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There is no way in solidity to check if an address is a contract. One of the goals of Ethereum is for humans and smart contracts to both be treated equally.3 N, _ L1 M( k$ d4 S
https://stackoverflow.com/a/37670490/7218912/ w; \- Y9 C R2 W! o+ _
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有时候确实有需要向智能合约转账:2 t( C# _+ Q0 u# S8 x( s: E6 a
但是大多数 smart contract 的 token 是不能转出的,只要合约里没写转出的逻辑,就不能转出。以太坊将智能合约看做是独立的个体,没人知道它的私钥,给不支持转出的智能合约转 token 就等于销毁了。
当我们要转账时,会转到交易方的地址,不会闲着没事给智能合约转 token。但是这种事情仍然会发生,不断有人给 EOSTokenContract 转 EOS(见下图),EOSTokenContract 账户下现在有 154,834 EOS,价值 $1,176,738.4(价格 $7.6)。类似的,QtumTokenContract 账户下现在有 22,293 QTUM,价值 $158,280.3(价格 $7.1)。+ ^- ?$ Q: a3 o* u3 h4 W
这些 token 都都都都销毁了 (⊙?⊙)
Dexaran 认识到了这个问题,
Contracts that are not designed to work with tokens must reject incoming token transactions. Otherwise, each token becomes a potential token trap.
ERC20 token standard issues.(google docs)8 k3 p2 B. _4 |5 h! t
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并在 github 创建了一个 issue。
tokenFallback 函数,当转账到不能转出的智能合约地址时,将自动取消。
contract ERC223 {5 o- e5 c, f# q9 Z: l. v$ B
function transfer(address to, uint value, bytes data) {8 N4 G( |2 O: |- z" a8 ~
uint codeLength;" h- C$ I2 C0 r! J
assembly {, I; ^0 Y- P5 N$ i6 m
codeLength := extcodesize(_to)
}
balances[msg.sender] = balances[msg.sender].sub(_value);4 E& g$ R6 i' q
balances[_to] = balances[_to].add(_value);* }: u$ S$ R1 a( A" b' F
if(codeLength>0) {8 f7 F0 r( |" m1 X, N" ^+ }
// Require proper transaction handling.. m6 @. O, T1 i3 b0 ]$ N# U
ERC223Receiver receiver = ERC223Receiver(_to);3 q" T# }8 @$ u! y, ^# r0 ^! l
receiver.tokenFallback(msg.sender, _value, _data);
} h$ m) H& F1 p; [+ |% k
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【todo】:代码解释
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The biggest change is that ERC223 no longer allow token to be transferred to a contract that does not allow token to be withdrawn.$ Z' V+ |9 q' @* T1 i$ f1 k
https://medium.com/cryptomover/what-are-erc20-and-erc223-tokens-307badcca5a