Interfaces and external calls

Everything so far has happened inside one contract. This lesson is where your code starts talking to code you did not write — and where the danger begins.

An interface is a shape, not a contract

interface ICounter {
    function increment() external;
    function count() external view returns (uint256);
}

No bodies, no state, no constructor. It tells the compiler how to encode a call to something at an address that claims to behave this way.

What an external call really is

Each counter.increment() is a whole separate execution:

• A new execution context, with its own storage and memory.
• msg.sender becomes your contract's address, not the original user.
• Control leaves your contract entirely until the call returns.

That third point is the important one.

Assume every external call is hostile

Three habits worth forming now:

1. Update your state before calling out. Always.
2. Check return values. A low-level .call() returns false rather than reverting.
3. Assume the callee can call you back, run out of gas, or revert on purpose to block you.

None of this is paranoia. Every one of these has been used to take real money.