Expressions

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Chapter 4. Expressions

You write expressions to determine values, to alter values stored in objects, and to

call functions that perform input and output. In fact, you express all computations

in the program by writing expressions. The translator must evaluate some of the

expressions you write to determine properties of the program. The translator or the

target environment must evaluate other expressions prior to program startup to

determine the initial values stored in objects with static duration. The program

evaluates the remaining expressions when it executes.

This document describes briefly just those aspect of expressions most relevant to

the use of the Standard C library:

An address constant expression specifies a value that has a pointer type and that

the translator or target environment can determine prior to program startup.

A constant expression specifies a value that the translator or target environment

can determine prior to program startup.

An integer constant expression specifies a value that has an integer type and that

the translator can determine at the point in the program where you write the

expression. (You cannot write a function call, assigning operator, or comma operator

except as part of the operand of a sizeof (page 16) operator.) In addition, you must

write only subexpressions that have integer type. You can, however, write a

floating-point constant as the operand of an integer type cast operator.

An lvalue expression An lvalue expression designates an object that has an object

type other than an array type. Hence, you can access the value stored in the object.

A modifiable lvalue expression designates an object that has an object type other

than an array type or a const type. Hence, you can alter the value stored in the

object. You can also designate objects with an lvalue expression that has an array

type or an incomplete type, but you can only take the address of such an

expression.

Promoting occurs for an expression whose integer type is not one of the

``computational'' types. Except when it is the operand of the sizeof operator, an

integer rvalue expression has one of four types: int, unsigned int, long, or unsigned

long. When you write an expression in an rvalue context and the expression has an

integer type that is not one of these types, the translator promotes its type to one of

these. If all of the values representable in the original type are also representable as

type int, then the promoted type is int. Otherwise, the promoted type is unsigned

int. Thus, for signed char, short, and any signed bitfield type, the promoted type is int.

For each of the remaining integer types (char, unsigned char, unsigned short, any

plain bitfield type, or any unsigned bitfield type), the effect of these rules is to favor

promoting to int wherever possible, but to promote to unsigned int if necessary to

preserve the original value in all possible cases.

An rvalue expression is an expression whose value can be determined only when

the program executes. The term also applies to expressions which need not be

determined until program execution.

You use the sizeof operator, as in the expression sizeof X to determine the size in

bytes of an object whose type is the type of X. The translator uses the expression

you write for X only to determine a type; it is not evaluated.

A void expression has type void.

Standard C++ Library

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