Operators in C and C++

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++. 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. 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. Bitwise C and C++ have the same bitwise operators and all can be overloaded in C++. 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. Member and pointer Other Synonyms C++ defines keywords to act as aliases for a number of operators: 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 iso646.h|. 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 ciso646| 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). Operator precedence is not affected by overloading. 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-expression while in C++ it is: logical-OR-expression ? expression : assignment-expression Hence, the expression: e = a 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 which is a valid expression. To use the comma operator in a function call argument expression, variable assignment, or a comma-separated list, use of parentheses is required. 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. 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 {{cpp|1= if (a==b & c) {...} }} 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. 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==