Design Patterns#

CRTP / Mixins#

Article by fluentcpp.com on CRTP

  • Mixins are a design pattern used to add additional functionality to an existing class

  • In this case, the TargetInfo and the Target class are meant to have a lot of setter APIs for user inputs.

  • Adding more APIs to the class makes its difficult to read and maintain.

  • For reference: See Serialization Schema

  • For example: In Target source_files have currently have several APIs

    • AddSource

    • AddSourceAbsolute

    • GlobSources

    • GlobSourcesAbsolute

  • In the future we might have additional APIs such as

    • AddSources that takes an std::initializer_list

    • AddSources that takes a std::vector<> or std::unordered_set<>

    • AddSource(s)ByPattern that takes a fmt string like {dir}/source.cpp

  • From our serialization schema we can see that each of these fields could have many APIs associated with them. Having 50+ APIs for different fields in a single header i.e target_info.h / target.h would not be readible and hard to maintain.

  • The Mixin / CRTP pattern is used to easily add functionality for a particular field in its own header / source file.

Friend classes#

  • Friend classes are used when 2 classes have strong coupling with each other, but still need to maintain flexibility for other purposes. For example: Unit testing.

  • In target.h we have 3 friend classes (non CRTP based)

    • CompilePch

    • CompileObject

    • LinkTarget

  • These 3 classes are made friend classes for 2 main purposes

    • Unit Testing

    • Flexibility / Maintaibility

  • Unit Testing

    • If these were not friend classes, the functions would’ve been private in scope within the Target class

    • Unit testing these individual private functions would not be possible would public interfaces

    • By making them friend classes, We can now unit test the public functions and embed this class in a private context with the Target class

  • Flexibility / Maintaibility

    • Each one of the classes mentioned above have their own information / states / tasks to hold.

    • Without this segregation all of the member variables, states and tasks would need to be present inside the Target class

  • Strong Coupling

    • The 3 friend classes have strong coupling with the Target class since it uses its internal member variables for setting / getting information.

    • The friend class can interact with the parent class and vice versa.