6.6 Conditional Operator

The conditional operator (?:) takes three operands. It tests the result of the first operand and then evaluates one of the other two operands based on the result of the first. Consider the following example:

E1 ? E2 : E3

If expression E1 is nonzero (true), then E2 is evaluated, and that is the value of the conditional expression. If E1 is 0 (false), E3 is evaluated, and that is the value of the conditional expression. Conditional expressions associate from right to left. In the following example, the conditional operator is used to get the minimum of x and y:

a = (x < y) ? x : y; /* a = min(x, y) */

There is a sequence point after the first expression (E1). The following example's result is predictable, and is not subject to unplanned side effects:

i++ > j ? y[i] : x[i];

The conditional operator does not produce an lvalue. Therefore, a statement such as a ? x : y = 10 is not valid.

The following restrictions apply:

• The first operand must have a scalar type.
• One of the following must hold for the second and third operands:
  • Both operands have an arithmetic type (the usual arithmetic conversions are performed to bring the second and third operands to a common type). The result has that type.
  • Both operands have compatible structure or union types.
  • Both operands have a type of void.
  • Both operands are pointers to qualified or unqualified versions of compatible types. The result has the composite type.
  • One operand is a pointer and the other is a null pointer constant. The result has the type of the pointer that is not a null pointer constant.
  • One operand is a pointer to an object or incomplete type and the other is a pointer to a qualified or unqualified version of void. The result has the type pointer to void.

6.7 Assignment Operators

There are several assignment operators. Assignments result in the value of the target variable after the assignment. They can be used as subexpressions in larger expressions. Assignment operators do not produce lvalues.

Assignment expressions have two operands: a modifiable lvalue on the left and an expression on the right. A simple assignment consists of the equal sign (=) between two operands:

E1 = E2;

The value of expression E2 is assigned to E1. The type is the type of E1, and the result is the value of E1 after completion of the operation.

A compound assignment consists of two operands, one on either side of the equal sign (=), in combination with another binary operator. For example:

E1 += E2;

This is equivalent to the following simple assignment (except that in the compound assignment E1 is evaluated once, while in the simple assignment E1 is evaluated twice):

E1 = E1 + E2;

In the following example, the following assignments are equivalent:

a *= b + 1; a = a * (b + 1);

In another example, the following expression adds 100 to the contents of number[1]:

number[1] += 100;

The result of this expression is the result after the addition and has the same type as number[1].

If both assignment operands are arithmetic, the right operand is converted to the type of the left before the assignment (see Section 6.11.1).

The assignment operator (=) can be used to assign values to structures and unions. In the VAX C compatibility mode of HP C, one structure can be assigned to another as long as the structures are defined to be the same size, in bytes. In ANSI mode, the structure values must also have the same type. With all compound assignment operators, all right operands and all left operands must be either pointers or evaluate to arithmetic values. If the operator is -= or +=, the left operand can be a pointer, and the right operand (which must be integral) is converted in the same manner as the right operand in the binary plus (+) and minus ( - ) operations.

Do not reverse the characters that comprise a compound assignment operator, as in the following example:

E1 =+ E2;

This is an obsolete form that is no longer supported, but it will pass through the compiler undetected. (It is interpreted as an assignment operator followed by the unary plus operator).

6.8 Comma Operator

When two or more expressions are separated by the comma operator, they evaluate from left to right. The result has the type and value of the rightmost expression (although side effects of the other expressions, if any, do take place). The result is not an lvalue. In the following example, the value 1 is assigned to R, and the value 2 is assigned to T:

R = T = 1, T += 2, T -= 1;

Side effects for each expression are completed before the next expression is evaluated.

A comma expression must be enclosed with parentheses if it appears where commas have some other meaning, as in argument and initializing lists. Consider the following expression:

f(a, (t=3,t+2), c)

This example calls the function f with the arguments a, 5, and c. In addition, variable t is assigned the value 3.

6.9 Constant Expressions

A constant expression is an expression that contains only constants. A constant expression can be evaluated during compilation rather than at run time, and can be used in any place that a constant can occur. In the following example, limit+1 is a constant expression, and is evaluated at compile time:

#define limit 500 char x[limit+1]

A constant expression cannot contain assignment, increment, decrement, function-call, or comma operators, except when they are within the operand of a sizeof operator. Each constant expression must evaluate to a constant that is in the range of representable values for its type.

There are several contexts in which C requires an expression that must evaluate to a constant:

• The size of a bit field
• The value of an enumeration constant
• The size of an array (and the second and subsequent dimensions in all array declarations)
• The value of a case label
• An integral constant expression used in conditional-inclusion preprocessing directives
• The initializer list for an object with static storage duration

6.9.1 Integral Constant Expressions

An integral constant expression has an integral type and contains only operands that are integer constants, enumeration constants, character constants, sizeof expressions whose operand does not have variable-length array type or a parenthesized name of such a type, or floating constants that are the immediate operands of casts. Cast operands in an integral constant expression only convert arithmetic types to integral types, except as part of an operand to the sizeof operator.

C allows more latitude for constant expressions in initializers. Such a constant expression can evaluate to one of the following:

• An arithmetic constant expression
• A null pointer constant
• An address constant
• An address constant for an object type plus or minus an integral constant expression

6.9.2 Arithmetic Constant Expressions

An arithmetic constant expression has an arithmetic type and contains only operands that are integer constants, floating constants, enumeration constants, character constants, or sizeof expressions whose operand does not have variable-length array type or a parenthesized name of such a type. Cast operators in an arithmetic constant expression only convert arithmetic types to arithmetic types, except as part of an operand to the sizeof operator.

6.9.3 Address Constants

An address constant is a pointer to an lvalue designating an object of static storage duration (see Section 2.10), or to a function designator. Address constants must be created explicitly by using the unary & operator, or implicitly by using an expression of array or function type. The array subscript [] and member access operators . and ->, the address & and indirection * unary operators, and pointer casts can be used to create an address constant, but the value of an object cannot be accessed by use of these operators.

6.10 Compound Literal Expressions

A compound literal, also called a constructor expression, is a form of expression that constructs the value of an object, including objects of array, struct, or union type.

In the C89 Standard, passing a struct value to a function typically involves declaring a named object of the type, initializing its members, and passing that object to the function. With the C99 Standard, this can now be done with a single compound literal expression. (Note that compound literal expressions are not supported in the common C, VAX C, and Strict ANSI89 modes of the HP C compiler.)

A compound literal is an unnamed object specified by a syntax consisting of a parenthesized type name (the same syntax as a cast operator¹) followed by a brace-enclosed list of initializers. The value of this unnamed object is given by the initializer list. The initializer list can use the designator syntax.

For example, to construct an array of 1000 ints that are all zero except for array element 9, which is to have a value of 5, you can write the following:

(int [1000]){[9] = 5}.

A compound literal object is an lvalue. The object it designates has static storage duration if it occurs outside all function definitions. Otherwise, it has automatic storage duration associated with the nearest enclosing block.

The following examples illustrate the use of compound literals.

Examples

int *p = (int []){2, 4}; 
      

This example initializes p to point to the first element of an array of two ints, the first having the value 2 and the second having the value 4. The expressions in this compound literal are required to be constant. The unnamed object has static storage duration.

void f(void) 
{ 
        int *p; 
        /*...*/ 
        p = (int [2]){*p}; 
        /*...*/ 
} 
      

In this example, p is assigned the address of the first element of an array of two ints, the first having the value previously pointed to by p and the second having the value zero. The expressions in this compound literal need not be constant. The unnamed object has automatic storage duration.

drawline((struct point){.x=1, .y=1}, 
        (struct point){.x=3, .y=4}); 
 
Or, if drawline instead expected pointers to struct point: 
 
drawline(&(struct point){.x=1, .y=1}, 
        &(struct point){.x=3, .y=4}); 
      

Initializers with designations can be combined with compound literals. Structure objects created using compound literals can be passed to functions without depending on member order.

(const float []){1e0, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6} 
      

A read-only compound literal can be specified through constructions like the one in this example.

"/tmp/testfile" 
(char []){"/tmp/testfile"} 
(const char []){"/tmp/testfile"} 
      

The three expressions in this example have different meanings:

The first always has static storage duration and has type "array of char", but need not be modifiable.

The last two have automatic storage duration when they occur within the body of a function, and the first of these two is modifiable.

(const char []){"abc"} == "abc" 
      

Like string literals, const-qualified compound literals can be placed into read-only memory and can even be shared. This example might yield 1 if the literal's storage is shared.

struct int_list { int car; struct int_list *cdr; }; 
struct int_list endless_zeros = {0, &endless_zeros}; 
eval(endless_zeros); 
      

Because compound literals are unnamed, a single compound literal cannot specify a circularly linked object. In this example, there is no way to write a self-referential compound literal that could be used as the function argument in place of the named object endless_zeros.

struct s { int i; }; 
 
int f (void) 
{ 
        struct s *p = 0, *q; 
        int j = 0; 
 
        while (j < 2) 
                 q = p, p = &((struct s){ j++ }); 
        return p == q && q->i == 1; 
} 
      

As shown in this example, each compound literal creates only a single * object in a given scope.

The function f() always returns the value 1.

6.11 Data-Type Conversions

C performs data-type conversions in the following four situations:

• When two or more operands of different types appear in an expression.
• When arguments of type char, short, and float are passed to a function using the old style declaration.
• When arguments that do not conform exactly to the parameters declared in a function prototype are passed to a function.
• When the data type of an operand is deliberately converted by the cast operator. See Section 6.4.6 for more information on the cast operator.

The following sections describe how operands and function arguments are converted.

6.11.1 Usual Arithmetic Conversions

The following rules---referred to as the usual arithmetic conversions---govern the conversion of all operands in arithmetic expressions. The effect is to bring operands to a common type, which is also the type of the result. The rules govern in the following order:

1. If either operand is not of arithmetic type, no conversion is performed.
2. If either operand has type long double, the other operand is converted to long double.
3. Otherwise, if either operand has type double, the other operand is converted to double.
4. Otherwise, if either operand has type float, the other operand is converted to float.
5. For complex numbers ( (ALPHA, I64)), the above conversions take place for the corresponding real type of the operand to be converted.
   For example, adding a double _Complex and a float, converts just the float operand to double, yielding a double _Complex result.
6. Otherwise, the integer promotions are performed on both operands, and the following rules apply:
   1. If both operands have the same type, then no further conversion is needed.
   2. Otherwise, if both operands have signed integer types or both have unsigned integer types, the operand with the type of lesser integer conversion rank is converted to the type of the operand with greater rank.
   3. Otherwise, If either operand has type unsigned long long int, the other operand is converted to unsigned long long int.
   4. If either operand has type unsigned long int, the other operand is converted to unsigned long int.
   5. Otherwise, if one operand has type long int and the other has type unsigned int, and if a long int can represent all values of an unsigned int, the operand of type unsigned int is converted to long int. If a long int cannot represent all the values of an unsigned int, both operands are converted to unsigned long int.
   6. Otherwise, if either operand has type long int, the other operand is converted to long int.
   7. Otherwise, if either operand has type unsigned int, the other operand is converted to unsigned int.
   8. Otherwise, both operands have type int.

The following sections elaborate on the usual arithmetic conversion rules.

6.11.1.1 Characters and Integers

A char, short int, or int bit field, either signed or unsigned, or an object that has enumeration type, can be used in an expression wherever an int or unsigned int is permitted. If an int can represent all values of the original type, the value is converted to an int. Otherwise, it is converted to an unsigned int. These conversion rules are called the integer promotions.

This implementation of integer promotion is called value preserving, as opposed to unsigned preserving in which unsigned char and unsigned short widen to unsigned int. HP C uses value-preserving promotions, as required by the ANSI C standard, unless the common C mode is specified.

To help locate arithmetic conversions that depend on unsigned preserving rules, HP C, with the check option enabled, flags any integer promotions of unsigned char and unsigned short to int that could be affected by the value-preserving approach for integer promotions.

All other arithmetic types are unchanged by the integer promotions.

In HP C, variables of type char are bytes treated as signed integers. When a longer integer is converted to a shorter integer or to char, it is truncated on the left; excess bits are discarded. For example:

int i; char c; i = 0xFFFFFF41; c = i;

This code assigns hex 41 ('A') to c. The compiler converts shorter signed integers to longer ones by sign extension.