CErc20: Token Integration

Key takeaway: Compound V2's CErc20 is the wrapper around an underlying ERC-20 token, providing doTransferIn and doTransferOut helpers that handle the underlying's specific transfer semantics. These helpers measure actual balance deltas before and after the transfer rather than trusting the input amount, which is critical for fee-on-transfer or rebasing tokens. Forks that skip this defensive measurement break on USDT in fee-on mode or on any cToken market that lists a non-standard ERC-20.

What You Are Building

You are building the final integration layer that turns all the internal logic into a deployable contract. CErc20 inherits the entire CToken chain (CTokenStorage, CTokenInterest, CTokenMint, CTokenRedeem, CTokenBorrow, CTokenRepay, CTokenLiquidate) and adds four things: the public entry points, the underlying token transfer implementation, standard ERC-20 functions, and initialization.

This is the contract users actually interact with. Everything built so far has been internal functions. CErc20 wraps them with external functions, reentrancy protection, and the concrete doTransferIn/doTransferOut implementations that move real ERC-20 tokens.