m04-zero-cost

name	m04-zero-cost
description	Mastering C++ Polymorphism: Templates, Concepts, and Virtual Functions. Triggers: templates, concepts, vtable, CRTP, static polymorphism, dynamic polymorphism, SFINAE.

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.