{"id":106836,"date":"2022-07-06T07:00:00","date_gmt":"2022-07-06T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=106836"},"modified":"2022-07-06T07:02:08","modified_gmt":"2022-07-06T14:02:08","slug":"20220706-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20220706-00\/?p=106836","title":{"rendered":"The empty Windows Runtime string is not just a pretty face"},"content":{"rendered":"

As I noted some time ago<\/a>, the empty Windows Runtime string is represented by a null pointer. This has natural but perhaps surprising consequences: Even though it is a null pointer, the empty Windows Runtime string is a real string, with hopes and dreams. Or at least a length and data.<\/p>\n

At the ABI level, Windows\u00adGet\u00adString\u00adLen<\/code> reports that a null pointer string has a length of zero, and Windows\u00adGet\u00adString\u00adRaw\u00adBuffer<\/code> gives you a buffer that consists of a single null terminator.<\/p>\n

Since an empty string and a null pointer are indistinguishable at the ABI layer, if you operate at the ABI layer (using raw HSTRING<\/code>s) or at a thin projection layer (such as C++\/CX and C++\/WinRT), you can take advantage of this equivalence.<\/p>\n

For starters, you don’t need to check for a null pointer before trying to use the string, because a null pointer is a perfectly valid HSTRING<\/code>.<\/p>\n\n\n\n\n\n
ABI<\/td>\nif (s != nullptr &&<\/span>
\n\u00a0\u00a0\u00a0\u00a0WindowsGetStringLen(s) == 1)<\/code><\/td>\n		if (s != nullptr &&<\/span>
\n\u00a0\u00a0\u00a0\u00a0s == HStringReference(L\"hi\").Get())<\/code><\/td>\n<\/tr>\n
C++\/CX<\/td>\nif (s != nullptr &&<\/span>
\n\u00a0\u00a0\u00a0\u00a0s->Length() == 1)<\/code><\/td>\n		if (s != nullptr &&<\/span>
\n\u00a0\u00a0\u00a0\u00a0s == L\"hi\")<\/code><\/td>\n<\/tr>\n
C++\/WinRT<\/td>\nif (s != hstring{} &&<\/span>
\n\u00a0\u00a0\u00a0\u00a0s.size() == 1)<\/code><\/td>\n		if (s != hstring{} &&<\/span>
\n\u00a0\u00a0\u00a0\u00a0s == L\"hi\"sv)<\/code><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

If you are checking for a nonempty string, you can just check for null. C++\/WinRT and C++\/CX even have special methods that tell you directly.<\/p>\n\n\n\n\n\n\n
 <\/td>\nSlower way<\/th>\nQuicker way<\/th>\n<\/tr>\n
ABI<\/td>\nif (WindowsGetStringLen(s) != 0)<\/code><\/td>\nif (s != nullptr)<\/code><\/td>\n<\/tr>\n
C++\/CX<\/td>\nif (s->Length() != 0)<\/code><\/td>\nif (!s->IsEmpty())<\/code><\/td>\n<\/tr>\n
C++\/WinRT<\/td>\nif (s.size() != 0)<\/code><\/td>\nif (!s.empty())<\/code><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Related<\/b>: The C++\/CX String^<\/code> is not an object, even though it wears a hat<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"

It really is an empty string.<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25],"class_list":["post-106836","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"

It really is an empty string.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/106836","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=106836"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/106836\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=106836"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=106836"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=106836"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}