Ethereum-Solidity Quiz Q17: What visibility modifiers does Solidity use?

Modifier	Within Contract	Derived Contract	Other Contract	External
public	✅	✅	✅	✅
internal	✅	✅	❌	❌
private	✅	❌	❌	❌
external	❌ (via this)	❌	✅	✅

Solidity uses four visibility modifiers to control how functions and state variables can be accessed:

1. Public - can be called from within the contract, other contracts, externally

2. External - can be called from other contracts, externally; NOT from within the same contract

3. Internal - can be called from within the same contract, derived contracts (inheritance), NOT from other contracts, NOT externally

4. Private - can be called from within the same contract, NOT from derived contracts, NOT from other contracts

Important notes:

• State variables can be public, internal, or private (NOT external)
• Functions can be public, internal, private, or external
• public state variables automatically generate getter functions
• external is more gas efficient than public because it doesn't create an internal function copy
• private and internal are NOT secure — blockchain data is always visible (just not accessible via function calls)

Best practices:

• Use external for functions meant to be called from outside
• Use internal for helper functions used by inheritance
• Use private with caution — usually only for helper functions
• Use public for state variables that need public access
• Be careful with private — it provides no security, just prevents accidental calls