C++ Zero-Cost Abstractions

Core Question

When do we determine the type?

• Compile Time (Static): Templates, Concepts, CRTP. Zero runtime cost, larger binary.
• Runtime (Dynamic): Virtual functions, std::any, std::variant. Flexible, vtable overhead.

Error → Design Question

Thinking Prompt

1. Is the set of types known at compile time?
   • Yes? → Templates or std::variant.
   • No? → Inheritance (Virtual functions).
2. Do I need to store them in a list?
   • Homogeneous? → std::vector<T>.
   • Heterogeneous? → std::vector<std::unique_ptr<Base>> or std::vector<std::variant<...>>.
3. Does the interface match exactly?
   • Duck typing needed? → Templates (Concepts).
   • Strict hierarchy? → Inheritance.

Trace Up / Down

• Trace Up:
  • Issue: "Virtual function call is too slow in tight loop."
  • Cause: Indirect branch misprediction.
  • Fix: Switch to Static Polymorphism (CRTP or Templates) if types are known.
• Trace Down:
  • Intent: "I want a function that accepts anything that has .draw()."
  • Code (C++20): void render(Drawable auto& item) { item.draw(); }

Quick Reference