github pdf url: https://github.com/dieforfree/edsebooks/blob/master/ebooks/Design%20Patterns%2C%20Elements%20of%20Reusable%20Object-Oriented%20Software.pdf http://c.biancheng.net/view/1320.html
github cpp code: https://github.com/JakubVojvoda/design-patterns-cpp resource : https://refactoring.guru/design-patterns