Frequently Asked Questions

This list was originally compiled by fivedogit.

Basic Questions

What is the transaction “payload”?

This is just the bytecode “data” sent along with the request.

Create a contract that can be killed and return funds

First, a word of warning: Killing contracts sounds like a good idea, because “cleaning up” is always good, but as seen above, it does not really clean up. Furthermore, if Ether is sent to removed contracts, the Ether will be forever lost.

If you want to deactivate your contracts, it is preferable to disable them by changing some internal state which causes all functions to throw. This will make it impossible to use the contract and ether sent to the contract will be returned automatically.

Now to answering the question: Inside a constructor, msg.sender is the creator. Save it. Then selfdestruct(creator); to kill and return funds.

example

Note that if you import "mortal" at the top of your contracts and declare contract SomeContract is mortal { ... and compile with a compiler that already has it (which includes Remix), then kill() is taken care of for you. Once a contract is “mortal”, then you can contractname.kill.sendTransaction({from:eth.coinbase}), just the same as my examples.

If I return an `enum`, I only get integer values in web3.js. How to get the named values?

Enums are not supported by the ABI, they are just supported by Solidity. You have to do the mapping yourself for now, we might provide some help later.

Can state variables be initialized in-line?

Yes, this is possible for all types (even for structs). However, for arrays it should be noted that you must declare them as static memory arrays.

Examples:

pragma solidity >=0.4.0 <0.6.0;

contract C {
    struct S {
        uint a;
        uint b;
    }

    S public x = S(1, 2);
    string name = "Ada";
    string[4] adaArr = ["This", "is", "an", "array"];
}

contract D {
    C c = new C();
}

What are some examples of basic string manipulation (`substring`, `indexOf`, `charAt`, etc)?

There are some string utility functions at stringUtils.sol which will be extended in the future. In addition, Arachnid has written solidity-stringutils.

For now, if you want to modify a string (even when you only want to know its length), you should always convert it to a bytes first:

pragma solidity >=0.4.0 <0.6.0;

contract C {
    string s;

    function append(byte c) public {
        bytes(s).push(c);
    }

    function set(uint i, byte c) public {
        bytes(s)[i] = c;
    }
}

Can I concatenate two strings?

Yes, you can use abi.encodePacked:

pragma solidity >=0.4.0 <0.6.0;

library ConcatHelper {
    function concat(bytes memory a, bytes memory b)
            internal pure returns (bytes memory) {
        return abi.encodePacked(a, b);
    }
}

Why is the low-level function `.call()` less favorable than instantiating a contract with a variable (`ContractB b;`) and executing its functions (`b.doSomething();`)?

If you use actual functions, the compiler will tell you if the types or your arguments do not match, if the function does not exist or is not visible and it will do the packing of the arguments for you.

See ping.sol and pong.sol.

What happens if you send ether along with a function call to a contract?

It gets added to the total balance of the contract, just like when you send ether when creating a contract. You can only send ether along to a function that has the payable modifier, otherwise an exception is thrown.

Advanced Questions

How do you get a random number in a contract? (Implement a self-returning gambling contract.)

Getting randomness right is often the crucial part in a crypto project and most failures result from bad random number generators.

If you do not want it to be safe, you build something similar to the coin flipper but otherwise, rather use a contract that supplies randomness, like the RANDAO.

Get return value from non-constant function from another contract

The key point is that the calling contract needs to know about the function it intends to call.

See ping.sol and pong.sol.

How do you create 2-dimensional arrays?

See 2D_array.sol.

Note that filling a 10x10 square of uint8 + contract creation took more than 800,000 gas at the time of this writing. 17x17 took 2,000,000 gas. With the limit at 3.14 million… well, there’s a pretty low ceiling for what you can create right now.

Note that merely “creating” the array is free, the costs are in filling it.

Note2: Optimizing storage access can pull the gas costs down considerably, because 32 uint8 values can be stored in a single slot. The problem is that these optimizations currently do not work across loops and also have a problem with bounds checking. You might get much better results in the future, though.

How do I initialize a contract with only a specific amount of wei?

Currently the approach is a little ugly, but there is little that can be done to improve it. In the case of a contract A calling a new instance of contract B, parentheses have to be used around new B because B.value would refer to a member of B called value. You will need to make sure that you have both contracts aware of each other’s presence and that contract B has a payable constructor. In this example:

pragma solidity >0.4.99 <0.6.0;

contract B {
    constructor() public payable {}
}

contract A {
    B child;

    function test() public {
        child = (new B).value(10)(); //construct a new B with 10 wei
    }
}

Can a contract pass an array (static size) or string or `bytes` (dynamic size) to another contract?¶

Sure. Take care that if you cross the memory / storage boundary, independent copies will be created:

pragma solidity >=0.4.16 <0.6.0;

contract C {
    uint[20] x;

    function f() public {
        g(x);
        h(x);
    }

    function g(uint[20] memory y) internal pure {
        y[2] = 3;
    }

    function h(uint[20] storage y) internal {
        y[3] = 4;
    }
}

The call to g(x) will not have an effect on x because it needs to create an independent copy of the storage value in memory. On the other hand, h(x) successfully modifies x because only a reference and not a copy is passed.

What does the following strange check do in the Custom Token contract?

require((balanceOf[_to] + _value) >= balanceOf[_to]);

Integers in Solidity (and most other machine-related programming languages) are restricted to a certain range. For uint256, this is 0 up to 2**256 - 1. If the result of some operation on those numbers does not fit inside this range, it is truncated. These truncations can have serious consequences, so code like the one above is necessary to avoid certain attacks.

Why are explicit conversions between fixed-size bytes types and integer types failing?

Since version 0.5.0 explicit conversions between fixed-size byte arrays and integers are only allowed, if both types have the same size. This prevents unexpected behaviour when truncating or padding. Such conversions are still possible, but intermediate casts are required that make the desired truncation and padding convention explicit. See Conversions between Elementary Types for a full explanation and examples.

Why can number literals not be converted to fixed-size bytes types?

Since version 0.5.0 only hexadecimal number literals can be converted to fixed-size bytes types and only if the number of hex digits matches the size of the type. See Conversions between Literals and Elementary Types for a full explanation and examples.