Other operators

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C
C language
Expressions

A collection of operators that do not fit into any of the other major categories.

This section is incomplete
Reason: consider a more general-purpose ToC for this and other tables that cover multiple topics
Operator Operator name Example Description
(...) function call f(...) call the function f(), with zero or more arguments
, comma operator a, b evaluate expression a, disregard its return value and complete any side-effects, then evaluate expression b, returning the type and the result of this evaluation
(type) type cast (type)a cast the type of a to type
? : conditional operator a ? b : c if a is logically true (does not evaluate to zero) then evaluate expression b, otherwise evaluate expression c
sizeof sizeof operator sizeof a the size in bytes of a
_Alignof (since C11) _Alignof operator _Alignof(type) the alignment required of type

Function call

The function call expression has the form

expression ( argument-list(optional) )

where

expression - any expression of pointer-to-function type (after lvalue conversions)
argument-list - comma-separated list of expressions (which cannot be comma operators) of any complete object type. May be omitted when calling functions that take no arguments.

The behavior of the function call expression depends on whether the prototype of the function being called is in scope at the point of call.

Call to a function with a prototype

1) The number of parameters must equal the number of arguments (unless the ellipsis parameter is used).
2) The type of each parameter must be a type such that implicit conversion as if by assignment exists that converts the unqualified type of the corresponding argument to the type of the parameter.
Additionally, for every parameter of array type that uses the keyword static between [ and ], the argument expression must designate a pointer to the element of an array with at least that many elements as specified in the size expression of the parameter.
 (since C99)
3) The arguments are evaluated in unspecified order and without sequencing.
4) Assignment (until C11)Initialization (since C11) is performed to copy the value of each argument to the corresponding function parameter (note; the function can modify its parameters, and those changes do not affect the arguments; C function calls are only call-by-value).
5) Function is executed, and the value it returns becomes the value of the function call expression (if the function returns void, the function call expression is a void expression) 
void f(char* p, int x) {}
int main(void)
{
    f("abc", 3.14); // array to pointer and float to int conversions
}

Call to a function without a prototype

1) The arguments are evaluated in unspecified order and without sequencing.
2) Default argument promotions are performed on every argument expression.
3) Assignment (until C11)Initialization (since C11) is performed to copy the value of each argument to the corresponding function parameter.
4) Function is executed, and the value it returns becomes the value of the function call expression (if the function returns void, the function call expression is a void expression) 
void f(); // no prototype
int main(void)
{
    f(1, 1.0f); // UB unless f is defined to take an int and a double
}
void f(int a, double c) {}

The behavior of a function call to a function without a prototype is undefined if

  • the number of arguments does not match the number of parameters.
  • the promoted types of the arguments are not compatible with the promoted types of the parameters except that
  • signed and unsigned versions of the same integer type are considered compatible if the value of the argument is representable by both types.
  • pointers to void and pointers to (possibly cvr-qualified) character types are considered compatible

Notes

The evaluations of expression that designates the function to be called and all arguments are unsequenced with respect to each other (but there is a sequence point before the body of the function begins executing) 

(*pf[f1()]) (f2(), f3() + f4()); // f1, f2, f3, f4 may be called in any order

Although function call is only defined for pointers to functions, it works with function designators due to the function-to-pointer implicit conversion. 

int f(void) { return 1; }
int (*pf)(void) = f;
int main(void)
{
    f();    // convert f to pointer, then call
    (&f)(); // create a pointer to function, then call

    pf();    // call the function
    (*pf)(); // obtain the function designator, convert to pointer, then calls

    (****f)(); // convert to pointer, obtain the function, repeat 4x, then call
    (****pf)(); // also OK
}

Functions that ignore unused arguments, such as printf, must be called with a prototype in scope (the prototype of such functions necessarily uses the trailing ellipsis parameter) to avoid invoking undefined behavior.

A post-C11 defect report DR 427 changes the semantics of preparing function parameters when calling a function with a prototype from assignment to initialization from their corresponding arguments, in order to allow parameters of const-qualified type (which are de-facto allowed). The permitted implicit conversions remain implicit conversions as if by assignment since that's what initialization also uses.

A function call expression where expression consists entirely of an identifier and that identifier is undeclared acts as though the identifier is declared as 

extern int identifier(); // returns int and has no prototype

So the following complete program is valid C89: 

main() {
    int n = atoi("123"); // implicitly declares atoi as int atoi()
}
 (until C99)

Comma operator

The comma operator expression has the form

lhs , rhs

where

lhs - any expression
rhs - any expression other than another comma operator (in other words, comma operator's associativity is left-to-right)

First, the left operand, lhs, is evaluated and its result value is discarded.

Then, a sequence point takes place, so that all side effects of lhs are complete.

Then, the right operand, rhs, is evaluated and its result is returned by the comma operator as a non-lvalue.

Notes

The type of the lhs may be void (that is, it may be a call to a function that returns void, or it can be an expression cast to void)

The comma operator may be lvalue in C++, but never in C

The comma operator may return a struct (the only other expressions that return structs are compound literals, function calls, assignments, and the conditional operator)

In the following contexts, the comma operator cannot appear at the top level of an expression because the comma has a different meaning:

If the comma operator has to be used in such context, it must be parenthesized: 

// int n = 2,3; // error, comma assumed to begin the next declarator
// int a[2] = {1,2,3}; // error: more initializers than elements
int n = (2,3), a[2] = {(1,2),3}; // OK

f(a, (t=3, t+2), c); // OK, first, stores 3 in t, then calls f with three arguments

Top-level comma operator is also disallowed in array bounds 

// int a[2,3]; // error
int a[(2,3)]; // OK, VLA array of size 3 (VLA because (2,3) is not a constant expression)

Comma operator is not allowed in constant expressions, regardless of whether it's on the top level or not 

// static int n = (1,2); // Error: constant expression cannot call the comma operator

Cast operator

See cast operator

Conditional operator

The conditional operator expression has the form

condition ? expression-true : expression-false

where

condition - an expression of scalar type
expression-true - the expression that will be evaluated if condition compares unequal to zero
expression-false - the expression that will be evaluated if condition compares equal to zero

Only the following expressions are allowed as expression-true and expression-false

  • two expressions of any arithmetic type
  • two expressions of the same struct or union type
  • two expressions of void type
  • two expressions of pointer type, pointing to types that are compatible, ignoring cvr-qualifiers
  • one expression is a pointer and the other is the null pointer constant (such as NULL)
  • one expression is a pointer to object and the other is a pointer to void (possibly qualified)
1) First, evaluates condition. There is a sequence point after this evaluation.
2) If the result of condition compares unequal to zero, executes expression-true, otherwise executes exception-false
3) Performs a conversion from the result of the evaluation to the common type, defined as follows:
1) if the expressions have arithmetic type, the common type is the type after usual arithmetic conversions
2) if the expressions have struct/union type, the common type is that struct/union type
3) if the expressions are both void, the entire conditional operator expression is a void expression
4) if one is a pointer and the other is a null pointer constant, the type is the type of that pointer
5) if both are pointers, the result is the pointer to the type that combines cvr-qualifiers of both pointed-to types (that is, if one is const int* and the other is volatile int*, the result is const volatile int*), and if the types were different, the pointed-to type is the composite type.
6) if one is a pointer to void, the result is a pointer to void with combined cvr-qualifiers
#define ICE(x) (sizeof(*(1? ((void*)((x) * 0l)): (int*)1)))

// if x is an Integer Constant Expression then macro expands to

 sizeof( 1 ? NULL : (int *) 1)  // (void *)((x)*0l)) -> NULL

// according to point (4) this further converts into

 sizeof(int)

// if x is not an Integer Constant Expression then macro expands to
// according to point (6)

 (sizeof(*(void *)(x))           // Error due incomplete type

Notes

The conditional operator is never an lvalue expression, although it may return objects of struct/union type. The only other expressions that may return stucts are assignment, comma, function call, and compound literal.

Note that in C++, it may be an lvalue expression.

See operator precedence for the details on the relative precedence of this operator and assignment.

Conditional operator has right-to-left associativity, which allows chaining 

struct vehicle v = arg == 'B' ? bus      :
                   arg == 'A' ? airplane :
                   arg == 'T' ? train    :
                   arg == 'C' ? car      :
                   arg == 'H' ? horse    :
                                feet;

sizeof operator

See sizeof operator

_Alignof operator

See _Alignof operator

References

  • C11 standard (ISO/IEC 9899:2011):
  • 6.5.2.2 Function calls (p: 81-82)
  • 6.5.3.4 The sizeof and _Alignof operators (p: 90-91)
  • 6.5.4 Cast operators (p: 91)
  • 6.5.15 Conditional operator (p: 100)
  • 6.5.17 Comma operator (p: 105)
  • C99 standard (ISO/IEC 9899:1999):
  • 6.5.2.2 Function calls (p: 71-72)
  • 6.5.3.4 The sizeof operator (p: 80-81)
  • 6.5.4 Cast operators (p: 81)
  • 6.5.15 Conditional operator (p: 90-91)
  • 6.5.17 Comma operator (p: 94)
  • C89/C90 standard (ISO/IEC 9899:1990):
  • 3.3.2.2 Function calls
  • 3.3.3.4 The sizeof operator
  • 3.3.4 Cast operators
  • 3.3.15 Conditional operator
  • 3.3.17 Comma operator

See also

Common operators
assignment increment
decrement 		arithmetic logical comparison member
access 		other

a = b
a += b
a -= b
a *= b
a /= b
a %= b
a &= b
a |= b
a ^= b
a <<= b
a >>= b

++a
--a
a++
a--

+a
-a
a + b
a - b
a * b
a / b
a % b
~a
a & b
a | b
a ^ b
a << b
a >> b

!a
a && b
a || b

a == b
a != b
a < b
a > b
a <= b
a >= b

a[b]
*a
&a
a->b
a.b

a(...)
a, b
(type) a
? :
sizeof
_Alignof (since C11)

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