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<HTML><HEAD><TITLE>Expressions</TITLE></HEAD><BODY><H1><A NAME="Expressions">Expressions</A></H1><HR><P>You write expressions to determine values, to alter values storedin objects, and to call functions that perform input and output. Infact, you express all computations in the program by writing expressions.The translator must evaluate some of the expressions you writeto determine properties of the program. The translator or the targetenvironment must evaluate other expressions prior to program startupto determine the initial values stored in objects with static duration.The program evaluates the remaining expressions when it executes.</P><P>This document describes briefly just those aspectof expressions most relevant to the use of the Standard C library:</P><P>An<B><A NAME="address constant expression">address constant expression</A></B>specifies a value that has a pointer typeand that the translator or target environment can determineprior to program startup.</P><P>A <B><A NAME="constant expression">constant expression</A></B>specifies a value that the translator or target environment can determineprior to program startup.</P><P>An<B><A NAME="integer constant expression">integer constant expression</A></B>specifies a value that has an integer typeand that the translator can determine at the point inthe program where you write the expression.(You cannot write a function call, assigning operator,or <I>comma</I> operator except as part of the operand of a<I><A HREF="#sizeof operator">sizeof</A></I> operator.)In addition, you must write only subexpressionsthat have integer type. You can, however,write a floating-point constant expression as the operand of an integer<I>type cast</I> operator.</P><P>A<B><A NAME="floating-point constant expression">floating-point constant expression</A></B>specifies a value that has a floating-point typeand that the translator can determine at the point inthe program where you write the expression.(You cannot write a function call, assigning operator,or <I>comma</I> operator except as part of the operand of a<I><A HREF="#sizeof operator">sizeof</A></I> operator.)In addition, you must write only subexpressionsthat have integer or floating-point type.</P><P>An <B><A NAME="lvalue expression">lvalue expression</A></B>An lvalue expression designates an object that hasan object type other than an array type. Hence, you can access thevalue stored in the object.A <I>modifiable</I> lvalue expression designates an object that hasan object type other than an array type or a <I>const</I> type. Hence,you can alter the value stored in the object.You can also designate objects with an lvalue expressionthat has an array type or an incomplete type,but you can only take the address of such an expression.</P><P><B><A NAME="Promoting">Promoting</A></B> occurs for anexpression whose integer type is not one of the ``computational'' types.Except when it is the operand of the<I><A HREF="#sizeof operator">sizeof</A></I> operator,an integer<A HREF="#rvalue expression">rvalue expression</A>has one of four types:<I>int, unsigned int, long,</I> or <I>unsigned long.</I>When you write an expression in an rvalue context and the expressionhas an integer type that is not one of these types, the translator<I>promotes</I> its type to one of these.If all of the values representable in theoriginal type are also representable as type <I>int,</I> then thepromoted type is <I>int.</I> Otherwise, the promoted type is<I>unsigned int.</I> Thus, for <I>signed char, short,</I>and any <I>signed bitfield</I> type, the promoted type is <I>int.</I>For each of the remaining integer types (<I>char, unsigned char,unsigned short,</I> any plain <I>bitfield</I> type,or any <I>unsigned bitfield</I> type), the effect of these rulesis to favor promoting to <I>int</I> wherever possible, but to promoteto <I>unsigned int</I> if necessary to preserve the original valuein all possible cases.</P><P>An <B><A NAME="rvalue expression">rvalue expression</A></B>is an expression whose value can be determined only when the program executes.The term also applies to expressions which <I>need not</I>be determined until program execution.</P><P>You use the <B><A NAME="sizeof operator">sizeof</A></B> operator,as in the expression <CODE>sizeof X</CODE>to determine the size in bytes of an object whose typeis the type of <CODE>X</CODE>. The translator uses the expressionyou write for <CODE>X</CODE> only to determine a type;it is not evaluated.</P><P>A <B><A NAME="void expression">void expression</A></B>has type <I>void.</I></P><HR><P>See also the<B><A HREF="index.html#Table of Contents">Table of Contents</A></B> and the<B><A HREF="_index.html">Index</A></B>.</P><P><I><A HREF="crit_pb.html">Copyright</A> © 1989-2002by P.J. Plauger and Jim Brodie. All rights reserved.</I></P><!--V4.01:1125--></BODY></HTML>⌨️ 快捷键说明
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