Frequently Asked Questions

This list was originally compiled by fivedogit.

Basic Questions

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.

How do structs work?

See struct_and_for_loop_tester.sol.

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 {

    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.

Are comments included with deployed contracts and do they increase deployment gas?

No, everything that is not needed for execution is removed during compilation. This includes, among others, comments, variable names and type names.

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 {

    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.

More Questions?

If you have more questions or your question is not answered here, please talk to us on gitter or file an issue.