how-not-to-cpp

Doing C++ the wrong way, on purpose.

A curated collection of C++ anti-patterns that compile, run, and disappoint. Every file demonstrates a real mistake found in real codebases — written deliberately, explained honestly, and organized so the wrongness is easy to find and learn from.

This is not random broken code. It is a serious engineering codebase built on systematically awful choices. The contrast is the joke. The explanation is the point.

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Build

Requires a C++17 compiler, meson, and ninja.

meson setup build
ninja -C build

Run the main showcase:

./build/src/how-not-to-cpp

Run the examples individually:

./build/examples/exception_safety_who
./build/examples/impossible_api_usage
./build/examples/thread_unsafety_demo

Run the tests:

meson test -C build

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Anti-pattern index

Each file has a header block explaining the bad idea, why someone reaches for it, why it is terrible, and what the fix looks like.

Headers

File	Anti-pattern
include/everything.hpp	Blanket #include of the entire standard library + using namespace std in a header
include/utilities_final_v2.hpp	Function implementations in headers with no inline or template reason (ODR violations)
include/global_state_manager_singleton_factory.hpp	Global mutable state wrapped in a singleton, exposed through a factory

Core source (src/)

File	Anti-pattern
memory_leaks.cpp	Manual new/delete, raw owning pointers, no RAII — leaks on every early return
singleton_registry.cpp	Singleton everything; singletons that register themselves into a singleton registry
bool_parameter_hell.cpp	12-bool function signatures; output parameters; mixed error strategies
inheritance_abuse.cpp	Dog, Cat, Vector3, and DatabaseConnection all inherit from EntityBaseComponentObject
dangling_reference_showcase.cpp	References to locals, string_view into temporaries, iterator invalidation
macros_are_templates_now.cpp	Macros replacing templates, generics, OOP, and error handling

Examples (examples/)

File	Anti-pattern
thread_unsafety_demo.cpp	Shared mutable state accessed from multiple threads with no synchronization
exception_safety_who.cpp	Manual resource acquisition followed by exceptions — leaks on every failure path
impossible_api_usage.cpp	Mixed error strategies, ambiguous ownership, overload traps

Tests (tests/)

File	Anti-pattern
test_that_depends_on_execution_order.cpp	Tests that share global mutable state and must run in a specific, unenforced order

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How each file is structured

// bad_idea.cpp
//
// BAD IDEA: what the pattern is
// WHY SOMEONE DOES IT: the honest reason a developer reaches for this
// WHY IT'S TERRIBLE: the concrete consequences
// HOW TO FIX IT: the modern C++ alternative

The fixes are usually simpler than the bad versions.

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What this is not

• A reason to avoid C++. Modern C++ has excellent idioms. This repo is a map of the potholes.
• Purely academic. Every anti-pattern here has caused production bugs somewhere.
• Exhaustive. These are representative samples, not a complete taxonomy.

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Further reading

• docs/rationale.md — why this repo exists and how to read it
• docs/style-guide.md — the official coding standards (read carefully)