Operators in C and C++

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Programming

Arithmetic operators

Operator name Syntax Overloadable Included
in C
Prototype examples (T is any type)
As member of T Outside class definitions
Basic assignment a = b Yes Yes ​T& T::operator =(const T&b); N/A
Addition a + b Yes Yes ​T T::operator +(const T& b)const; ​T operator +(const T& a, constT& b);
Subtraction a - b Yes Yes ​T T::operator (const T& b)const; ​T operator (const T& a, constT& b);
Unary plus (integer promotion) +a Yes Yes ​T T::operator +() const; ​T operator +(const T& a);
Unary minus (additive inverse) -a Yes Yes ​T T::operator () const; ​T operator (const T& a);
Multiplication a * b Yes Yes ​T T::operator *(const T& b)const; ​T operator *(const T &a, constT& b);
Division a / b Yes Yes ​T T::operator /(const T& b)const; ​T operator /(const T& a, constT& b);
Modulo (remainder) a % b Yes Yes ​T T::operator %(const T& b)const; ​T operator %(const T& a, constT& b);
Increment Prefix ++a Yes Yes ​T& T::operator ++(); ​T& operator ++(T& a);
Suffix a++ Yes Yes ​T T::operator ++(int); ​T operator ++(T& a, int);
Note: C++ uses the unnamed dummy-parameter int to differentiate between prefix and Suffix increment operators.
Decrement Prefix --a Yes Yes ​T& T::operator (); ​T& operator (T& a);
Suffix a-- Yes Yes ​T T::operator (int); ​T operator (T& a, int);
Note: C++ uses the unnamed dummy-parameter int to differentiate between prefix and Suffix decrement operators.

[edit]Comparison operators/Relational operators

Operator name Syntax Overloadable Included
in C
Prototype examples (T is any type)
As member of T Outside class definitions
Equal to a ==b Yes Yes bool T::operator ==(const T&b) const; bool operator ==(const T& a,const T& b);
Not equal to a !=b Yes Yes bool T::operator !=(const T&b) const; bool operator !=(const T& a,const T& b);
Greater than a > b Yes Yes bool T::operator >(const T&b) const; bool operator >(const T& a,const T& b);
Less than a < b Yes Yes bool T::operator <(const T&b) const; bool operator <(const T& a,const T& b);
Greater than or equal to a >=b Yes Yes bool T::operator >=(const T&b) const; bool operator >=(const T& a,const T& b);
Less than or equal to a <=b Yes Yes bool T::operator <=(const T&b) const; bool operator <=(const T& a,const T& b);

[edit]Logical operators

Operator name Syntax Overloadable Included
in C
Prototype examples (T is any type)
As member of T Outside class definitions
Logical negation (NOT) !a Yes Yes bool T::operator !() const; bool operator !(const T& a);
Logical AND a &&b Yes Yes bool T::operator &&(const T&b) const; bool operator &&(const T& a,const T& b);
Logical OR a ||b Yes Yes bool T::operator ||(const T&b) const; bool operator ||(const T& a,const T& b);

[edit]Bitwise operators

Operator name Syntax Overloadable Included
in C
Prototype examples (T is any type)
As member of T Outside class definitions
Bitwise NOT ~a Yes Yes ​T T::operator ~() const; ​T operator ~(const T& a);
Bitwise AND a & b Yes Yes ​T T::operator &(const T& b)const; ​T operator &(const T& a, constT& b);
Bitwise OR a | b Yes Yes ​T T::operator |(const T& b)const; ​T operator |(const T& a, constT& b);
Bitwise XOR a ^ b Yes Yes ​T T::operator ^(const T& b)const; ​T operator ^(const T& a, constT& b);
Bitwise left shift[note 1] a <<b Yes Yes ​T T::operator <<(const T& b)const; ​T operator <<(const T& a, constT& b);
Bitwise right shift[note 1] a >>b Yes Yes ​T T::operator >>(const T& b)const; ​T operator >>(const T& a, constT& b);

[edit]Compound-assignment operators

Operator name Syntax Overloadable Included
in C
Prototype examples (T is any type)
As member of T Outside class definitions
Addition assignment a += b Yes Yes ​T& T::operator +=(const T&b); ​T& operator +=(T& a, const T&b);
Subtraction assignment a -= b Yes Yes ​T& T::operator =(const T&b); ​T& operator =(T& a, const T&b);
Multiplication assignment a *= b Yes Yes ​T& T::operator *=(const T&b); ​T& operator *=(T& a, const T&b);
Division assignment a /= b Yes Yes ​T& T::operator /=(const T&b); ​T& operator /=(T& a, const T&b);
Modulo assignment a %= b Yes Yes ​T& T::operator %=(const T&b); ​T& operator %=(T& a, const T&b);
Bitwise AND assignment a &= b Yes Yes ​T& T::operator &=(const T&b); ​T& operator &=(T& a, const T&b);
Bitwise OR assignment a |= b Yes Yes ​T& T::operator |=(const T&b); ​T& operator |=(T& a, const T&b);
Bitwise XOR assignment a ^= b Yes Yes ​T& T::operator ^=(const T&b); ​T& operator ^=(T& a, const T&b);
Bitwise left shift assignment a <<=b Yes Yes ​T& T::operator <<=(constT& b); ​T& operator <<=(T& a, constT& b);
Bitwise right shift assignment a >>=b Yes Yes ​T& T::operator >>=(constT& b); ​T& operator >>=(T& a, constT& b);

[edit]Member and pointer operators

Operator name Syntax Overloadable Included
in C
Prototype examples
As member of T or T1 Outside class definitions
Array subscript a[b] Yes Yes ​R T1::operator[](T2); N/A
Indirection (“variable pointed by a“) *a Yes Yes ​R T::operator *(); ​R operator *(T);
Reference (“address of a“) &a Yes Yes ​T* T::operator &(); ​T* operator &(T& a);
member b of object pointed to by a a->b Yes Yes ​T2* T1::operator>(); N/A
member b of object a a.b No Yes N/A
of object pointed to by a, the member pointed to by b a->*b Yes No ​R T1::operator>*(T2); ​R operator>*(T1, T2);
of object a, the member pointed to by b a.*b No No N/A

[edit]Other operators

Operator name Syntax Overloadable Included
in C
Prototype examples
As member of T or T1 Outside class definitions
Function call
See Function object.
a() Yes Yes ​R T1::operator ()(T2, T3, …); N/A
comma a, b Yes Yes ​T2& T1::operator ,(T2& b)const; ​T2& operator ,(const T1&a, T2& b);
ternary conditional a ? b : c No Yes N/A
Scope resolution a::b No No N/A
size-of sizeof(a)
sizeof(type)
No Yes N/A
Type identification typeid(a)
typeid(type)
No No N/A
cast (type) a Yes Yes ​T1::operator T2() const; N/A
Note: for user-defined conversions, the return type implicitly and necessarily matches the operator name.
Allocate storage new type Yes No void* T::operatornew(size_t x); void* operator new(size_tx);
Allocate storage (array) new type[n] Yes No void* T::operatornew[](size_t x); void* operatornew[](size_t x);
Deallocate storage delete a Yes No void T::operatordelete(void* x); void operator delete(void*x);
Deallocate storage (array) delete[] a Yes No void T::operatordelete[](void* x); void operatordelete[](void* x);

Notes:

  1. ^ a b In the context of iostreams, writers often will refer to << and >> as the “put-to” or “stream insertion” and “get-from” or “stream extraction” operators, respectively.

[edit]Operator precedence

The following is a table that lists the precedence and associativity of all the operators in the C++ and C programming languages (when the operators also exist in Java, Perl, PHP and many other recent languages the precedence is the same as that given). Operators are listed top to bottom, in descending precedence. Descending precedence refers to the priority of evaluation. Considering an expression, an operator which is listed on some row will be evaluated 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 evaluated 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 context-free grammar.[citation needed] 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.”

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). 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 :: Scope resolution (C++ only) Left-to-right
2 ++ -- Suffix increment and 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 and decrement Right-to-left
+ - Unary plus and minus
! ~ Logical NOT and bitwise NOT
(type) Type cast
* Indirection (dereference)
& Address-of
sizeof Size-of
new, new[] Dynamic memory allocation (C++ only)
delete, delete[] Dynamic memory deallocation (C++ only)
4 .* ->* Pointer to member (C++ only) Left-to-right
5 * / % Multiplication, division, and modulus (remainder)
6 + - Addition and subtraction
7 << >> Bitwise left shift and right shift
8 < <= For relational operators < and ≤ respectively
> >= For relational operators > and ≥ respectively
9 == != For relational = and ≠ respectively
10 & Bitwise AND
11 ^ Bitwise XOR (exclusive or)
12 | Bitwise OR (inclusive or)
13 && Logical AND
14 || Logical OR
15 c ? t : f Ternary conditional (see ?:) Right-to-Left
16 = Direct assignment (provided by default for C++ classes)
+= -= Assignment by sum and difference
*= /= %= Assignment by product, quotient, and remainder
<<= >>= Assignment by bitwise left shift and right shift
&= ^= |= Assignment by bitwise AND, XOR, and OR
17 throw Throw operator (exceptions throwing, C++ only)
18 , Comma Left-to-right

[edit]Notes

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); it is functionally equivalent to something like (tmp=3*x, x++, tmp) withtmp being a temporary value.

Precedence and bindings

  • Abstracting the issue of precedence or binding, consider the diagram above. 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’s 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 Suffix ++ (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, but many compilers parse it as:

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

which is a semantic error, since the result of the conditional-expression (which might be a++) is not an lvalue. In C++, it is parsed as:

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

which is a valid expression.

The precedence of the bitwise logical operators has been criticized.[1] 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.

[edit]C++ operator synonyms

C++ defines[1] keywords to act as aliases for a number of symbols that function as operators: and (&&), bitand (&), and_eq (&=), or (||), bitor (|), or_eq (|=), xor (^), xor_eq (^=), not (!), not_eq (!=), compl (~). These are parsed exactly like their symbolic equivalents, and can be used in place of the symbol they replace. It is the character or string that is aliased, not the operator. As a result, bitand is used to replace the bitwise AND operator, the address-of operator, and can be used to specify reference types (e.g. int bitand ref = n;).

The ANSI C specification makes allowance for these keywords as preprocessor macros in the header file iso646.h. For compatibility with C, C++ provides the header ciso646; inclusion of which has no effect.

[edit]References

  1. ^ ISO/IEC JTC1/SC22/WG21 – The C++ Standards Committee (1 September 1998). ISO/IEC 14882:1998(E) Programming Language C++. International standardization working group for the programming language C++. pp. 40–41.

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