# Expressions

<head><title>Expressions</title></head>

# 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](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_express.html#sizeof%20operator)* operator.)
In addition, you must write only subexpressions
that have integer type. You can, however,
write a floating-point constant expression as the operand of an integer
*type cast* operator.

A
**floating-point constant expression**
specifies a value that has a floating-point 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](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_express.html#sizeof%20operator)* operator.)
In addition, you must write only subexpressions
that have integer or floating-point type.

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](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_express.html#sizeof%20operator)* operator,
an integer
[rvalue expression](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_express.html#rvalue%20expression)
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.*

* * *

See also the
**[Table of Contents](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_index.html#Table%20of%20Contents)** and the
**[Index](https://docs.qualcomm.com/doc/80-N3040-23/topic/_cpp_index.html)**.

*[Copyright](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_crit_pjp.html) © 1992-2006by P.J. Plauger and Jim Brodie. All rights reserved.*

<!--V5.01:1566-->

Last Published: Oct 06, 2026

[Previous Topic
Functions](https://docs.qualcomm.com/bundle/publicresource/80-N3040-23/topics/cpp_function.md) [Next Topic
Preprocessing](https://docs.qualcomm.com/bundle/publicresource/80-N3040-23/topics/cpp_preproc.md)