Operators in C and C++

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This is a list of operators in the C++ and C programming languages. All the operators listed exist in C++; the third column indicates whether an operator is also present in C. It should also be noted that C does not support operator overloading.

The following operators are sequence points in both languages (when not overloaded): &&, ||, ?:, and , (the comma operator).

C++ also contains the type conversion operators const_cast, static_cast, dynamic_cast, and reinterpret_cast which are not listed in the table for brevity. The formatting of these operators means that their precedence level is unimportant.

Those operators that are in C, with the exception of the comma operator and the arrow operator, are also in Java, Perl, C#, and PHP with the same precedence, associativity, and semantics, with one exception: ternary operator associativity in PHP is left-to-right.

[edit] Table

For the purposes of this table, a, b, and c represent valid values (literals, values from variables, or return value), object names, or lvalues, as appropriate.

"Overloadable" means that the operator is overloadable in C++. "Included in C" means that the operator exists and has a semantic meaning in C (operators are not overloadable in C).

[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]}

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).

[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).
• 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).

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 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 ? b : c = d

is parsed differently in the two languages. In C, this expression is parsed as:

e = ((a ? b : c) = d)

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

e = (a ? b : (c = 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 punctuation that is aliased, not the operators. For example, bitand can replace both the bitwise AND operator and the address-of operator.

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.

All comparison operators (See tables at top) return a bool.

    bool a;
    int b = 1;
    int c = 2;
    a = b == c;

That will work since == returns a boolean, never requiring it to be inside of an if statement or something similar. a = b == c; is also shorthand for

    bool a;
    int b = <Some Number Here>;
    int c = <Some Number Here>;
    if(b == c)
        a = true;
    else
        a = false;

[edit] External links

• Basic types & Operators
• Prefix vs. postfix operators in C and C++

[edit] References

1. ^ [|SC22 Committee, Working group] (1998). ISO/IEC 14882:1998(E), Programming Languages - C++. International standardization subcommittee for programming languages. pp. 40-41. http://plumber.gnu-darwin.org/home/pub/Documents/isoiec14882-c%2B%2B-standard.pdf.