Operators in C and C++

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

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]

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]

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

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.

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). 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.[11][12]

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,[13][14] e.g.:

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

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.

C++ defines[17] certain keywords to act as aliases for a number of operators:

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++ provides the header ciso646, the inclusion of which has no effect.