m03-mutability

Core Question

Who is allowed to change this state?

C++ defaults to Mutable. You must opt-in to safety with const.

• API Contract: const methods promise not to change visible state.
• Physical Constness: The compiler ensures bits don't change.
• Logical Constness: You use mutable to change hidden state (like caches) inside const methods.

Error → Design Question

| Issue | Design Question | | ------------------------- | ----------------------------------------------------------------------- | | Discarded qualifiers | Are you trying to modify member data in a const function? | | Data Race | Did you modify a mutable field from multiple threads without locking? | | Iterator Invalidation | Are you modifying a container while reading it? |

Thinking Prompt

1. Should this be const?

   • Variables: Yes, default to const or constexpr.
   • Methods: Yes, unless it must modify state.
   • Pointers: const T* (ptr to const) vs T* const (const ptr).

2. Is internal state 'invariant' or 'cache'?

   • Cache/Memoization? → Use mutable + std::mutex.
   • Actual state? → Non-const method.

3. Is it thread-safe?

   • const methods implies safe for concurrent readers.
   • mutable members MUST be protected by synchronization in const methods.

Trace Up / Down

• Trace Up:

  • Issue: "Undefined behavior in multithreaded app."
  • Cause: A const method modified a mutable cache without a lock, assuming const meant safe.
  • Fix: Add std::mutex to the class, lock it in the const method.

• Trace Down:

  • Intent: "I want a thread-safe lookup table."
  • Code: std::shared_mutex allowing multiple readers (shared_lock) and one writer (unique_lock).

Quick Reference

| Keyword | Meaning | Use When | | ----------------------- | --------------------- | -------------------------------- | | const | Read-only access | Parameters, local vars, getters. | | constexpr | Compile-time constant | Constants, array sizes. | | mutable | Modifiable in const | Caches, Mutexes within a class. | | std::mutex | Exclusive Lock | Protecting mutable state. | | std::shared_mutex | Read/Write Lock | Rare updates, frequent reads. |