![\"CreationalPatterns\"](\"https:\/\/www.modernescpp.com\/wp-content\/uploads\/2022\/09\/CreationalPatterns.png\")
<\/h3>\nIn the last installment of this blog, I introduced the Factory Method: Creational Patterns: Factory Method 1. My implementation had two serious issues: slicing and ownership semantics. Today, I fix these issues.<\/p>\n
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<\/h3>\nTo remind you, here is a simplified and slightly modified implementation of the Factory Method in my last post. First, this implementation support only the member function <\/p>\n <\/p>\n The program produces the expected polymorphic behavior.<\/p>\n First: What is slicing?<\/p>\n Slicing is one of the darkest corners of C++. Let me give you a simple example:<\/p>\n <\/p>\n <\/p>\n The expressions (1), (2), and (3) have all the same effect: The <\/p>\n You may ask yourself. Why is slicing an issue for the example factoryMethodWindowIssues.cpp.<\/span> Let me quote the C++ Core Guidelines: C.67: A polymorphic class should suppress public copy\/move<\/a>.<\/p>\n Okay, there is a new question: What is a polymorphic class?<\/p>\n Here is the issue. <\/p>\nclone<\/code>; second, also the base class Window<\/code> should be cloneable.<\/p>\n\n\/\/ factoryMethodWindowIssues.cpp<\/span>\r\n\r\n#include <iostream><\/span>\r\n\r\n\/\/ Product<\/span>\r\nclass<\/span> Window<\/span>{ \r\n public<\/span>:<\/span> \r\n virtual<\/span> Window*<\/span> clone() { \r\n std::<\/span>cout <<<\/span> \"Clone Window\"<\/span> <<<\/span> '\\n'<\/span>;\r\n return<\/span> new<\/span> Window<\/span>(*<\/span>this<\/span>);\r\n } \r\n virtual<\/span> ~<\/span>Window() {};\r\n};\r\n\r\n\/\/ Concrete Products <\/span>\r\nclass<\/span> DefaultWindow<\/span>:<\/span> public<\/span> Window { \r\n DefaultWindow*<\/span> clone() override { \r\n std::<\/span>cout <<<\/span> \"Clone DefaultWindow\"<\/span> <<<\/span> '\\n'<\/span>;\r\n return<\/span> new<\/span> DefaultWindow<\/span>(*<\/span>this<\/span>);\r\n } \r\n};\r\n\r\nclass<\/span> FancyWindow<\/span>:<\/span> public<\/span> Window { \r\n FancyWindow*<\/span> clone() override { \r\n std::<\/span>cout <<<\/span> \"Clone FancyWindow\"<\/span> <<<\/span> '\\n'<\/span>;\r\n return<\/span> new<\/span> FancyWindow<\/span>(*<\/span>this<\/span>);\r\n } \r\n};\r\n\r\n\/\/ Concrete Creator or Client <\/span>\r\nWindow*<\/span> cloneWindow<\/span>(Window&<\/span> oldWindow) { \r\n return<\/span> oldWindow.clone();\r\n}\r\n \r\nint<\/span> main<\/span>() {\r\n\r\n std::<\/span>cout <<<\/span> '\\n'<\/span>;\r\n\r\n Window window;\r\n DefaultWindow defaultWindow;\r\n FancyWindow fancyWindow;\r\n\r\n const<\/span> Window*<\/span> window1 =<\/span> cloneWindow(window);\r\n const<\/span> Window*<\/span> defaultWindow1 =<\/span> cloneWindow(defaultWindow);\r\n const<\/span> Window*<\/span> fancyWindow1 =<\/span> cloneWindow(fancyWindow);\r\n \r\n delete<\/span> window1;\r\n delete<\/span> defaultWindow1;\r\n delete<\/span> fancyWindow1;\r\n\r\n std::<\/span>cout <<<\/span> '\\n'<\/span>;\r\n \r\n}\r\n<\/pre>\n<\/div>\n![\"factoryMethodWindowIssues\"](\"https:\/\/www.modernescpp.com\/wp-content\/uploads\/2022\/09\/factoryMethodWindowIssues.png\")
<\/p>\n<\/p>\nSlicing<\/h3>\n
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\n\/\/ slice.cpp<\/span>\r\n\r\n#include <iostream><\/span>\r\n#include <typeinfo><\/span>\r\n\r\nstruct<\/span> Base { };\r\n \r\nstruct<\/span> Derived :<\/span> Base { };\r\n\r\nvoid<\/span> displayTypeinfo<\/span>(const<\/span> Base&<\/span> b) {\r\n std::<\/span>cout <<<\/span> typeid<\/span>(b).name() <<<\/span> '\\n'<\/span>; \r\n}\r\n\r\nvoid<\/span> needB<\/span>(Base b) {\r\n displayTypeinfo(b);\r\n};\r\n \r\nint<\/span> main<\/span>() {\r\n\r\n Derived d;\r\n \r\n Base b =<\/span> d; \r\n displayTypeinfo(b); \/\/ (1)<\/span>\r\n \r\n Base b2(d); \r\n displayTypeinfo(b2); \/\/ (2)<\/span>\r\n \r\n needB(d); \/\/ (3)<\/span>\r\n\r\n}\r\n<\/pre>\n<\/div>\nDerived<\/code> part of d<\/code> is removed. I assume that was not your intention.<\/p>\n![\"slice\"](\"https:\/\/www.modernescpp.com\/wp-content\/uploads\/2017\/03\/slice.png\")
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Window<\/code> in the program factoryMethodWindowIssues.cpp<\/span> is a polymorphic class, and it does not suppress public copy\/move. A Window<\/code> can be a victim of slicing. The following program exemplifies this. I just removed the reference from the function signature of the function cloneWindow<\/code>:<\/p>\n
![\"factoryMethodWindowIssuesSlicing\"](\"https:\/\/www.modernescpp.com\/wp-content\/uploads\/2022\/09\/factoryMethodWindowIssuesSlicing.png\")
<\/p>\n![\"factoryMethodWindowsSlicingFixed\"](\"https:\/\/www.modernescpp.com\/wp-content\/uploads\/2022\/09\/factoryMethodWindowsSlicingFixed.png\")
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