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.
https://stackoverflow.com/a/37670490/7218912
有时候确实有需要向智能合约转账：
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)。
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)
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 {
function transfer(address to, uint value, bytes data) {
uint codeLength;
assembly {
codeLength := extcodesize(_to)
}
balances[msg.sender] = balances[msg.sender].sub(_value);
balances[_to] = balances[_to].add(_value);
if(codeLength>0) {
// Require proper transaction handling.
ERC223Receiver receiver = ERC223Receiver(_to);
receiver.tokenFallback(msg.sender, _value, _data);
}
}
}
【todo】：代码解释
The biggest change is that ERC223 no longer allow token to be transferred to a contract that does not allow token to be withdrawn.
https://medium.com/cryptomover/what-are-erc20-and-erc223-tokens-307badcca5a
