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Solidity idiosyncrasies

Solidity gotchas, pitfalls, limitations, and idiosyncrasies.

This is a list of things that have caused me to bang my head against a brick wall when coming across them in solidity, especially when starting out as a beginner.

Notice! These examples are from Solidity v0.4.x. Some of these example may no longer be relevant in newer versions of Solidity.

Contents

• Examples
• Contributing
• Issues
• Credits
• Resources
• License

Examples

In no particular order:

• Using delete on an array leaves a gap; need to shift items manually and update the length property.

  contract MyContract {
    uint[] array = [1,2,3];
  
    function removeAtIndex(uint index) returns (uint[]) {
      if (index >= array.length) return;
  
      for (uint i = index; i < array.length-1; i++) {
        array[i] = array[i+1];
      }
  
      array.length--;
  
      return array;
    }
  }
  

• Can't return struct for external methods; need to return multiple values.

  contract MyContract {
    struct MyStruct {
      string str;
      uint i;
    }
  
    MyStruct myStruct;
  
    function MyContract() {
      myStruct = MyStruct("foo", 1);
    }
  
    function myMethod() external returns (string, uint) {
      return (myStruct.str, myStruct.i);
    }
  }
  

• Can't compare two strings; one easy workaround is to compare the sha3 hashes of the strings.

  contract MyContract {
    function compare(string s1, string s2) returns (bool) {
      return (sha3(s1) == sha3(s2));
    }
  }
  

  or compare byte by byte which is more performant. Utility libraries such as solidity-stringutils and solidity-bytes-utils provide helper functions for string comparison and have good examples.

• Can't compare address to 0 to check if it's empty; need to compare to address(0).

  contract MyContract {
    function isEmptyAddress(address addr) returns (bool) {
      return (addr == address(0));
    }
  }
  

• string has no length property; need to manually check string length in characters.

  contract MyContract {
    string str = "foo";
  
    function MyContract() {
      assert(size(str) == 3);
    }
  
    function size(string s) returns (uint) {
      uint length = 0;
      uint i = 0;
      bytes memory strBytes = bytes(s);
  
      while (i < strBytes.length) {
        if (strBytes[i]>>7 == 0) {
          i+=1;
        } else if (strBytes[i]>>5 == 0x6) {
          i+=2;
        } else if (strBytes[i]>>4 == 0xE) {
          i+=3;
        } else if (strBytes[i]>>3 == 0x1E) {
          i+=4;
        } else {
          i+=1;
        }
  
        length++;
      }
    }
  }
  

  Don't use bytes(str).length to check string length because encoded strings can differ in length, for example, a single character encoded in UTF-8 can be more than a byte long.

• Can't pass array of strings as argument to external function (from web3); need to do manual serializing and deserializing.

• Can't typecast address to string; need to manually convert using bytes.

  contract MyContract {
    string public str;
  
    function MyContract() {
      str = toString(msg.sender);
    }
  
    function toString(address addr) returns (string) {
      bytes memory b = new bytes(20);
      for (uint i = 0; i < 20; i++) {
        b[i] = byte(uint8(uint(addr) / (2**(8*(19 - i)))));
      }
      return string(b);
    }
  }
  

• Can't typecast bytes to string; need to do manual conversion.

  contract MyContract {
    bytes32 bts = "foo";
    string str;
  
    function MyContract() {
      str = bytes32ToString(bts);
    }
  
    function bytes32ToString(bytes32 x) constant returns (string) {
      bytes memory bytesString = new bytes(32);
      uint charCount = 0;
      for (uint j = 0; j < 32; j++) {
        byte char = byte(bytes32(uint(x) * 2 ** (8 * j)));
        if (char != 0) {
          bytesString[charCount] = char;
          charCount++;
        }
      }
      bytes memory bytesStringTrimmed = new bytes(charCount);
      for (j = 0; j < charCount; j++) {
        bytesStringTrimmed[j] = bytesString[j];
      }
      return string(bytesStringTrimmed);
    }
  }
  

  The helper library solidity-stringutils has more string typecasting examples.

• Can't easily convert bytes to address: need to manually convert each byte to uint160:

  function bytesToAddress(bytes _address) public returns (address) {
    uint160 m = 0;
    uint160 b = 0;
  
    for (uint8 i = 0; i < 20; i++) {
      m *= 256;
      b = uint160(_address[i]);
      m += (b);
    }
  
    return address(m);
  }
  

• The type is only deduced from the first assignment when using var; so this can be dangerous in certain scenarios where it's initialized to a smaller data type then expected, causing undesired consequences, like the following:

  contract MyContract {
    function loop() {
      /* `i` will have max a max value of 255 (initialized as uint8),
       * causing an infinite loop.
       */
      for (var i = 0; i < 1000; i++) {
  
      }
    }
  }
  

  It's best practice to use an explicit type (var is now deprecated).

• uint is alias to uint256.

• byte is alias to bytes1.

• sha3 is alias to keccak256. (keccak256 is preferred)

• now is alias to block.timestamp.

• bytes is the same byte[] but packed tightly (more expensive).

• string is the same as bytes but doesn't allow length or index access.

• Any type that can be converted to uint160 can be converted to address.

• address is equivalent to uint160**.

• public vs external vs internal vs private

  • external: function is part of the contract interface, which means it can be called from other contracts and via transactions. External functions are sometimes more efficient when they receive large arrays of data. Use external if you expect that the function will only ever be called externally. For external functions, the compiler doesn't need to allow internal calls, and so it allows arguments to be read directly from calldata, saving the copying step, which will save more gas. Also note that external functions cannot be inherited by other contracts!

  • public: function can either be called internally or externally. For public state variables, an automatic getter function is generated.

  • internal: function or state variables can only be accessed internally (i.e. from within the current contract or contracts deriving from it), without using this.

  • private: function or state variable is only visible for the contract they are defined in and not in derived contracts.

• msg.sender is the contract caller; (aka contract creator if in the constructor).

• There's a limit to how many variables can be in a function; this includes parameter and return variables. The limit is 16 variables, otherwise you get the StackTooDeepException error "Internal compiler error: Stack too deep, try removing local variables.".

  However if you need that many variables, then you're probably doing something wrong. You can break up the function into smaller functions, and set global variables to public to generate getters.

• enum variable type get compiled down to an int8; (unless the enum has more than 8 options, in which case it walks up the int type scale).

  contract MyContract {
    enum MyEnum {
      Foo,
      Bar,
      Qux
    }
  
    function MyContract() {
      assert(uint(MyEnum.Foo) == 0);
      assert(uint(MyEnum.Bar) == 1);
      assert(uint(MyEnum.Qux) == 2);
    }
  }
  

• You have to use new keyword for creating variable length in-memory arrays. As opposed to storage arrays, it's not possible to resize memory arrays by assigning to the length member.

  contract MyContract {
    uint[] foo;
  
    function MyContract() {
      uint[] memory bar = new uint[](5);
      bytes memory qux = new bytes(5);
  
      // dynamically resize storage array
      foo.length = 6;
      foo[5] = 1;
      assert(foo[5] == 1);
  
      // doesn't work, will throw
      bar.length = 6;
      qux.length = 6;
    }
  }
  

• The type of an array literal is a memory array of fixed size whose base type is the common type of the given elements; e.g. the type of [1, 2, 3] is uint8[3] memory, because the type of each of these constants is uint8. Fixed size memory arrays can't be assigned to dynamically-sized memory arrays, e.g. the following is not possible:

  contract MyContract {
    function MyContract() {
      /* This creates a `TypeError` because uint8[3] memory
       * can't be converted to uint256[] memory.
       */
      uint[3] memory x = [1, 2, 3];
  
      // This works, because it's the same common type.
      uint8[3] memory y = [1, 2, 3];
  
      // This works, because it's the same common type.
      uint16[3] memory z = [256, 2, 3];
    }
  }
  

• Declaring a local array and assuming it'll be created in memory but it actually overwrites storage; e.g. the type of the local variable x is uint[] storage, but it has to be assigned from a state variable before it can be used because storage is not dynamically allocated, so it functions only as an alias for a pre-existing variable in storage. What happens is that the compiler interprets x as a storage pointer and will make it point to the storage slot 0 by default, which in this case is variable foo, and is modified by x.push(1) causing an error.

  contract MyContract {
    uint foo;
    uint[] bar;
  
    // This will not work!
    function MyContract() {
      uint[] x;
      x.push(1);
      bar = x;
    }
  }
  

  do this instead:

  contract MyContract() {
    uint foo;
    uint[] bar;
  
    function MyContract() {
      uint[] memory x = new uint[](5);
      x[0] = 1;
      bar = x;
  
      assert(foo == 0);
      assert(bar[0] == 1);
    }
  }
  

• Solidity inherits scoping rules from JavaScript; a variable declared anywhere within a function will be in scope for the entire function, regardless of where it's declared. There is no block scoping, e.g. the following examples:

  contract MyContract {
    function MyContract() {
      uint i = 0;
  
      while (i++ < 1) {
        uint foo = 0;
      }
  
      while (i++ < 2) {
        // Illegal, second declaration of variable.
        uint foo = 0;
      }
    }
  }
  

  contract MyContract {
    function MyContract() {
      for (uint i = 0; i < 1; i ++) {
  
      }
  
      // Illegal, second declaration of variable.
      for (uint i = 0; i < 1; i++) {
  
      }
    }
  }
  

• Integers will be truncated if they don't fit with the type range; e.g. for uint256 the range is 0 up to 2^256 - 1, so if the result of an operation does not fit within the range then it's trucated and there can be serious consequences. Always perform assertions before modifying state e.g. making sure sender has enough token balance before sending to recipient.

  require((balanceOf[to] + value) >= balanceOf[to]);
  

• Exceptions consume all the gas; EVM's only exception is Out of Gas, typically caused by an invalid JUMP error.

• throw is being deprecated in favor of revert(), require(), assert().

• Calls to external functions can fail, so always check return value; like when using send().

• Use transfer() instead of send(); transfer() is equivalent of require(x.send(y)) (will throw if not successful).

  There are some dangers in using send: The transfer fails if the call stack depth is at 1024 (this can always be forced by the caller) and it also fails if the recipient runs out of gas.

  A better solution is to use the pattern where the recipient withdraws the money.

• Calls are limited to a depth of 1024; which means that for more complex operations, loops should be preferred over recursive calls.

• Have to declare the source file compiler version at the top of the contract file; the ^ means to use the latest patch release (0.4.x), so it uses the most update to date version without any breaking changes.

  pragma solidity ^0.4.4;
  
  contract MyContract {
  
  }
  

• All primitive data types are initialized with default values; there is no "null" data type (like in JavaScript).

  contract MyContract {
    int n; // 0
    string str; // ""
    address addr; // 0x0000000000000000000000000000000000000000
    bool b; // false
  }
  

• Date suffixes after literal numbers can be used to convert between units of time, where seconds are the base unit and units are considered naively in the following way:

  contract MyContract {
    function MyContract() {
      assert(1 == 1 seconds);
      assert(1 minutes == 60 seconds);
      assert(1 hours == 60 minutes);
      assert(1 days == 24 hours);
      assert(1 weeks == 7 days);
      assert(1 years == 365 days);
    }
  }
  

  But take care if you perform calendar calculations using these units; not every year equals 365 days and not even every day has 24 hours because of leap seconds. Due to the fact that leap seconds cannot be predicted, an exact calendar library has to be updated by an external oracle.

• Date suffixes can't be applied to variables; here's how you can interpret some input variable in, e.g days:

  contract MyContract {
    function hasStarted(uint start, uint daysAfter) returns (bool) {
      return (now >= start + daysAfter * 1 days);
    }
  }
  

• Generating random numbers is hard; because Ethereum a deterministic system. You can generate a "random" number based on the block hash and block number, but keep in mind that miners have influence on these values.

  contract MyContract {
    function rand(uint min, uint max) public returns (uint) {
      return uint(block.blockhash(block.number-1))%(min+max)-min;
    }
  }
  

• Have to use indexed keyword for events parameters to allow events them to be searchable; it allows you to search for the events using the indexed parameters as filters.

  contract MyContract {
    event Transfer(address indexed sender, address indexed recipient, uint256 amount);
  }
  

  // Filter by indexed parameter.
  myContract.events.Transfer({sender: '0x123...abc'}, (error, events) => {})
  
  /* Can't filter by un-indexed parameter,
   * so this won't work.
   */
  myContract.events.Transfer({amount: 1}, (error, events) => {})
  

• Only up to three parameters can receive the attribute indexed for event parameters.

• You can specify named output parameters in returns signature which creates new local variables.

  contract MyContract {
    function MyContract() {
      assert(myMethod() == 10);
    }
  
    function myMethod() returns (uint num) {
      num = 10;
      return num;
    }
  }
  

  but make sure to rename it different than storage variables since it can override them.

• Need to use payable modifier to allow function to receive ether; otherwise the transaction will be rejected.

• assert will use up all remaining gas; after Metropolis, require behaves like revert which refunds remaining gas which is preferable. Use assert for runtime error catching where conditions should never ever be possible.

• Contracts can't activate themselves; they need a "poke", e.g. a contract can't automatically do something when it reaches a certain block number (like a cron job). There needs to be a call from the outside for the contract to do something; an external poke.

Remix

• Need to pass an array of single bytes instead of string for addresses; e.g. "0x2680EA4C9AbAfAa63C2957DD3951017d5BBAc518" will be interpreted as a string rather than hex bytes. To pass an address represented in bytes you need to break up the address into an array of single bytes, e.g. ["0x26", "0x80", "0xEA", ... "0xBA", "0xc5", "0x18"], when sending in via Remix browser interface.

Example code

Example code available in the contracts/ directory.

Contributing

Pull requests are always welcomed for explaining or showing a solidity feature that is not intuitive.

Issues

Ethereum and Solidity are quickly evolving so some things may no longer be relevant in the future.

Please submit an issue or make a pull request if something in incorrect.

Credits

• Credits to the people contributing on Ethereum Stack Exchange, where I read a lot of the solutions from.

Resources

• Solidity

• Truffle

• Smart Contract Best Practices

• Solidity Baby Steps

• Ethereum Stack Exchange

License

MIT