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erc20.md

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ERC20


  • defines a common interface for contracts implementing this token, such that any compatible token can be accessed and used in the same way.
  • a transaction sending ether to an address changes the state of an address. a transaction transferring a token to an address only changes the state of the token contract, not the state of the recipient address.
  • one of the main reasons for the success of EIP-20 tokens is in the interplay between approve and transferFrom, which allows for tokens to not only be transferred between externally owned accounts (EOA).
    • but to be used in other contracts under application specific conditions by abstracting away msg.sender as the mechanism for token access control.
  • a limiting factor lies from the fact that the EIP-20 approve function is defined in terms of msg.sender.
    • this means that user’s initial action involving EIP-20 tokens must be performed by an EOA.
    • if the user needs to interact with a smart contract, then they need to make 2 transactions (approve and the smart contract internal call transferFrom), with gas costs.

ERC20-compliant token contract


  • totalSupply: Returns the total units of this token that currently exist. ERC20 tokens can have a fixed or a variable supply.
  • balanceOf: Given an address, returns the token balance of that address.
  • transfer: Given an address and amount, transfers that amount of tokens to that address, from the balance of the address that executed the transfer.
  • transferFrom: Given a sender, recipient, and amount, transfers tokens from one account to another. Used in combination with approve.
  • approve: given a recipient address and amount, authorizes that address to execute several transfers up to that amount, from the account that issued the approval.
  • allowance: given an owner address and a spender address, returns the remaining amount that the spender is approved to withdraw from the owner.
  • Transfer: event triggered upon a successful transfer (call to transfer or transferFrom) (even for zero-value transfers).
  • Approval: event logged upon a successful call to approve.

ERC20 optional functions


  • in addition to the required functions listed in the previous section, the following optional functions are also defined by the standard:
    • name: returns the human-readable name (e.g., "US Dollars") of the token.
    • symbol: returns a human-readable symbol (e.g., "USD") for the token.
    • decimals: returns the number of decimals used to divide token amounts. For example, if decimals is 2, then the token amount is divided by 100 to get its user representation.

the ERC20 interface


contract ERC20 {
   function totalSupply() constant returns (uint theTotalSupply);
   function balanceOf(address _owner) constant returns (uint balance);
   function transfer(address _to, uint _value) returns (bool success);
   function transferFrom(address _from, address _to, uint _value) returns
      (bool success);
   function approve(address _spender, uint _value) returns (bool success);
   function allowance(address _owner, address _spender) constant returns
      (uint remaining);
   event Transfer(address indexed _from, address indexed _to, uint _value);
   event Approval(address indexed _owner, address indexed _spender, uint _value);
}