\nA customer encountered the following problem:\n<\/p>\n
\nclass Shape\n{\npublic:\n virtual bool Is2D() { return false; }\n};\nclass Shape2D : public Shape\n{\npublic:\n virtual bool Is2D() { return true; }\n};\nShape *FindShape(Cookie cookie);\nvoid BuyPaint(Cookie cookie)\n{\n Shape2D *shape = static_cast<Shape2D *>(FindShape(cookie));\n if (shape->Is2D()) {\n .. do all sorts of stuff ...\n }\n}\n<\/pre>\n\nThe BuyPaint<\/code> function converts the cookie back\nto a Shape<\/code> object, and then checks if the object\nis a Shape2D<\/code> object by calling Is2D<\/code>.\nIf so, then it does some more stuff to figure out what type of paint\nto buy.\n<\/p>\n\n(Note to nitpickers:\nThe actual scenario was not like this, but I presented it this way\nto illustrate the point.\nIf you say “You should’ve\nused RTTI” or “You should’ve had a BuyPaint method on the Shape class”,\nthen you’re missing the point.)\n<\/p>\n
\nThe programmers figured they’d save some typing by casting the\nresult of FindShape<\/code> to a Shape2D<\/code>\nright away,\nbecause after all, since Is2D<\/code> is a virtual\nmethod, it will call the right version of the function,\neither Shape::Is2D<\/code>\nor Shape2D::Is2D<\/code>,\ndepending on the actual type of the underlying object.\n<\/p>\n\nBut when compiler optimizations were turned on, they discovered\nthat the call to Is2D<\/code> was optimized away,\nand the BuyPaint<\/code> function merely assumed\nthat it was always operating on a Shape2D<\/code> object.\nIt then ended up trying to buy paint even for one-dimensional objects\nlike points and lines.\n<\/p>\n\nCompiler optimization bug?\nNope.\nCode bug due to reliance on undefined behavior.\n<\/p>\n
\nThe C++ language says (9.3.1)\n“If a nonstatic member function of a class X<\/code>\nis called for an object that is not of type X<\/code>,\nor of a type derived from X<\/code>, the behavior is undefined.”\nIn other words,\nif you are invoking a method on an object of type X<\/code>,\nthen you are promising that it really is of type X<\/code>,\nor a class derived from it.\n<\/p>\n\nThe code above violates this rule,\nbecause it is invoking the Is2D<\/code> method\non a Shape2D*<\/code>,\nwhich therefore comes with the promise\n“This really is a Shape2D<\/code> object\n(or something derived from it).”\nBut this is a promise the code cannot deliver on, because the\nobject returned by FindShape<\/code> might be a simple\nShape<\/code>.\n<\/p>\n\nThe compiler ran with the (false) promise and said,\n“Well, since you are guaranteeing that the object is at least\na Shape2D<\/code>,\nand since I have studied your code and determined that no\nclasses which further derive from Shape2D<\/code> override\nthe Is2D<\/code> method,\nI have therefore proved that the final overrider<\/i> is\nShape2D::Is2D<\/code> and can therefore\ninline that method.”\n<\/p>\n\nResult: The compiler inlines Shape2D::Is2D<\/code>,\nwhich returns true<\/code>, so the “if” test can be optimized out.\nThe compiler can assume that BuyPaint<\/code> is always\ncalled with cookies that represent two-dimensional objects.\n<\/p>\n\nThe fix is to do the annoying typing that the original authors\nwere trying to avoid:\n<\/p>\n
\nvoid BuyPaint(Cookie cookie)\n{\n Shape *shape = FindShape(cookie);\n if (shape->Is2D()) {\n Shape2D *shape2d = static_cast<Shape2D *>(shape);\n .. do all sorts of stuff (with shape2d) ...\n }\n}\n<\/pre>\n","protected":false},"excerpt":{"rendered":"A customer encountered the following problem: class Shape { public: virtual bool Is2D() { return false; } }; class Shape2D : public Shape { public: virtual bool Is2D() { return true; } }; Shape *FindShape(Cookie cookie); void BuyPaint(Cookie cookie) { Shape2D *shape = static_cast<Shape2D *>(FindShape(cookie)); if (shape->Is2D()) { .. do all sorts of stuff … […]<\/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-6713","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
A customer encountered the following problem: class Shape { public: virtual bool Is2D() { return false; } }; class Shape2D : public Shape { public: virtual bool Is2D() { return true; } }; Shape *FindShape(Cookie cookie); void BuyPaint(Cookie cookie) { Shape2D *shape = static_cast<Shape2D *>(FindShape(cookie)); if (shape->Is2D()) { .. do all sorts of stuff … […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/6713","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=6713"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/6713\/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=6713"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=6713"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=6713"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}