13eholder/m04-zero-cost

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

| Issue | Design Question | | ------------------ | ---------------------------------------------- | | Template spew | Are you missing Concepts constraints? | | Linker error | Did you define template in .cpp instead of .h? | | Object slicing | Did you assign Derived to Base value? | | Slow build | Are you overusing headers/templates? |

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

| Pattern | Dispatch | Cost | Use When | | ------------------- | ---------------- | ------------------- | ---------------------------- | | virtual | Dynamic | Vtable + Cache miss | Plugins, Runtime extensions. | | Template | Static | Code bloat | High perf, Type deduction. | | std::function | Dynamic | Alloc + Indirect | Storing callbacks. | | std::variant | Static branching | Branch switch | Closed set of types. | | CRTP | Static | Zero | Static inheritance. |