Solidity v0.5.0 Breaking Changes

This section highlights the main breaking changes introduced in Solidity version 0.5.0, along with the reasoning behind the changes and how to update affected code. For the full list check the release changelog.

Note

Contracts compiled with Solidity v0.5.0 can still interface with contracts and even libraries compiled with older versions without recompiling or redeploying them. Changing the interfaces to include data locations and visibility and mutability specifiers suffices.

Semantic Only Changes

This section lists the changes that are semantic-only, thus potentially hiding new and different behavior in existing code.

  • Signed right shift now uses proper arithmetic shift, i.e. rounding towards negative infinity, instead of rounding towards zero. Signed and unsigned shift will have dedicated opcodes in Constantinople, and are emulated by Solidity for the moment.
  • The continue statement in a do...while loop now jumps to the condition, which is the common behavior in such cases. It used to jump to the loop body. Thus, if the condition is false, the loop terminates.
  • The functions .call(), .delegatecall() and .staticcall() do not pad anymore when given a single bytes parameter.
  • Pure and view functions are now called using the opcode STATICCALL instead of CALL if the EVM version is Byzantium or later. This disallows state changes on the EVM level.
  • The ABI encoder now properly pads byte arrays and strings from calldata (msg.data and external function parameters) when used in external function calls and in abi.encode. For unpadded encoding, use abi.encodePacked.
  • The ABI decoder reverts in the beginning of functions and in abi.decode() if passed calldata is too short or points out of bounds. Note that dirty higher order bits are still simply ignored.
  • Forward all available gas with external function calls starting from Tangerine Whistle.

Semantic and Syntactic Changes

This section highlights changes that affect syntax and semantics.

  • The functions .call(), .delegatecall(), staticcall(), keccak256(), sha256() and ripemd160() now accept only a single bytes argument. Moreover, the argument is not padded. This was changed to make more explicit and clear how the arguments are concatenated. Change every .call() (and family) to a .call("") and every .call(signature, a, b, c) to use .call(abi.encodeWithSignature(signature, a, b, c)) (the last one only works for value types). Change every keccak256(a, b, c) to keccak256(abi.encodePacked(a, b, c)). Even though it is not a breaking change, it is suggested that developers change x.call(bytes4(keccak256("f(uint256)"), a, b) to x.call(abi.encodeWithSignature("f(uint256)", a, b)).
  • Functions .call(), .delegatecall() and .staticcall() now return (bool, bytes memory) to provide access to the return data. Change bool success = otherContract.call("f") to (bool success, bytes memory data) = otherContract.call("f").
  • Solidity now implements C99-style scoping rules for function local variables, that is, variables can only be used after they have been declared and only in the same or nested scopes. Variables declared in the initialization block of a for loop are valid at any point inside the loop.

Explicitness Requirements

This section lists changes where the code now needs to be more explicit. For most of the topics the compiler will provide suggestions.

  • Explicit function visibility is now mandatory. Add public to every function and constructor, and external to every fallback or interface function that does not specify its visibility already.
  • Explicit data location for all variables of struct, array or mapping types is now mandatory. This is also applied to function parameters and return variables. For example, change uint[] x = m_x to uint[] storage x = m_x, and function f(uint[][] x) to function f(uint[][] memory x) where memory is the data location and might be replaced by storage or calldata accordingly. Note that external functions require parameters with a data location of calldata.
  • Contract types do not include address members anymore in order to separate the namespaces. Therefore, it is now necessary to explicitly convert values of contract type to addresses before using an address member. Example: if c is a contract, change c.transfer(...) to address(c).transfer(...), and c.balance to address(c).balance.
  • The address type was split into address and address payable, where only address payable provides the transfer function. An address payable can be directly converted to an address, but the other way around is not allowed. Converting address to address payable is possible via conversion through uint160. If c is a contract, address(c) results in address payable only if c has a payable fallback function. If you use the withdraw pattern, you most likely do not have to change your code because transfer is only used on msg.sender instead of stored addresses and msg.sender is an address payable.
  • Conversions between bytesX and uintY of different size are now disallowed due to bytesX padding on the right and uintY padding on the left which may cause unexpected conversion results. The size must now be adjusted within the type before the conversion. For example, you can convert a bytes4 (4 bytes) to a uint64 (8 bytes) by first converting the bytes4 variable to bytes8 and then to uint64. You get the opposite padding when converting through uint32.
  • Using msg.value in non-payable functions (or introducing it via a modifier) is disallowed as a security feature. Turn the function into payable or create a new internal function for the program logic that uses msg.value.
  • For clarity reasons, the command line interface now requires - if the standard input is used as source.

Deprecated Elements

This section lists changes that deprecate prior features or syntax. Note that many of these changes were already enabled in the experimental mode v0.5.0.

Command Line and JSON Interfaces

  • The command line option --formal (used to generate Why3 output for further formal verification) was deprecated and is now removed. A new formal verification module, the SMTChecker, is enabled via pragma experimental SMTChecker;.
  • The command line option --julia was renamed to --yul due to the renaming of the intermediate language Julia to Yul.
  • The --clone-bin and --combined-json clone-bin command line options were removed.
  • Remappings with empty prefix are disallowed.
  • The JSON AST fields constant and payable were removed. The information is now present in the stateMutability field.
  • The JSON AST field isConstructor of the FunctionDefinition node was replaced by a field called kind which can have the value "constructor", "fallback" or "function".
  • In unlinked binary hex files, library address placeholders are now the first 36 hex characters of the keccak256 hash of the fully qualified library name, surrounded by $...$. Previously, just the fully qualified library name was used. This recudes the chances of collisions, especially when long paths are used. Binary files now also contain a list of mappings from these placeholders to the fully qualified names.

Constructors

  • Constructors must now be defined using the constructor keyword.
  • Calling base constructors without parentheses is now disallowed.
  • Specifying base constructor arguments multiple times in the same inheritance hierarchy is now disallowed.
  • Calling a constructor with arguments but with wrong argument count is now disallowed. If you only want to specify an inheritance relation without giving arguments, do not provide parentheses at all.

Functions

  • Function callcode is now disallowed (in favor of delegatecall). It is still possible to use it via inline assembly.
  • suicide is now disallowed (in favor of selfdestruct).
  • sha3 is now disallowed (in favor of keccak256).
  • throw is now disallowed (in favor of revert, require and assert).

Conversions

  • Explicit and implicit conversions from decimal literals to bytesXX types is now disallowed.
  • Explicit and implicit conversions from hex literals to bytesXX types of different size is now disallowed.

Literals and Suffixes

  • The unit denomination years is now disallowed due to complications and confusions about leap years.
  • Trailing dots that are not followed by a number are now disallowed.
  • Combining hex numbers with unit denominations (e.g. 0x1e wei) is now disallowed.
  • The prefix 0X for hex numbers is disallowed, only 0x is possible.

Variables

  • Declaring empty structs is now disallowed for clarity.
  • The var keyword is now disallowed to favor explicitness.
  • Assignments between tuples with different number of components is now disallowed.
  • Values for constants that are not compile-time constants are disallowed.
  • Multi-variable declarations with mismatching number of values are now disallowed.
  • Uninitialized storage variables are now disallowed.
  • Empty tuple components are now disallowed.
  • Detecting cyclic dependencies in variables and structs is limited in recursion to 256.
  • Fixed-size arrays with a length of zero are now disallowed.

Syntax

  • Using constant as function state mutability modifier is now disallowed.
  • Boolean expressions cannot use arithmetic operations.
  • The unary + operator is now disallowed.
  • Literals cannot anymore be used with abi.encodePacked without prior conversion to an explicit type.
  • Empty return statements for functions with one or more return values are now disallowed.
  • The “loose assembly” syntax is now disallowed entirely, that is, jump labels, jumps and non-functional instructions cannot be used anymore. Use the new while, switch and if constructs instead.
  • Functions without implementation cannot use modifiers anymore.
  • Function types with named return values are now disallowed.
  • Single statement variable declarations inside if/while/for bodies that are not blocks are now disallowed.
  • New keywords: calldata and constructor.
  • New reserved keywords: alias, apply, auto, copyof, define, immutable, implements, macro, mutable, override, partial, promise, reference, sealed, sizeof, supports, typedef and unchecked.

Example

The following example shows a contract and its updated version for Solidity v0.5.0 with some of the changes listed in this section.

Old version:

// This will not compile
pragma solidity ^0.4.25;

contract OtherContract {
   uint x;
   function f(uint y) external {
      x = y;
   }
   function() payable external {}
}

contract Old {
   OtherContract other;
   uint myNumber;

   // Function mutability not provided, not an error.
   function someInteger() internal returns (uint) { return 2; }

   // Function visibility not provided, not an error.
   // Function mutability not provided, not an error.
   function f(uint x) returns (bytes) {
      // Var is fine in this version.
      var z = someInteger();
      x += z;
      // Throw is fine in this version.
      if (x > 100)
         throw;
      bytes b = new bytes(x);
      y = -3 >> 1;
      // y == -1 (wrong, should be -2)
      do {
         x += 1;
         if (x > 10) continue;
         // 'Continue' causes an infinite loop.
      } while (x < 11);
      // Call returns only a Bool.
      bool success = address(other).call("f");
      if (!success)
         revert();
      else {
         // Local variables could be declared after their use.
         int y;
      }
      return b;
   }

   // No need for an explicit data location for 'arr'
   function g(uint[] arr, bytes8 x, OtherContract otherContract) public {
      otherContract.transfer(1 ether);

      // Since uint32 (4 bytes) is smaller than bytes8 (8 bytes),
      // the first 4 bytes of x will be lost. This might lead to
      // unexpected behavior since bytesX are right padded.
      uint32 y = uint32(x);
      myNumber += y + msg.value;
   }
}

New version:

pragma solidity >0.4.99 <0.6.0;

contract OtherContract {
   uint x;
   function f(uint y) external {
      x = y;
   }
   function() payable external {}
}

contract New {
   OtherContract other;
   uint myNumber;

   // Function mutability must be specified.
   function someInteger() internal pure returns (uint) { return 2; }

   // Function visibility must be specified.
   // Function mutability must be specified.
   function f(uint x) public returns (bytes memory) {
      // The type must now be explicitly given.
      uint z = someInteger();
      x += z;
      // Throw is now disallowed.
      require(x > 100);
      int y = -3 >> 1;
      // y == -2 (correct)
      do {
         x += 1;
         if (x > 10) continue;
         // 'Continue' jumps to the condition below.
      } while (x < 11);

      // Call returns (bool, bytes).
      // Data location must be specified.
      (bool success, bytes memory data) = address(other).call("f");
      if (!success)
         revert();
      return data;
   }

   using address_make_payable for address;
   // Data location for 'arr' must be specified
   function g(uint[] memory arr, bytes8 x, OtherContract otherContract, address unknownContract) public payable {
      // 'otherContract.transfer' is not provided.
      // Since the code of 'OtherContract' is known and has the fallback
      // function, address(otherContract) has type 'address payable'.
      address(otherContract).transfer(1 ether);

      // 'unknownContract.transfer' is not provided.
      // 'address(unknownContract).transfer' is not provided
      // since 'address(unknownContract)' is not 'address payable'.
      // If the function takes an 'address' which you want to send
      // funds to, you can convert it to 'address payable' via 'uint160'.
      // Note: This is not recommended and the explicit type
      // 'address payable' should be used whenever possible.
      // To increase clarity, we suggest the use of a library for
      // the conversion (provided after the contract in this example).
      address payable addr = unknownContract.make_payable();
      require(addr.send(1 ether));

      // Since uint32 (4 bytes) is smaller than bytes8 (8 bytes),
      // the conversion is not allowed.
      // We need to convert to a common size first:
      bytes4 x4 = bytes4(x); // Padding happens on the right
      uint32 y = uint32(x4); // Conversion is consistent
      // 'msg.value' cannot be used in a 'non-payable' function.
      // We need to make the function payable
      myNumber += y + msg.value;
   }
}

// We can define a library for explicitly converting ``address``
// to ``address payable`` as a workaround.
library address_make_payable {
   function make_payable(address x) internal pure returns (address payable) {
      return address(uint160(x));
   }
}