Anonymous function in Solidity example code

Question

I'm following this link - https://ethereum.org/token to learn Solidity. The example contract implementation has the following line which I don't understand.

Can someone please explain what is this function intended to do? When would it be called? Is it an internal function or external function call?

Answer 1

This is deprecated syntax that isn't required as of Solidity 0.4.0

function () is the 'default function' or 'fallback function'. If a transaction is sent to an address without transaction data, or if that data doesn't call a valid function on a contract, the default function is called instead.

throw is a deprecated keyword to halt the computation and revert any changed state.

The function as written will force all transactions to fail if they don't call a legitimate function.

In the later versions of Solidity (>= v0.4.0), a transaction will throw if the default function is not explicitly implemented and the transaction has not called a legitimate function.

If a contract is to accept trivial transactions, such as being sent ether, the default function must be implemented as payable. The amount of gas sent with a trivial send is not enough to change a state variable but is enough to log an event. So a typical implementation may be:

function () payable {
    Deposit(msg.sender, msg.value);
}

As regards throw, this keyword compiles to an illegal opcode which forces the EVM to halt the transaction and consumes all gas sent with the transaction. This has been replaced with the precompiled functions revert(), assert() and require() to test for exceptions and halt the transaction on a failure.

function () payable {
    // only owner can send
    require(msg.sender == owner);

    // Prevent overflow
    assert(balance[owner] + msg.value >= balance[owner]);

    // Accept only upto an amount
    if(balance[owner] + msg.value > maximum) revert();
    
    // Log the deposit
    Deposit(msg.sender, msg.value);

    // Careful, no guarantee there will be enough gas for this
    balance[owner] += msg.value;
}
    

Answer 2

That function is called the "fallback" function. It is the function that is invoked when a transaction with no data field is made to a contract.

Notice that there is no "payable" modifier on that contract. When you label a function as "payable" it is a function that invokable with a non-zero Ether value in the "value" field of the transaction.

The reason for that empty fallback function is to say, if someone tries to send Ether to this contract, don't accept it, and throw an exception.