SOLID Principles C++ Quick Reference¶

→ src/solid/solid_principles.cpp

Summary¶

Principle	Meaning	C++ Gotcha
S Single Responsibility	One class, one reason to change	God-class anti-pattern; split into composition
O Open/Closed	Open for extension, closed for modification	Use virtual interfaces or templates, not `if/else` chains
L Liskov Substitution	Subtypes must be substitutable for base types	Don't tighten preconditions or weaken postconditions in derived
I Interface Segregation	Don't force clients to depend on unused interfaces	Prefer many small abstract classes over one fat interface
D Dependency Inversion	Depend on abstractions, not concretions	Inject `unique_ptr<Interface>` instead of concrete class

SRP: Separate OrderParser, OrderValidator, OrderExecutor instead of one OrderManager
OCP: Strategy pattern for new pricing models without touching existing engine
LSP: If Square inherits Rectangle, setting width shouldn't break height assumptions
ISP: Split IDatabase into IReader + IWriter read-only clients shouldn't know about writes
DIP: Trading system depends on IMarketFeed interface, not SpecificExchangeAPI

Violation	What's Wrong	Fix
`switch` on type enum	OCP must modify switch for new types	Polymorphism or `std::variant` + `std::visit`
Derived class throws on base method	LSP breaks substitutability	Redesign hierarchy or use composition
Constructor creates its own dependencies	DIP hard to test	Inject via constructor parameter
Class with 20+ methods	SRP + ISP doing too much	Split into focused components
`#include` of concrete implementation	DIP tight coupling	Forward-declare, depend on interface header