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<html><!-- Mirrored from c-faq.com/aryptr/ptrary2.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 08:02:57 GMT --><head><title>declaring pointers to arrays, etc.</title></head><body><p>[Someone questioned the statement,``If you really need to declare a pointer to an entire array,use something like int (*ap)[N];''.This was my response.]<p>From: scs@eskimo.com (Steve Summit)<br>Subject: Re: question about answer to 6.13<br>Date: Mon, 25 Mar 2002 23:17:44 -0500<br>Message-Id: <2002Mar25.2317.scs.001@aeroroot.scs.ndip.eskimo.net><p>You wrote:<br>> i was wondering about something stated in the answer<br>> to question 6.13 of the c-faq:<br>><br>> ``If you really need to declare a pointer to an entire<br>> array, use something like <TT>int (*ap)[N];</TT> where <TT>N</TT> is<br>> the size of the array.''<br>><br>> doesn't <TT>(int (*ap)[n])</TT> mean ``array of pointers to <TT>int</TT>''?<br><p>Nope!<p>> if not, how why and how would you declare an<br>> array of pointers to int, then.<br><p>That'd be simply<p><pre> int *ap[N];</pre>The difference is in the parentheses.In C, there are precedence relationships in declarations, too,and when the default precedence isn't what you want, you useexplicit parentheses to override it.<p>To see very clearly how this works, let's first look at twoexamples from the world of expressions, one simple, one a bitmore complicated.<p>When you write<p><pre> x = 1 + 2 * 3;</pre>how is that interpreted? Multiplication has higher precedencethan addition, so 2 is multiplied by 3 and the result added to 1.If you wanted 1 to be added to 2 and then multiplied by 3,you'd use parentheses:<p><pre> x = (1 + 2) * 3;</pre>When you write<p><pre> int a[] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10}; int *ip = &a[5]; x = *ip++;</pre><p>how is that interpreted? Does <TT>*ip++</TT> mean to follow the pointer(that is, take the pointed-to contents of) <TT>ip</TT> and increment whatit points to, or increment the pointer <TT>ip</TT> and (because this isthe postincrement form) take the contents of what it used topoint to? Postfix <TT>++</TT> has higher precedence than unary prefix <TT>*</TT>,so the interpretation is that the pointer is incremented -- <TT>x</TT>gets set to 5, and <TT>ip</TT> ends up pointing to <TT>a[6]</TT>. If you wantedto increment the pointed-to location, you'd again use explicitparentheses to override the default precedence: saying<p><pre> x = (*ip)++;</pre>sets <TT>x</TT> to 5, and leaves <TT>ip</TT> pointing at <TT>a[5]</TT>, and increments <TT>a[5]</TT>to contain 6.<p>(This is probably not the best example, because you may have ahard time seeing that <TT>x</TT> gets set to 5 in both cases, not 6.Trust me, it does. Postfix <TT>++</TT> means that the ``old'' value of thething being incremented always gets used, whether the thing beingincremented is the pointer <TT>ip</TT> or the pointed-to array element <TT>a[5]</TT>.)<p>Finally, let's look at declarations. If you write<p><pre> int *ap[10];</pre>are you declaring an array of pointers, or a pointer to an array?In declarations, the brackets <TT>[]</TT> which describe arrays havehigher precedence than the <TT>*</TT> which describes pointers. It lookslike the <TT>*</TT> and the <TT>[]</TT> are both next to the identifier <TT>ap</TT>, butsince <TT>[]</TT> has higher precedence it means that the brackets are``closer'' -- <TT>ap</TT> is an array first, and what it's an array of ispointers. If you really wanted a pointer to an array (thoughusually you do not) you once again override the defaultprecedence using explicit parentheses:<p><pre> int (*pa)[10];</pre>says that <TT>pa</TT> is a pointer first, and what it's a pointer to is anarray.<p>Fortunately (and this is no accident) pointers to arrays aremuch, much rarer than arrays of pointers, so the natural syntax(without the parentheses) is almost always what you want.(Outside of examples for the FAQ list, I'm not sure I've useda pointer to an array in all my years of C programming.)</body><!-- Mirrored from c-faq.com/aryptr/ptrary2.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 08:02:57 GMT --></html>⌨️ 快捷键说明
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