{"id":995,"date":"2017-11-08T21:19:08","date_gmt":"2017-11-08T21:19:08","guid":{"rendered":"http:\/\/danielschlegel.org\/wp\/?page_id=995"},"modified":"2017-11-14T21:56:45","modified_gmt":"2017-11-14T21:56:45","slug":"diamond-problem","status":"publish","type":"page","link":"https:\/\/danielschlegel.org\/wp\/teaching\/diamond-problem\/","title":{"rendered":"Diamond Problem"},"content":{"rendered":"

The “Deadly Diamond of Death”<\/h2>\n

Consider a set of classes with the following inheritance hierarchy. <\/p>\n

 \r\n   A\r\n \/   \\\r\nB     C\r\n \\   \/\r\n   D\r\n<\/pre>\n
diamond.cpp<\/h3>\n
\r\n#include\r\nusing namespace std;\r\n\r\nclass A {\r\n\tint value;\r\n\r\npublic:\r\n\tA() {\r\n\t\tvalue = 5;\r\n\t\tcout << \"A's constructor called\" << endl;\r\n\t}\r\n\tvoid printValue(){\r\n\t\tcout << value << endl;\r\n\t}\r\n};\r\n\r\nclass B: public A {\r\npublic:\r\n\tB() {\r\n\t\tcout << \"B's constructor called\" << endl;\r\n\t}\r\n};\r\n\r\nclass C: public A {\r\npublic:\r\n\tC() {\r\n\t\tcout << \"C's constructor called\" << endl;\r\n\t}\r\n};\r\n\r\nclass D: public B, public C {\r\npublic:\r\n\tD() {\r\n\t\tcout << \"D's constructor called\" << endl;\r\n\t}\r\n};\r\n\r\nint main() {\r\n\tD d;\r\n\treturn 0;\r\n}\r\n<\/pre>\n
Outputs: <\/p>\n
\r\nA's constructor called\r\nB's constructor called\r\nA's constructor called\r\nC's constructor called\r\nD's constructor called\r\n<\/pre>\n
Our class hierarchy ended up looking more like this: <\/p>\n
\r\nA     A\r\n|     |\r\nB     C\r\n \\   \/\r\n   D\r\n<\/pre>\n
This is because C++ by default uses replicated inheritance<\/em>. <\/p>\n
Let's investigate further. Now add printValue() to D's main: <\/p>\n
\r\nint main() {\r\n\tD d;\r\n\td.printValue();\r\n\treturn 0;\r\n}\r\n<\/pre>\n
Output:<\/p>\n
\r\n..\/diamond.cpp:47:4: error: non-static member 'printValue' found in multiple base-class subobjects of type 'A':\r\n    class D -> class B -> class A\r\n    class D -> class C -> class A\r\n        d.printValue();\r\n          ^\r\n..\/diamond.cpp:19:7: note: member found by ambiguous name lookup\r\n        void printValue(){\r\n             ^\r\n1 error generated.\r\n<\/pre>\n
The same thing will happen if we try to add<\/p>\n
\r\nA a = d;\r\n<\/pre>\n
to the main. We could try to tell the compiler which A we mean by saying something like: <\/p>\n
\r\nA a = static_cast(d);\r\na.printValue();\r\n<\/pre>\n
which works, but doesn't solve the problem that we have two different A's. <\/p>\n
Solution: use shared multiple inheritance<\/em> - make B and C inherit from virtual A. <\/p>\n
\r\nclass B: virtual public A { ... }\r\nclass C: virtual public A { ... }\r\n<\/pre>\n
\r\nA's constructor called\r\nB's constructor called\r\nC's constructor called\r\nD's constructor called\r\n5\r\n<\/pre>\n
The actual machinery behind all of this is quite complex, but you can read about it a bit here<\/a>. <\/p>\n","protected":false},"excerpt":{"rendered":"
The “Deadly Diamond of Death” Consider a set of classes with the following inheritance hierarchy. A \/ \\ B C \\ \/ D diamond.cpp #include using namespace std; class A { int value; public: A() { value = 5; cout<\/p>\n
Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"parent":11,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"_editorskit_title_hidden":false,"_editorskit_reading_time":0,"_editorskit_is_block_options_detached":false,"_editorskit_block_options_position":"{}","footnotes":""},"class_list":["post-995","page","type-page","status-publish","hentry"],"yoast_head":"\nDiamond Problem - Daniel R. Schlegel<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/danielschlegel.org\/wp\/teaching\/diamond-problem\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Diamond Problem - Daniel R. 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