Wikipedia

Operators in C and C++

(Redirected from Function call operator)

This is a list of operators in the C and C++ programming languages. All the operators (except typeof) listed exist in C++; the column "Included in C", states whether an operator is also present 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.

C++ also contains the type conversion operators const_cast, static_cast, dynamic_cast, and reinterpret_cast. The formatting of these operators means that their precedence level is unimportant.

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.

Table

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For the purposes of these tables, a, b, and c represent valid values (literals, values from variables, or return value), object names, or lvalues, as appropriate. R, S and T stand for any type(s), and K for a class type or enumerated type. Some of the operators have alternative spellings using digraphs and trigraphs or operator synonyms.

Arithmetic operators

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All arithmetic operators exist in C and C++ and can be overloaded in C++.

Operator name Syntax C++ prototype examples
As member of K Outside class definitions
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 (integer remainder)[a] a % b R K::operator %(S b); R operator %(K a, S b);
Increment Prefix ++a R& K::operator ++(); R& operator ++(K& a);
Postfix a++ R K::operator ++(int); R operator ++(K& a, int);
Note: C++ uses the unnamed dummy-parameter int to differentiate between prefix and postfix increment operators.
Decrement Prefix --a R& K::operator --(); R& operator --(K& a);
Postfix a-- R K::operator --(int); R operator --(K& a, int);
Note: C++ uses the unnamed dummy-parameter int to differentiate between prefix and postfix decrement operators.

Comparison operators/relational operators

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All 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]

Operator name Syntax Included
in C 		Prototype examples
As member of K Outside class definitions
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[b] a <=> b No auto K::operator <=>(const S &b); auto operator <=>(const K &a, const S &b);
The operator has a total of 3 possible return types: std::weak_ordering, std::strong_ordering and std::partial_ordering to which they all are convertible to.

Logical operators

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All logical operators exist in C and C++ and can be overloaded in C++, albeit the overloading of the logical AND and logical OR is discouraged, because as overloaded operators they behave as ordinary function calls, which means that both of their operands are evaluated, so they lose their well-used and expected short-circuit evaluation property.[2]

Operator name Syntax C++ prototype examples
As member of K Outside class definitions
Logical negation (NOT) !a bool K::operator !(); bool operator !(K a);
Logical AND a && b bool K::operator &&(S b); bool operator &&(K a, S b);
Logical OR a || b bool K::operator ||(S b); bool operator ||(K a, S b);

Bitwise operators

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All bitwise operators exist in C and C++ and can be overloaded in C++.

Operator name Syntax Prototype examples
As member of K Outside class definitions
Bitwise NOT ~a
 R K::operator ~(); R operator ~(K a);
Bitwise AND a & b R K::operator &(S b); R operator &(K a, S b);
Bitwise OR a | b R K::operator |(S b); R operator |(K a, S b);
Bitwise XOR a ^ b R K::operator ^(S b); R operator ^(K a, S b);
Bitwise left shift[c] a << b R K::operator <<(S b); R operator <<(K a, S b);
Bitwise right shift[c][d] a >> b R K::operator >>(S b); R operator >>(K a, S b);

Assignment operators

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All assignment expressions exist in C and C++ and can be overloaded in C++.

For the given operators the semantic of the built-in combined assignment expression a ⊚= b is equivalent to a = a ⊚ b, except that a is evaluated only once.

Operator name Syntax C++ prototype examples
As member of K Outside class definitions
Direct assignment a = b R& K::operator =(S b); —
Addition assignment a += b R& K::operator +=(S b); R& operator +=(K& a, S b);
Subtraction assignment a -= b R& K::operator -=(S b); R& operator -=(K& a, S b);
Multiplication assignment a *= b R& K::operator *=(S b); R& operator *=(K& a, S b);
Division assignment a /= b R& K::operator /=(S b); R& operator /=(K& a, S b);
Modulo assignment a %= b R& K::operator %=(S b); R& operator %=(K& a, S b);
Bitwise AND assignment a &= b R& K::operator &=(S b); R& operator &=(K& a, S b);
Bitwise OR assignment a |= b R& K::operator |=(S b); R& operator |=(K& a, S b);
Bitwise XOR assignment a ^= b R& K::operator ^=(S b); R& operator ^=(K& a, S b);
Bitwise left shift assignment a <<= b R& K::operator <<=(S b); R& operator <<=(K& a, S b);
Bitwise right shift assignment[d] a >>= b R& K::operator >>=(S b); R& operator >>=(K& a, S b);

Member and pointer operators

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Operator name Syntax Can overload in C++ Included
in C 		C++ prototype examples
As member of K Outside class definitions
Subscript a[b]a<:b:>[4] Yes Yes R& K::operator [](S b);
R& K::operator [](S b, ...); // since C++23
 —
Indirection ("object pointed to by a") *a Yes Yes R& K::operator *(); R& operator *(K a);
Address-of ("address of a") &a Yes[e] 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 ->();[f]
 —
Structure reference ("member b of object a") a.b No Yes —
Member selected by pointer-to-member b of object pointed to by a[g] 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 operators

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Operator name Syntax Can overload in C++ Included
in C 		Prototype examples
As member of K Outside class definitions
Function call
See Function object. 		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[h] No No —
User-defined literals[i]
since C++11 		"a"_b Yes No — R operator "" _b(T a)
Sizeof sizeof a[j]
sizeof (R)		 No Yes —
Size of parameter pack
since C++11 		sizeof...(Args) No No —
Alignof
since C++11 		alignof(R)
or _Alignof(R)[k]		 No Yes —
Decltype
since C++11 		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 R(a)
R{a}since C++11
auto(a)since C++23
auto{a}since C++23		 No No Note: 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. [6]
static_cast conversion static_cast<R>(a) Yes No K::operator R();
explicit K::operator R(); since C++11 		—
Note: 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.).
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[l] Yes No void* K::operator new(size_t x); void* operator new(size_t x);
Allocate storage (array) new R[n][m] 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 storage (array) delete[] a Yes No void K::operator delete[](void* a); void operator delete[](void* a);
Exception check
since C++11 		noexcept(a) No No —

Notes:

  1. ^ The modulus operator works just with integer operands, for floating point numbers a library function must be used instead (like fmod).
  2. ^ About C++20 three-way comparison
  3. ^ 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.
  4. ^ a b 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.
  5. ^ The actual address of an object with an overloaded operator & can be obtained with std::addressof
  6. ^ 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.
  7. ^ Meyers, Scott (October 1999), "Implementing operator->* for Smart Pointers" (PDF), Dr. Dobb's Journal, Aristeia.
  8. ^ Although a :: punctuator exists in C as of C23, it is not used as a scope resolution operator.
  9. ^ About C++11 User-defined literals
  10. ^ 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]
  11. ^ C++ defines alignof operator, whereas C defines _Alignof (C23 defines both). Both operators have the same semantics.
  12. ^ The type name can also be inferred (e.g new auto) if an initializer is provided.
  13. ^ The array size can also be inferred if an initializer is provided.

Operator precedence

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The following is a table that lists the precedence and associativity of all the operators in the C and C++ languages. Operators are listed top to bottom, in descending precedence. Descending precedence refers to the priority of the grouping of operators and operands. Considering an expression, an operator which is listed on some row will be grouped prior to any operator that is listed on a row further below it. Operators that are in the same cell (there may be several rows of operators listed in a cell) are grouped with the same precedence, in the given direction. An operator's precedence is unaffected by overloading.

The syntax of expressions in C and C++ is specified by a phrase structure grammar.[7] The table given here has been inferred from the grammar.[citation needed] 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:

"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."[8]

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 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, note that the immediate, unparenthesized 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).

Precedence 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

[9][10][11]

Notes

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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).

Many of the operators containing multi-character sequences are given "names" built from the operator name of each character. 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". The binding of operators in C and C++ is specified (in the corresponding Standards) 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.[12][13]

If you want to use comma-as-operator within a single function argument, variable assignment, or other comma-separated list, you need to use parentheses,[14][15] e.g.: 

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.[16] 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.[17]

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.

C++ operator synonyms

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C++ defines[18] certain keywords to act as aliases for a number of operators:

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

These can be used exactly the same way as the punctuation symbols they replace, as they are not the same operator under a different name, but rather simple token replacements for the name (character string) of the respective operator. This means that the expressions (a > 0 and not flag) and (a > 0 && !flag) have identical meanings. It also means that, for example, the bitand keyword may be used to replace not only the bitwise-and operator but also the address-of operator, and it can even 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.

See also

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References

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  1. ^ "Operator overloading§Comparison operators". cppreference.com.
  2. ^ "Standard C++".
  3. ^ "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 9899:201x Programming Languages - C. open-std.org – The C Standards Committee. 19 December 2011. p. 465.
  8. ^ the ISO C 1999 standard, section 6.5.6 note 71 (Technical report). ISO. 1999.
  9. ^ "C Operator Precedence - cppreference.com". en.cppreference.com. Retrieved 16 July 2019.
  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 16 July 2019.
  12. ^ "C Operator Precedence - cppreference.com". en.cppreference.com. Retrieved 10 April 2020.
  13. ^ "Does the C/C++ ternary operator actually have the same precedence as assignment operators?". Stack Overflow. Retrieved 22 September 2019.
  14. ^ "Other operators - cppreference.com". en.cppreference.com. Retrieved 10 April 2020.
  15. ^ "c++ - How does the Comma Operator work". Stack Overflow. Retrieved 1 April 2020.
  16. ^ C history § Neonatal C, Bell labs.
  17. ^ "Re^10: next unless condition". www.perlmonks.org. Retrieved 23 March 2018.
  18. ^ ISO/IEC 14882:1998(E) Programming Language C++. open-std.org – The C++ Standards Committee. 1 September 1998. pp. 40–41.
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