CRTP: A Curious Polymorphism Idiom

Polymorphism in C++

• Definition: the use of a single symbol to represent multiple different types
• Compile-time (static) polymorphism
  • Function overloading: implemented by name mangling, not available in C
  • Operator overloading: similar as function overloading
  • Templates
    • CRTP <- today we focus on
• Runtime (dynamic) polymorphism
  • Function overriding (virtual) -> also can be inferred early at compile time / constexpr virtual (C++20)
  • std::optional (C++17)
  • std::variant (C++17)
  • ...
• A takeaway here: look into if variables can be inferred during compile time
• We prefer compile-time polymorphism in common, but still need to be aware of the overhead
  • Binary size
  • Codebase complexity

A Curious Idiom: CRTP

• CRTP stands for Curiously Recurring Template Pattern
• a class X derives from a class template instantiation using X itself as a template argument

  template <class T> 
  class X { … }; 
  
  class A : public X<A> {...};
  

• Used for enhancement or semantic extension
• No virtual (virtual is not bad fairly speaking)

Why can CRTP compile

• Laziness of template instantiation: instantiate iff needed
• Incomplete type
• static_cast: perform downcast to pointer to complete class D from its non-virtual base B
• Instantiation of member function
• <godbolt>

When to use CRTP

• Mixin, when you feel like
  • it’s more like an enhancement to existing classes
  • it can be "included" rather than "inherited"
  • you don’t need type erasure
• Demos
  • Add more operators to class with operator<<godbolt>
  • Singleton <godbolt>
• In practices
  • std::enable_shared_from_this (example)
  • COW in apache/doris

Miscellaneous

• Pitfalls
  • Type erasure
    • virtual comes with type erasure, but CRTP not
    • without using a general base class like AbstractShape from the above example, derived classes cannot be stored homogeneously
      • std::vector<Shape*> doesn’t work
      • solution: inherit from a shared base class with a virtual destructor (like AbstractShape, but back to virtual again…)
• Best Practices
  • use “friend trick” to avoid potential mistakes
• CRTP in C++23
  • Deducing this
  • Abbreviated function template: use auto on parameter type
  • <godbolt>
• Benchmarks
  • https://github.com/nitingupta910/crtp_bench
  • https://eli.thegreenplace.net/2013/12/05/the-cost-of-dynamic-virtual-calls-vs-static-crtp-dispatch-in-c