction
7&lt;&lt;Bitwise left shiftLeft-to-right
&gt;&gt;Bitwise right shift
8&lt;=&gt;Three-way comparison (Introduced in C++20 - C++ only)Left-to-right
9&lt;Less thanLeft-to-right
&lt;=Less than or equal to
&gt;Greater than
&gt;=Greater than or equal to
10==Equal toLeft-to-right
=Not equal to
11&Bitwise ANDLeft-to-right
12^Bitwise XOR (exclusive or)Left-to-right
13<nowiki>|</nowiki>| Bitwise OR (inclusive or)| Left-to-right|-! 14| &&| Logical AND| Left-to-right|-! 15| <nowiki>||</nowiki>Logical ORLeft-to-right
16co_awaitCoroutine processing (C++ only)Right-to-left
co_yield
17?:Ternary conditional operatorRight-to-left
=Direct assignment
+=Assignment by sum
-=Assignment by difference
*=Assignment by product
/=Assignment by quotient
%=Assignment by remainder
&lt;&lt;=Assignment by bitwise left shift
&gt;&gt;=Assignment by bitwise right shift
&=Assignment by bitwise AND
^=Assignment by bitwise XOR
<nowiki>|</nowiki>=| style="border-bottom-style: none; border-top-style: none" | Assignment by bitwise OR|-| style="border-top-style: none" | throw| style="border-top-style: none" | Throw operator (exceptions throwing, C++ only)|-! 18lowest| ,| 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 a ? b, c : d is interpreted as a ? (b, c) : d, and not as the meaningless (a ? b), (c : d). So, the expression in the middle of the conditional operator (between '''?''' and ''':''') is parsed as if parenthesized. Also, the immediate, un-parenthesized result of a C cast expression cannot be the operand of sizeof. Therefore, sizeof (int) * x is interpreted as (sizeof(int)) * x and not sizeof ((int) * x).

Chained expressions

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

  • For example, ++x*3 is ambiguous without some precedence rule(s). The precedence table tells us that: is 'bound' more tightly to than to, so that whatever does (now or later—see below), it does it ONLY to (and not to x*3); it is equivalent to (++x, x*3).
  • Similarly, with 3*x++, where though the post-fix is designed to act AFTER the entire expression is evaluated, the precedence table makes it clear that ONLY gets incremented (and NOT 3*x). In fact, the expression (tmp=x++, 3*tmp) is evaluated with being a temporary value. It is functionally equivalent to something like (tmp=3*x, ++x, tmp).
  • 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:logical-OR-expression ? expression : conditional-expressionwhile in C++ it is:logical-OR-expression ? expression : assignment-expressionHence, the expression:e = a < d ? a++ : a = dis parsed differently in the two languages. In C, this expression is a syntax error, because the syntax for an assignment expression in C is:unary-expression '=' assignment-expressionIn C++, it is parsed as:e = (a < d ? a++ : (a = d))which is a valid expression.[10] [11]

To use the comma operator in a function call argument expression, variable assignment, or a comma-separated list, use of parentheses is required.[12] [13] For example,

int a = 1, b = 2, weirdVariable = (++a, b), d = 4;

Criticism of bitwise and equality operators precedence

The precedence of the bitwise logical operators has been criticized.[14] Conceptually, & and | are arithmetic operators like * and +.

The expression is syntactically parsed as whereas the expression is parsed as . 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 and early C, the operators didn't exist. Instead 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 it behaved as a logical operator, but in it behaved as a bitwise one). It was retained so as to keep backward compatibility with existing installations.[15]

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

External links

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Notes and References

  1. Web site: Operator overloading§Comparison operators. cppreference.com.
  2. Web site: Standard C++.
  3. Web site: ISO/IEC 9899:1999 specification, TC3 . p. 64, § 6.4.6 Ponctuators para. 3.
  4. Web site: user-defined conversion. 5 April 2020.
  5. https://en.cppreference.com/w/cpp/language/explicit_cast Explicit type conversion
  6. Book: ISO/IEC 14882:1998(E) Programming Language C++ . 1 September 1998 . open-std.org – The C++ Standards Committee . 40–41.
  7. Book: ISO/IEC 9899:201x Programming Languages - C . 19 December 2011 . open-std.org – The C Standards Committee . 465.
  8. the ISO C 1999 standard, section 6.5.6 note 71 . ISO . 1999 .
  9. Web site: C++ Built-in Operators, Precedence and Associativity . docs.microsoft.com . 11 May 2020 . en-us.
  10. Web site: C Operator Precedence - cppreference.com . en.cppreference.com . 10 April 2020.
  11. Web site: Does the C/C++ ternary operator actually have the same precedence as assignment operators?. Stack Overflow. 2019-09-22.
  12. Web site: Other operators - cppreference.com . en.cppreference.com . 10 April 2020.
  13. Web site: c++ - How does the Comma Operator work . Stack Overflow . 1 April 2020.
  14. .
  15. Web site: Re^10: next unless condition. www.perlmonks.org. 23 March 2018.

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