{"id":1383,"date":"2014-03-27T07:00:00","date_gmt":"2014-03-27T07:00:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/2014\/03\/27\/functions-that-return-gdi-regions-rarely-actually-return-regions\/"},"modified":"2014-03-27T07:00:00","modified_gmt":"2014-03-27T07:00:00","slug":"functions-that-return-gdi-regions-rarely-actually-return-regions","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20140327-00\/?p=1383","title":{"rendered":"Functions that return GDI regions rarely actually return regions"},"content":{"rendered":"

\nFor reasons I don’t quite understand,\nthe only functions in GDI and USER which create GDI regions\nare the functions with Create<\/code> in their name,\nlike\nCreate­Rect­Rgn<\/code> or\nCreate­Polygon­Rgn<\/code>.\nAll the other functions which return a region require you\nto pass an existing region to use as the output.\n<\/p>\n

\nI can see this being useful for\nCombine­Rgn<\/code>,\nbecause you can set your output region to be equal\nto one of the input regions in order to update a region\nin place.\n<\/p>\n

\n\/\/ hrgnTotal |= hrgnSegment\nCombineRgn(hrgnTotal, hrgnTotal, hrgnSegment, RGN_OR);\n\/\/ hrgnTotal &= hrgnClip\nCombineRgn(hrgnTotal, hrgnTotal, hrgnClip, RGN_AND);\n<\/pre>\n
\nBut for all of the Get<\/code> functions,\nhaving to create an output region is usually just\nan annoyance.\n<\/p>\n
\n\/\/ Create a dummy region - contents not important\nHRGN hrgnClip = CreateRectRgn(0, 0, 0, 0);\n\/\/ Ask for the clipping region to be copied to the dummy region\nint status = GetClipRgn(hdc, hrgnClip);\n<\/pre>\n
\nI guess it lets you reuse a single dummy region over and over\nagain, but in practice, you’re just going to destroy the region\nwhen you’re done to free up the GDI region memory.\n<\/p>\n
\nWhatever the historical reason for this, we’re stuck with it.\nIt may be an ugly pattern, but at least it’s a pattern.\n<\/p>\n
\nThe things I do for you<\/b>:\nI asked a colleague who worked on Windows 3.0\nif he knew the reason for this design pattern.\nHe didn’t know but suggested that I ask another person\nwho retired from Microsoft many, many years ago.\nFortunately, I happened to have his email address even though we\naren’t really that close.\nAnd the second person also didn’t know.\n“This behavior was already in place when I joined the Windows 1.03 project.\nMaybe you can ask\n\nRao<\/a>.”\nUnfortunately, I don’t have Rao’s email address,\nso the trail ran cold.\nBut I burned a favor for you guys.<\/p>\n","protected":false},"excerpt":{"rendered":"
For reasons I don’t quite understand, the only functions in GDI and USER which create GDI regions are the functions with Create in their name, like Create­Rect­Rgn or Create­Polygon­Rgn. All the other functions which return a region require you to pass an existing region to use as the output. I can see this being useful […]<\/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-1383","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
For reasons I don’t quite understand, the only functions in GDI and USER which create GDI regions are the functions with Create in their name, like Create­Rect­Rgn or Create­Polygon­Rgn. All the other functions which return a region require you to pass an existing region to use as the output. I can see this being useful […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/1383","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=1383"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/1383\/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=1383"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=1383"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=1383"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}