Sending coins from contract A to contract B

Question

I have two contracts.

Contract A is an ERC20 token with initial_supply all owned by the contract itself.

And Contract B which exposes a deposit method.

When a user sends a transaction to ContractB.deposit() I want it to:

1. Send Contract A coins to contract B (from contract B i.e, contract B needs to request the coins from contract A to B)<- not sure how to do
2. Send Contract A coins which are now stored in contract B to msg.sender <- not sure how to do

How can I achieve this? My own attempts have largely failed.

Answer 1

Use delegateCall in contract B to execute contract A. This executes contract A code with contract B storage.

Not sure what deposit does, it seems more like a withdraw to me.

Nonetheless, contract B needs to have enough allowance from contract A then you can call a delegateCall with transfer from contract B to execute transfer from contact A, and this sends the tokens to msg.sender

This is basically the same as when EOA with enough allowance calls a transfer from the contract to itself, the only difference is that you are using another contract instead.

Read these to understand DelegateCall;

https://solidity-by-example.org/delegatecall/ https://medium.com/coinmonks/delegatecall-calling-another-contract-function-in-solidity-b579f804178c

Edit after a long time:

1. On the ERC-20 constructor give max allowance to contract B
2. On deposit do a transferFrom(A, address(this), amount) now B has A amount tokens
3. Then do transferFrom(address(this), msg.sender, amount) now B uses the amount of tokens that it just got from A and send it to the msg.sender

PD: Its kinda pointless to sent tokens from A to B and then B to sender. You could simple to a transferFrom(A, msg.sender, amount). B must have max allowance on itself to send the second txn