[Ethereum] How to get the return values

contract-developmentsolidity

I have a function with 4 return values which are in different types. Within the same contract, I want another function get this return values. For example,

function myFunction1() returns (uint, string, address){
       ......
}
funtion myFunction2(){
    String s = myFunction1();
}

I want string s get the second value returned by myFunction1. How can I do that?

Best Answer

See the section on "destructuring assignments and returning multiple values" in the Solidity documentation.

As an example:

    function f() returns (uint, bool, uint) {
        return (7, true, 2);
    }

    function g() {
        // Declares and assigns the variables. Specifying the type explicitly is not possible.
        var (x, b, y) = f();
    }

However, it should be noted that you can't return dynamically sized values, as per this previous answer.

Related Solutions

[Ethereum] How to return multiple strings from a contract function

There are a few types of string values that you may be referring to.

bytes1 - bytes32: fixed size.
bytes or string: dynamically sized.

Solidity supports functions with multiple return values.

contract MultiReturner {
    function getData() constant returns (bytes32, bytes32) {
        bytes32 a = "abcd";
        bytes32 b = "wxyz";
        return (a, b);
    }

    function getDynamicData() constant returns (bytes, bytes) {
        bytes a;
        a.push('a');
        a.push('b');
        a.push('c');
        bytes b;
        b.push('x');
        b.push('y');
        b.push('z');
        return (a, b);
    }
}

You can do the same with bytes or string but with the limitation that solidity does not support returning dynamically sized values to other functions. This means that you would be able to use the getData function from within another contract and retrieve the return values but you would not be able to retrieve the return values from getDynamicData from within a contract.

contract UsesMultiReturner {
    function doIt() {
        mr = MultiReturner(0x1234);

        // this will work
        var (a, b) = mr.getData();

        // this won't work
        var (a, b) = mr.getDynamicData();
    }
}

You can however retrieve the return values from both getData and getDynamicData when call from outside of the blockchain.

[Ethereum] get the return value of a .call from a contract from the Blockchain

Low-level calls are just a tiny bit above inline assembly, so you can use inline assembly to emulate this feature.

Something like this:

function get(address _addr, string _func) public view returns (bytes data) {
    uint len = 32;
    uint ptr;

    bytes4 sig = bytes4(keccak256(_func));
    assembly {
        ptr := mload(0x40)       
        mstore(ptr, sig)

        let result := call(5000, _addr, 0, ptr, 0x4, ptr, add(len, 0x40))

        if eq(result, 0) {
            revert(0, 0)
        }

        ptr := add(ptr, 0x40)
        mstore(0x40, add(ptr, add(len, 0x40)))
    }

    data = toBytes(ptr, len); 
}

function toBytes(uint _addr, uint _len) internal pure returns (bytes memory bts) {
    bts = new bytes(_len);
    uint btsptr;
    assembly {
        btsptr := add(bts, 0x20)
    }
    copy(_addr, btsptr, _len);
}

Best Answer

Related Solutions

[Ethereum] How to return multiple strings from a contract function

[Ethereum] get the return value of a .call from a contract from the Blockchain

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