solidity语言12

View Functions

函数声明为视图，将无权修改状态

pragma solidity ^0.4.16;

contract C {
    function f(uint a, uint b) public view returns (uint) {
        return a * (b + 42) + now;
    }
}

以下操作属违规操作
1. Writing to state variables.
2. Emitting events.
3. Creating other contracts.
4. Using selfdestruct.
5. Sending Ether via calls.
6. Calling any function not marked view or pure.
7. Using low-level calls.
8. Using inline assembly that contains certain opcodes.

Pure Functions

函数声明为视图，将无权读取或者修改状态

pragma solidity ^0.4.16;

contract C {
    function f(uint a, uint b) public pure returns (uint) {
        return a * (b + 42);
    }
}

以下操作属违规操作
1. Reading from state variables.
2. Accessing this.balance or <address>.balance.
3. Accessing any of the members of block, tx, msg (with the exception of msg.sig and msg.data).
4. Calling any function not marked pure.
5. Using inline assembly that contains certain opcodes.

Fallback Function

合约可以有一个匿名函数，这个函数不能有参数，也不能返回结果。假如没有函数匹配给定的函数标识，合约将调用。
函数在合约收到以太后执行，另外，为了能接收以太，回退函数必须使用关键字payable。这次调用花费很少，大约2300 gas

pragma solidity ^0.4.0;

contract Test {
// This function is called for all messages sent to
// this contract (there is no other function).

// Sending Ether to this contract will cause an exception,
// because the fallback function does not have the `payable`
// modifier.
    uint x;

    function() public {
        x = 1;
    }
}

// This contract keeps all Ether sent to it with no way to get it back.
contract Sink {
    function() public payable {}
}

contract Caller {
    function callTest(Test test) public {
        test.call(0xabcdef01);
        // hash does not exist results in test.x becoming == 1.

        // The following will not compile, but even if someone sends ether to that contract,
        // the transaction will fail and reject the Ether.
        //test.send(2 ether);
    }
}