| name | m03-mutability |
| description | Mastering C++ Const Correctness and Mutability. Triggers: const, mutable, logically const, bitwise const, data race, mutex, shared_mutex, thread safety. |
C++ Mutability & Const Correctness
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
-
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).
-
Is internal state 'invariant' or 'cache'?
- Cache/Memoization? → Use
mutable + std::mutex.
- Actual state? → Non-const method.
-
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. |
