C++ Specifiers and Attributes Cheatsheet for Embedded Systems

When creating a function for my embedded projects, I am often overwhelmed by the many possible ways to define methods or variables. Maybe this summary helps you as well!

Specifiers Overview

• inline: For small, often-used functions. Example: High-frequency sensor data processing functions.
• static: For shared class members or persistent local variables. Example: Counter in a function tracking the number of calls.
• constexpr: For compile-time constants. Example: Predefined configuration parameters like buffer sizes.
• volatile: For variables modified outside the program, such as by interrupts. Example: Flags set in an ISR (Interrupt Service Routine).
• register: Suggests storing a variable in a register. Example: Rarely specified, best left to compiler’s optimization.
• restrict: For performance optimization in pointer-intensive functions. Example: Signal processing functions using non-overlapping buffers.
• noexcept: For functions guaranteed not to throw exceptions. Example: Critical error handlers in embedded systems.
• [[noreturn]]: For functions that never return. Example: System reboot or fatal error handling functions.
• const: For immutable values and methods that do not modify the object. Example: Getters in classes representing device settings.
• mutable: For members modified in const member functions. Example: Cache variables in a const method.
• override: Ensures a member function overrides a virtual function in the base class. Example: Derived class methods that modify base class behavior.
• final: Prevents further derivation of classes or overriding of methods. Example: Critical base classes in a control system.
• explicit: Prevents implicit conversion by constructors. Example: Constructors that should not be called with a single argument implicitly.
• thread_local: For variables that are local to a thread. Example: Error state variable specific to each thread in a multithreaded application.
• [[nodiscard]]: Ensures function results are not ignored. Example: Functions returning error status that must always be checked.
• [[maybe_unused]]: Suppresses warnings for unused entities. Example: Parameters that are only used in debug builds.
• [[deprecated]]: Marks functions or entities as outdated. Example: Old API functions superseded by newer versions.
• [[likely]] and [[unlikely]]: Provides hints about branch prediction. Example: Conditions in performance-critical loops that are rarely true.

Practical Applications in Embedded Systems

• Prioritize consistent, predictable behaviors and execution times.
• Use constexpr and inline to reduce runtime overhead.
• Leverage volatile for hardware interaction to ensure real-time responsiveness.
• Avoid dynamic memory allocation where possible; instead, use static storage duration.
• Disable exceptions and use noexcept to clarify this intent.
• Minimize use of register, instead relying on the compiler for register allocation.

Best Practices for Specifier Usage

• Use const and explicit to enforce immutability and prevent implicit behaviors.
• Utilize thread_local for per-thread data to avoid costly synchronization in multi-threaded environments.
• Mark critical functions with [[nodiscard]] to ensure that their results are handled correctly.
• Employ [[deprecated]] to manage evolving firmware and API changes smoothly.

This has been summarized with the help of the almighty ChatGPT. For specific examples, take a look at the Roboost firmware, where I try to incorporate these practices.

So Long, and Thanks for All the Fish