{"id":183,"date":"2008-12-16T19:29:59","date_gmt":"2008-12-17T00:29:59","guid":{"rendered":"http:\/\/vgable.com\/blog\/2008\/12\/16\/isempty\/"},"modified":"2009-04-22T18:47:46","modified_gmt":"2009-04-22T23:47:46","slug":"isempty","status":"publish","type":"post","link":"https:\/\/vgable.com\/blog\/2008\/12\/16\/isempty\/","title":{"rendered":"isEmpty?"},"content":{"rendered":"

Checking if a Cocoa object is empty is a little harder then in other languages, say C++, (but easier in some ways). Because every object in Objective-C is actually a pointer<\/em> to an object, there are two ways, obj<\/code>, can be empty. <\/p>\n

obj = {}<\/code><\/strong><\/p>\n

\nobj<\/code> points to an object that is empty. Say an array with 0 items, or the string \"\"<\/code>, etc..<\/p>\n

obj = nil<\/code><\/strong><\/p>\n

\nobj<\/code>, the pointer obj<\/code>, is NULL<\/code>, nil<\/code>, 0<\/code>, or whatever you want to call it. You might argue that obj<\/code> isn’t really an object, but it is<\/em> empty, because there’s nothing in it.<\/p>\n

Bug:<\/h3>\n

\nWhen I first started writing Objective-C, I made the mistake of writing code like: if([name isEqualToString:@\"\"]){ ... }<\/code>, to test for empty strings. And this code would work for a while until I used it in a situation where name<\/code> was nil<\/code>, and then, because sending any method called on nil<\/code> “returns” NO<\/code><\/a>, I would have mysterious errors. (Worse then a crash<\/a>, because it’s harder to track down.)<\/p>\n

Bug:<\/H3> <\/p>\n

\nIt’s tempting to avoid the previous bug, by explicitly testing for nil<\/code> and<\/em> {}<\/code>. Say with code like:<\/p>\n

if (email == nil || ![email isEqualTo:@\"\"] )\n   email = @\"An email address is required\";<\/pre>\n
But generally this is a bad idea.  It means more code, which means more places for a bug.  I know it’s only one trivial test, but I’m serious, when I say it’s asking for a bug — like the bug in the example above, which sets  email <\/code> to @\"An email address is required\"<\/code>, whenever it is not<\/em> the empty string, rather then when it is<\/em> empty.  (Values have been changed tho protect the innocent but it’s a bug I’ve seen.)<\/p>\n
Solutions:<\/H3><\/p>\n
Wil Shipley suggests using the global function:<\/a>
\n
\nstatic inline BOOL IsEmpty(id thing) {
\n    return thing == nil
\n        || ([thing respondsToSelector:@selector(length)]
\n        && [(NSData *)thing length] == 0)
\n        || ([thing respondsToSelector:@selector(count)]
\n        && [(NSArray *)thing count] == 0);
\n}
\n<\/code><\/p>\n
I’ve been using his IsEmpty()<\/code> for about a year. I’ve had zero problems with it, while it’s made my code more readable and concise.<\/p>\n
Another solution is to take advantage of what happens when you send a message to nil<\/code><\/a>.  (To over-simplify, you get back 0 or NO<\/code>.)  So you can just say “if ([obj count] == 0)<\/code> then obj<\/code> is empty.”  This often means reversing your thinking, and testing “IsNotEmpty()” instead of “IsEmpty()”.  I don’t think it’s as clear is IsEmpty()<\/code> in general, but in cases where it is, there you have it.<\/p>\n","protected":false},"excerpt":{"rendered":"
Checking if a Cocoa object is empty is a little harder then in other languages, say C++, (but easier in some ways). Because every object in Objective-C is actually a pointer to an object, there are two ways, obj, can be empty. obj = {} obj points to an object that is empty. Say an […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[18,7,6,5,4,13,8],"tags":[110,395,106,107,256,108,109,78,105],"class_list":["post-183","post","type-post","status-publish","format-standard","hentry","category-bug-bite","category-c","category-cocoa","category-objective-c","category-programming","category-sample-code","category-usability","tag-containers","tag-isequal","tag-nil","tag-nsarray","tag-nsdata","tag-nsdictionary","tag-nsset","tag-nsstring","tag-null"],"_links":{"self":[{"href":"https:\/\/vgable.com\/blog\/wp-json\/wp\/v2\/posts\/183","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/vgable.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/vgable.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/vgable.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/vgable.com\/blog\/wp-json\/wp\/v2\/comments?post=183"}],"version-history":[{"count":0,"href":"https:\/\/vgable.com\/blog\/wp-json\/wp\/v2\/posts\/183\/revisions"}],"wp:attachment":[{"href":"https:\/\/vgable.com\/blog\/wp-json\/wp\/v2\/media?parent=183"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/vgable.com\/blog\/wp-json\/wp\/v2\/categories?post=183"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/vgable.com\/blog\/wp-json\/wp\/v2\/tags?post=183"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}