docs/050-breaking-changes.rst
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 <https://github.com/argotorg/solidity/releases/tag/v0.5.0>_.
.. 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. See the
:ref:Interoperability With Older Contracts <interoperability> section below.
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.
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.
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 = z to uint[] storage x = z, 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.
Explicit conversions between unrelated contract types are now disallowed. You can only
convert from a contract type to one of its base or ancestor types. If you are sure that
a contract is compatible with the contract type you want to convert to, although it does not
inherit from it, you can work around this by converting to address first.
Example: if A and B are contract types, B does not inherit from A and
b is a contract of type B, you can still convert b to type A using A(address(b)).
Note that you still need to watch out for matching payable fallback functions, as explained below.
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 :ref:withdraw pattern<withdrawal_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. Before v0.5.0 any
conversion between bytesX and uintY would go through uint8X. For
example uint8(bytes3(0x291807)) would be converted to uint8(uint24(bytes3(0x291807)))
(the result is 0x07).
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.
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.
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 reduces 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 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.
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).
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.
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.
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.
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.
.. _interoperability:
It is still possible to interface with contracts written for Solidity versions prior to v0.5.0 (or the other way around) by defining interfaces for them. Consider you have the following pre-0.5.0 contract already deployed:
.. code-block:: solidity
// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.4.25;
// This will report a warning until version 0.4.25 of the compiler
// This will not compile after 0.5.0
contract OldContract {
function someOldFunction(uint8 a) {
//...
}
function anotherOldFunction() constant returns (bool) {
//...
}
// ...
}
This will no longer compile with Solidity v0.5.0. However, you can define a compatible interface for it:
.. code-block:: solidity
// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.5.0 <0.9.0;
interface OldContract {
function someOldFunction(uint8 a) external;
function anotherOldFunction() external returns (bool);
}
Note that we did not declare anotherOldFunction to be view, despite it being declared constant in the original
contract. This is due to the fact that starting with Solidity v0.5.0 staticcall is used to call view functions.
Prior to v0.5.0 the constant keyword was not enforced, so calling a function declared constant with staticcall
may still revert, since the constant function may still attempt to modify storage. Consequently, when defining an
interface for older contracts, you should only use view in place of constant in case you are absolutely sure that
the function will work with staticcall.
Given the interface defined above, you can now easily use the already deployed pre-0.5.0 contract:
.. code-block:: solidity
// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.5.0 <0.9.0;
interface OldContract {
function someOldFunction(uint8 a) external;
function anotherOldFunction() external returns (bool);
}
contract NewContract {
function doSomething(OldContract a) public returns (bool) {
a.someOldFunction(0x42);
return a.anotherOldFunction();
}
}
Similarly, pre-0.5.0 libraries can be used by defining the functions of the library without implementation and
supplying the address of the pre-0.5.0 library during linking (see :ref:commandline-compiler for how to use the
commandline compiler for linking):
.. code-block:: solidity
// This will not compile after 0.6.0
// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.5.0;
library OldLibrary {
function someFunction(uint8 a) public returns(bool);
}
contract NewContract {
function f(uint8 a) public returns (bool) {
return OldLibrary.someFunction(a);
}
}
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:
.. code-block:: solidity
// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.4.25;
// This will not compile after 0.5.0
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 memory 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:
.. code-block:: solidity
// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.5.0;
// This will not compile after 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;
require(y == -2);
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 AddressMakePayable 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.makePayable();
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 AddressMakePayable {
function makePayable(address x) internal pure returns (address payable) {
return address(uint160(x));
}
}