CALLCODE is deprecated due to unsafe context handling. It remains callable for backward compatibility but should never be used in new designs.

EXTCODESIZE returns zero for EOAs and non-zero for deployed contracts. This distinction is commonly used for contract detection, though it has edge cases.

GASLEFT returns the amount of gas remaining in the current execution context. It is commonly used for gas-aware logic and debugging.

bytes32 always occupies a full 32-byte storage slot. Smaller types may be packed together, but bytes32 always stands alone.

Solidity packs struct variables tightly into storage slots based on declaration order. Reordering fields can change slot boundaries and break upgrade compatibility.

The base slot of a mapping is derived using keccak256(key . slot). This hashing ensures unique storage locations per key without collisions.

Changing variable types alters how Solidity packs them into storage slots. This can shift offsets and corrupt existing storage layouts in upgradeable contracts.

Calldata is immutable and cannot be modified by the callee. This makes it gas-efficient and safe for external function inputs

Storage is not automatically wiped when a contract self-destructs. The data remains on-chain and can be accessed again if a contract is redeployed at the same address.

bytes32 has a fixed 32-byte size and fits into a single storage slot. string is dynamically sized and requires additional storage pointers and length metadata.

Using delete on a dynamic array sets its length to zero. Storage slots may remain allocated but become inaccessible through the array.

Deleting a mapping key resets its value to the default type value. The key itself still exists conceptually since mappings do not track keys.

Storage persists on-chain across transactions and blocks. Memory and calldata are temporary and cleared after execution completes.

Public variables automatically generate getter functions. If a function shares the same name, the compiler prioritizes the variable-generated getter, shadowing the function definition.

Solidity does not support a final or sealed keyword for contracts. Any contract can be inherited unless architectural constraints are enforced manually.