Eris Industries tutorial: Call contract object from other contract

Question

In the Eris Industries tutorial, they present a banking and fund management contract system.

The FundManager controls the bank contract. I don't understand the following line from the deposit function used to check if the function call was successful:

bool success = Bank(bank).deposit.value(msg.value)(msg.sender);

Why Bank(bank) and what is it? The FundManager has an address bank attribute, instantiated by the constructor.

So, essentially, my question is:

Bank(bank): Get a reference object of type `Bank` at address `bank`
Bank(bank).deposit: call the `deposit` function on that object
Bank(bank).deposit.value(msg.value): What is happening here and what is returned?
Bank(bank).deposit.value(msg.value)(msg.sender): What is happening here?

Answer 1

The FundManager constructor creates a new Bank contract object...

bank = new Bank();

...and stores its address in this variable:

// This holds a reference to the current bank contract.
    address bank;

When the FundManager's deposit function is called, it calls the Bank contract using its address, as explained in this previous thread:

Bank(bank)...

In full, the call is:

bool success = Bank(bank).deposit.value(msg.value)(msg.sender);

Where the deposit function in the Bank contract is defined as:

// This will take the value of the transaction and add to the senders account.
function deposit(address customer) returns (bool res)

The customer address parameter equates to the msg.sender passed by the call in FundManager.

Edit:

Bank(bank).deposit.value(msg.value): What is happening here and what is returned?

I don't think this is a valid call by itself. It'd probably complain about a missing argument.

Bank(bank).deposit.value(msg.value)(msg.sender): What is happening here?

Bank(bank).deposit() is an external function call: it's calling a function in a different contract.

The parameter for the deposit function in the second contract is address customer, so we'd expect the call in the first contract to be:

Bank(bank).deposit(msg.sender)

...where msg.sender equates to address customer. So far so clear.

However, for external function calls, if we want the second contract to know the original value of msg.value from the first contract, then we must explicitly pass it. The reason being (from this page in the Solidity docs (part-way down)):

The values of all members of msg, including msg.sender and msg.value can change for every external function call. This includes calls to library functions.

To explicitly pass the value we need a value(msg.value) in there as well, which by convention goes before the actual parameter of the function we're calling:

          +------------------------------------+
          |                                    |
          |                                    V
Bank(bank).deposit    .value(msg.value)      (msg.sender)
                            ^
                            |
                  shove it in the middle