Solidity Basics: Your First Lines of Code — Deep dive (why it matters)

ABI as contractOnce deployed, your ABI is your product. Adding a public variable or function exposes callable selectors forever. Breaking changes require new deployments. Decide surface area deliberately.

Visibility defaults biteFunctions default to public in some older examples; explicitly set visibility to avoid surprise entry points. State defaults to internal, but marking it public creates getters that frontends may depend on.

Data location correctnessPassing storage pointers when you meant memory can mutate shared state by accident. Calldata saves gas for read-only inputs. Knowing the difference is both a safety and cost skill.

Compiler pragmasUsing floating pragmas invites supply-chain risk if a new compiler version changes behavior. Lock ranges that match your audits and tests.