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Operators in C and C++

(Redirected from Operator precedence in C and C++)

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

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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

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

Operation Syntax C++ prototype
in class K outside class
Addition a + b R K::operator +(S b); R operator +(K a, S b);
Subtraction a - b R K::operator -(S b); R operator -(K a, S b);
Unary plus; integer promotion +a R K::operator +(); R operator +(K a);
Unary minus; additive inverse -a R K::operator -(); R operator -(K a);
Multiplication a * b R K::operator *(S b); R operator *(K a, S b);
Division a / b R K::operator /(S b); R operator /(K a, S b);
Modulo[a] a % b R K::operator %(S b); R operator %(K a, S b);
Prefix increment ++a R& K::operator ++(); R& operator ++(K& a);
Postfix increment a++ R K::operator ++(int); [b] R operator ++(K& a, int); [b]
Prefix decrement --a R& K::operator --(); R& operator --(K& a);
Postfix decrement a-- R K::operator --(int); [b] R operator --(K& a, int); [b]

Relational

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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 C C++ prototype
in class K outside class
Equal to a == b Yes bool K::operator ==(S const& b) const; bool operator ==(K const& a, S const& b);
Not equal to a != b Yes bool K::operator !=(S const& b) const; bool operator !=(K const& a, S const& b);
Greater than a > b Yes bool K::operator >(S const& b) const; bool operator >(K const& a, S const& b);
Less than a < b Yes bool K::operator <(S const& b) const; bool operator <(K const& a, S const& b);
Greater than or equal to a >= b Yes bool K::operator >=(S const& b) const; bool operator >=(K const& a, S const& b);
Less than or equal to a <= b Yes bool K::operator <=(S const& b) const; bool operator <=(K const& a, S const& b);
Three-way comparison[c][d] a <=> b No auto K::operator <=>(const S &b); auto operator <=>(const K &a, const S &b);

Logical

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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]

Operation Syntax C++ prototype
in class K outside class
NOT !a bool K::operator !(); bool operator !(K a);
AND a && b bool K::operator &&(S b); bool operator &&(K a, S b);
OR a || b bool K::operator ||(S b); bool operator ||(K a, S b);

Bitwise

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

Operation Syntax C++ prototype
in class K outside class
NOT ~a
 R K::operator ~(); R operator ~(K a);
AND a & b R K::operator &(S b); R operator &(K a, S b);
OR a | b R K::operator |(S b); R operator |(K a, S b);
XOR a ^ b R K::operator ^(S b); R operator ^(K a, S b);
Shift left[e] a << b R K::operator <<(S b); R operator <<(K a, S b);
Shift right[e][f] a >> b R K::operator >>(S b); R operator >>(K a, S b);

Assignment

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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.

Operation Syntax C++ prototype
in class K outside class
Assignment a = b R& K::operator =(S b); —
Addition combination a += b R& K::operator +=(S b); R& operator +=(K& a, S b);
Subtraction combination a -= b R& K::operator -=(S b); R& operator -=(K& a, S b);
Multiplication combination a *= b R& K::operator *=(S b); R& operator *=(K& a, S b);
Division combination a /= b R& K::operator /=(S b); R& operator /=(K& a, S b);
Modulo combination a %= b R& K::operator %=(S b); R& operator %=(K& a, S b);
Bitwise AND combination a &= b R& K::operator &=(S b); R& operator &=(K& a, S b);
Bitwise OR combination a |= b R& K::operator |=(S b); R& operator |=(K& a, S b);
Bitwise XOR combination a ^= b R& K::operator ^=(S b); R& operator ^=(K& a, S b);
Bitwise left shift combination a <<= b R& K::operator <<=(S b); R& operator <<=(K& a, S b);
Bitwise right shift combination[g] a >>= b R& K::operator >>=(S b); R& operator >>=(K& a, S b);

Member and pointer

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Operation Syntax Can overload In C C++ prototype
in class K outside class
Subscript a[b]a<:b:>[4] Yes Yes R& K::operator [](S b);
R& K::operator [](S b, ...);[h] 		—
Indirection
(object pointed to by a) 		*a Yes Yes R& K::operator *(); R& operator *(K a);
Address-of
(address of a) 		&a Yes[i] Yes R* K::operator &(); R* operator &(K a);
Structure dereference
(member b of object pointed to by a) 		a->b Yes Yes R* K::operator ->();[j]
 —
Structure reference
(member b of object a) 		a.b No Yes —
Member selected by pointer-to-member b of object pointed to by a[k] a->*b Yes No R& K::operator ->*(S b); R& operator ->*(K a, S b);
Member of object a selected by pointer-to-member b a.*b No No —

Other

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Operation Syntax Can overload In C C++ prototype
in class K outside class
Function call a(a1, a2) Yes Yes R K::operator ()(S a, T b, ...); —
Comma a, b Yes Yes R K::operator ,(S b); R operator ,(K a, S b);
Ternary conditional a ? b : c No Yes —
Scope resolution a::b[l] No No —
User-defined literals[m][n] "a"_b Yes No — R operator "" _b(T a)
Sizeof sizeof a[o]
sizeof (R)		 No Yes —
Size of parameter pack[n] sizeof...(Args) No No —
Alignof[n] alignof(R)
or _Alignof(R)[p]		 No Yes —
Decltype[n] decltype (a)
decltype (R)		 No No —
Type identification typeid(a)
typeid(R)		 No No —
Conversion
(C-style cast) 		(R)a Yes Yes K::operator R();[5] —
Conversion [q][6] R(a)
R{a}[n]
auto(a)[h]
auto{a}[h]		 No No —
static_cast conversion [r] static_cast<R>(a) Yes No K::operator R();
explicit K::operator R();[n] 		—
dynamic cast conversion dynamic_cast<R>(a) No No —
const_cast conversion const_cast<R>(a) No No —
reinterpret_cast conversion reinterpret_cast<R>(a) No No —
Allocate storage new R[s] Yes No void* K::operator new(size_t x); void* operator new(size_t x);
Allocate array new R[n][t] Yes No void* K::operator new[](size_t a); void* operator new[](size_t a);
Deallocate storage delete a Yes No void K::operator delete(void* a); void operator delete(void* a);
Deallocate array delete[] a Yes No void K::operator delete[](void* a); void operator delete[](void* a);
Exception check[n] noexcept(a) No No —

Synonyms

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C++ defines keywords to act as aliases for a number of operators:[7]

Keyword Operator
and &&
and_eq &=
bitand &
bitor |
compl ~
not !
not_eq !=
or ||
or_eq |=
xor ^
xor_eq ^=

Each keyword is a different way to specify an operator and as such can be used instead of the corresponding symbolic variation. For example, (a > 0 and not flag) and (a > 0 && !flag) 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., int bitand ref = n).

The ISO C specification makes allowance for these keywords as preprocessor macros in the header file iso646.h. For compatibility with C, C++ also provides the header iso646.h, the inclusion of which has no effect. Until C++20, it also provided the corresponding header ciso646 which had no effect as well.

Expression evaluation order

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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).[8][9][10]

Operator precedence is not affected by overloading.

Order Operator Description Associativity
1

highest

:: Scope resolution (C++ only) None
2 ++ Postfix increment Left-to-right
-- Postfix decrement
() Function call
[] Array subscripting
. Element selection by reference
-> Element selection through pointer
typeid() Run-time type information (C++ only) (see typeid)
const_cast Type cast (C++ only) (see const_cast)
dynamic_cast Type cast (C++ only) (see dynamic cast)
reinterpret_cast Type cast (C++ only) (see reinterpret_cast)
static_cast Type cast (C++ only) (see static_cast)
3 ++ Prefix increment Right-to-left
-- Prefix decrement
+ Unary plus
- Unary minus
! Logical NOT
~ Bitwise NOT (ones' complement)
(type) Type cast
* Indirection (dereference)
& Address-of
sizeof Sizeof
_Alignof Alignment 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 * Multiplication Left-to-right
/ Division
% Modulo (remainder)
6 + Addition Left-to-right
- Subtraction
7 << Bitwise left shift Left-to-right
>> Bitwise right shift
8 <=> Three-way comparison (Introduced in C++20 - C++ only) Left-to-right
9 < Less than Left-to-right
<= Less than or equal to
> Greater than
>= Greater than or equal to
10 == Equal to Left-to-right
!= Not equal to
11 & Bitwise AND Left-to-right
12 ^ Bitwise XOR (exclusive or) Left-to-right
13 | Bitwise OR (inclusive or) Left-to-right
14 && Logical AND Left-to-right
15 || Logical OR Left-to-right
16 co_await Coroutine processing (C++ only) Right-to-left
co_yield
17 ?: Ternary conditional operator Right-to-left
= Direct assignment
+= Assignment by sum
-= Assignment by difference
*= Assignment by product
/= Assignment by quotient
%= Assignment by remainder
<<= Assignment by bitwise left shift
>>= Assignment by bitwise right shift
&= Assignment by bitwise AND
^= Assignment by bitwise XOR
|= Assignment by bitwise OR
throw Throw operator (exceptions throwing, C++ only)
18

lowest

, Comma Left-to-right

Details

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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

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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: x is 'bound' more tightly to ++ than to *, so that whatever ++ does (now or later—see below), it does it ONLY to x (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 x gets incremented (and NOT 3*x). In fact, the expression (tmp=x++, 3*tmp) is evaluated with tmp being a temporary value. It is functionally equivalent to something like (tmp=3*x, ++x, tmp).
 
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

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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-expression

while in C++ it is: 

logical-OR-expression ? expression : assignment-expression

Hence, the expression: 

e = a < d ? a++ : a = d

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: 

unary-expression '=' assignment-expression

In C++, it is parsed as: 

e = (a < d ? a++ : (a = d))

which is a valid expression.[11][12]

To use the comma operator in a function call argument expression, variable assignment, or a comma-separated list, use of parentheses is required.[13][14] For example, 

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

Criticism of bitwise and equality operators precedence

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The precedence of the bitwise logical operators has been criticized.[15] Conceptually, & and | are arithmetic operators like * and +.

The expression a & b == 7 is syntactically parsed as a & (b == 7) whereas the expression a + b == 7 is parsed as (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 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 if (a==b & c) {...} it behaved as a logical operator, but in c = a & b it behaved as a bitwise one). It was retained so as to keep backward compatibility with existing installations.[16]

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.

Notes

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  1. ^ The modulus operator only supports integer operands; for floating point, a function such as fmod can be used.
  2. ^ a b c d The int is a dummy parameter to differentiate between prefix and postfix.
  3. ^ About C++20 three-way comparison
  4. ^ Possible return types: std::weak_ordering, std::strong_ordering and std::partial_ordering to which they all are convertible to.
  5. ^ a b In the context of iostreams in C++, writers often will refer to << and >> as the "put-to" or "stream insertion" and "get-from" or "stream extraction" operators, respectively.
  6. ^ According to the C99 standard, the right shift of a negative number is implementation defined. Most implementations, e.g., the GCC,[3] use an arithmetic shift (i.e., sign extension), but a logical shift is possible.
  7. ^ According to the C99 standard, the right shift of a negative number is implementation defined. Most implementations, e.g., the GCC,[3] use an arithmetic shift (i.e., sign extension), but a logical shift is possible.
  8. ^ a b c since C++23
  9. ^ The actual address of an object with an overloaded operator & can be obtained with std::addressof
  10. ^ The return type of operator->() must be a type for which the -> operation can be applied, such as a pointer type. If x is of type C where C overloads operator->(), x->y gets expanded to x.operator->()->y.
  11. ^ Meyers, Scott (October 1999), "Implementing operator->* for Smart Pointers" (PDF), Dr. Dobb's Journal, Aristeia.
  12. ^ Although a :: punctuator exists in C as of C23, it is not used as a scope resolution operator.
  13. ^ About C++11 User-defined literals
  14. ^ a b c d e f g since C++11
  15. ^ The parentheses are not necessary when taking the size of a value, only when taking the size of a type. However, they are usually used regardless.[citation needed]
  16. ^ C++ defines alignof operator, whereas C defines _Alignof (C23 defines both). Both operators have the same semantics.
  17. ^ Behaves like const_cast/static_cast/reinterpret_cast. In the last two cases, the auto specifier is replaced with the type of the invented variable x declared with auto x(a); (which is never interpreted as a function declaration) or auto x{a};, respectively.
  18. ^ For user-defined conversions, the return type implicitly and necessarily matches the operator name unless the type is inferred (e.g. operator auto(), operator decltype(auto)() etc.).
  19. ^ The type name can also be inferred (e.g new auto) if an initializer is provided.
  20. ^ The array size can also be inferred if an initializer is provided.

See also

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  • Bitwise operations in C – Operations transforming individual bits of integral data types
  • Bit manipulation – Algorithmically modifying data below the word level
  • Logical operator – Symbol connecting sentential formulas in logic
  • Boolean algebra (logic) – Algebraic manipulation of "true" and "false"
  • Table of logic symbols – List of symbols used to express logical relations

References

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  1. ^ "Operator overloading§Comparison operators". cppreference.com.
  2. ^ "Standard C++".
  3. ^ a b "Integers implementation", GCC 4.3.3, GNU.
  4. ^ "ISO/IEC 9899:1999 specification, TC3" (PDF). p. 64, § 6.4.6 Ponctuators para. 3.
  5. ^ "user-defined conversion". Retrieved 5 April 2020.
  6. ^ Explicit type conversion in C++
  7. ^ ISO/IEC 14882:1998(E) Programming Language C++. open-std.org – The C++ Standards Committee. 1 September 1998. pp. 40–41.
  8. ^ ISO/IEC 9899:201x Programming Languages - C. open-std.org – The C Standards Committee. 19 December 2011. p. 465.
  9. ^ the ISO C 1999 standard, section 6.5.6 note 71 (Technical report). ISO. 1999.
  10. ^ "C++ Built-in Operators, Precedence and Associativity". docs.microsoft.com. Retrieved 11 May 2020.
  11. ^ "C Operator Precedence - cppreference.com". en.cppreference.com. Retrieved 10 April 2020.
  12. ^ "Does the C/C++ ternary operator actually have the same precedence as assignment operators?". Stack Overflow. Retrieved 22 September 2019.
  13. ^ "Other operators - cppreference.com". en.cppreference.com. Retrieved 10 April 2020.
  14. ^ "c++ - How does the Comma Operator work". Stack Overflow. Retrieved 1 April 2020.
  15. ^ C history § Neonatal C, Bell labs.
  16. ^ "Re^10: next unless condition". www.perlmonks.org. Retrieved 23 March 2018.
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