C++ backend patterns — minimal examples (CMake, C++20)

Tiny, realistic C++20 examples of common backend patterns and design patterns, each in its own folder under examples/.

Requirements

• CMake 3.20+
• A C++20 compiler (Clang/GCC/MSVC)
• Optional: clang-format (for ./scripts/format.sh)

Patterns (ranked backend focus)

Done	Rank	Pattern / Idiom	Category	Backend C++ focus
[x]	1	RAII	C++ idiom	Leak-free sockets/files/locks; exception-safe cleanup.
[x]	2	Ownership with smart pointers	C++ idiom	Makes lifetimes explicit across async + callbacks.
[x]	3	Rule of 0 / Rule of 5	C++ idiom	Prevents accidental copies/moves in request/response objects.
[x]	4	Thread pool / work queue	Concurrency	Core throughput primitive for RPC/HTTP handlers and background work.
[x]	5	Producer–consumer (bounded queue)	Concurrency	Backpressure; protects latency under load.
[x]	6	Futures/promises (task-based async)	Concurrency	Composable async workflows; cancellation/timeout integration.
[x]	7	Reactor (event loop)	Concurrency/IO	Scalable network IO (epoll/kqueue-style architectures).
[x]	8	Facade	GoF (Structural)	Wraps subsystems (DB, cache, metrics) behind clean APIs.
[x]	9	Adapter	GoF (Structural)	Normalizes 3rd-party clients/libs into your interfaces.
[x]	10	Strategy	GoF (Behavioral)	Swappable policies (retry, routing, load balancing, parsing).
[x]	11	Factory Method	GoF (Creational)	Pluggable creation for clients/handlers; test-friendly DI.
[x]	12	Abstract Factory	GoF (Creational)	Environment-based wiring (prod vs test vs staging stacks).
[x]	13	Proxy	GoF (Structural)	Client wrappers for caching, auth, rate limits, retries, tracing.
[x]	14	Decorator	GoF (Structural)	Layer cross-cutting concerns around handlers/clients.
[x]	15	Type erasure	C++ idiom	Store heterogeneous callables/handlers without template bloat.
[x]	16	Observer (pub/sub)	GoF (Behavioral)	Metrics/logging hooks and internal eventing (mind lifetimes).
[x]	17	State	GoF (Behavioral)	Connection/session/protocol state machines without giant switch.
[x]	18	Command	GoF (Behavioral)	Queueable jobs; background tasks; audit/redo semantics.
[x]	19	Pimpl	C++ idiom	Cuts rebuild times; hides heavy deps in service libraries.
[x]	20	Bridge	GoF (Structural)	Decouple interface from platform/impl (TLS, DNS, IO backends).
[x]	21	Composite	GoF (Structural)	Middleware chains / routing trees / filter graphs.
[x]	22	NVI (Non-Virtual Interface)	C++ idiom	Enforce invariants in extensible service components.
[x]	23	Traits/tag dispatch/(concepts-style selection)	C++ idiom	Efficient generic code in serialization, hashing, parsing.
[x]	24	Visitor (often via std::variant + std::visit)	GoF (Behavioral)	Clean handling of protocol/message variants.
[x]	25	Flyweight	GoF (Structural)	Interning/shared immutable data to reduce memory in hot paths.

Build

cmake -S . -B build
cmake --build build -j

Run

# Run one example:
./build/examples/<pattern>/<pattern>_example

# List built example binaries:
ls -1 ./build/examples/*/*_example

Format

./scripts/format.sh

Optional: enforce formatting on commit (git hook)

git config core.hooksPath .githooks