🌟 Solidity Interfaces — Complete Notes

1. What is an Interface?

An interface in Solidity is a purely declarative structure that lists function signatures without any implementation.

• Think of it as a menu or a protocol:

  “These are the functions you can call, but I won’t tell you how they work.”

Key points:

• Declared with interface keyword
• Contains only function signatures
• No function bodies (no logic)
• Functions are implicitly external
• No state variables
• No constructors
• Often start with I (e.g., IERC20)
• All functions in an interface are implicitly virtual
• Interfaces can inherit from other interfaces (not from contracts)

2. Why do we use interfaces?

👉 To interact with deployed contracts without copying their entire source code.

Example:

• If a token contract is already deployed on Ethereum, you only need its interface to talk to it.