******************************** Solidity v0.7.0 Breaking Changes ******************************** This section highlights the main breaking changes introduced in Solidity version 0.7.0, along with the reasoning behind the changes and how to update affected code. For the full list check `the release changelog `_. Silent Changes of the Semantics =============================== * Exponentiation and shifts of literals by non-literals (e.g. ``1 << x`` or ``2 ** x``) will always use either the type ``uint256`` (for non-negative literals) or ``int256`` (for negative literals) to perform the operation. Previously, the operation was performed in the type of the shift amount / the exponent which can be misleading. Changes to the Syntax ===================== * In external function and contract creation calls, Ether and gas is now specified using a new syntax: ``x.f{gas: 10000, value: 2 ether}(arg1, arg2)``. The old syntax -- ``x.f.gas(10000).value(2 ether)(arg1, arg2)`` -- will cause an error. * The global variable ``now`` is deprecated, ``block.timestamp`` should be used instead. The single identifier ``now`` is too generic for a global variable and could give the impression that it changes during transaction processing, whereas ``block.timestamp`` correctly reflects the fact that it is just a property of the block. * NatSpec comments on variables are only allowed for public state variables and not for local or internal variables. * The token ``gwei`` is a keyword now (used to specify, e.g. ``2 gwei`` as a number) and cannot be used as an identifier. * String literals now can only contain printable ASCII characters and this also includes a variety of escape sequences, such as hexadecimal (``\xff``) and unicode escapes (``\u20ac``). * Unicode string literals are supported now to accommodate valid UTF-8 sequences. They are identified with the ``unicode`` prefix: ``unicode"Hello 😃"``. * State Mutability: The state mutability of functions can now be restricted during inheritance: Functions with default state mutability can be overridden by ``pure`` and ``view`` functions while ``view`` functions can be overridden by ``pure`` functions. At the same time, public state variables are considered ``view`` and even ``pure`` if they are constants. Inline Assembly --------------- * Disallow ``.`` in user-defined function and variable names in inline assembly. It is still valid if you use Solidity in Yul-only mode. * Slot and offset of storage pointer variable ``x`` are accessed via ``x.slot`` and ``x.offset`` instead of ``x_slot`` and ``x_offset``. Removal of Unused or Unsafe Features ==================================== Mappings outside Storage ------------------------ * If a struct or array contains a mapping, it can only be used in storage. Previously, mapping members were silently skipped in memory, which is confusing and error-prone. * Assignments to structs or arrays in storage does not work if they contain mappings. Previously, mappings were silently skipped during the copy operation, which is misleading and error-prone. Functions and Events -------------------- * Visibility (``public`` / ``external``) is not needed for constructors anymore: To prevent a contract from being created, it can be marked ``abstract``. This makes the visibility concept for constructors obsolete. * Type Checker: Disallow ``virtual`` for library functions: Since libraries cannot be inherited from, library functions should not be virtual. * Multiple events with the same name and parameter types in the same inheritance hierarchy are disallowed. * ``using A for B`` only affects the contract it is mentioned in. Previously, the effect was inherited. Now, you have to repeat the ``using`` statement in all derived contracts that make use of the feature. Expressions ----------- * Shifts by signed types are disallowed. Previously, shifts by negative amounts were allowed, but reverted at runtime. * The ``finney`` and ``szabo`` denominations are removed. They are rarely used and do not make the actual amount readily visible. Instead, explicit values like ``1e20`` or the very common ``gwei`` can be used. Declarations ------------ * The keyword ``var`` cannot be used anymore. Previously, this keyword would parse but result in a type error and a suggestion about which type to use. Now, it results in a parser error. Interface Changes ================= * JSON AST: Mark hex string literals with ``kind: "hexString"``. * JSON AST: Members with value ``null`` are removed from JSON output. * NatSpec: Constructors and functions have consistent userdoc output. How to update your code ======================= This section gives detailed instructions on how to update prior code for every breaking change. * Change ``x.f.value(...)()`` to ``x.f{value: ...}()``. Similarly ``(new C).value(...)()`` to ``new C{value: ...}()`` and ``x.f.gas(...).value(...)()`` to ``x.f{gas: ..., value: ...}()``. * Change ``now`` to ``block.timestamp``. * Change types of right operand in shift operators to unsigned types. For example change ``x >> (256 - y)`` to ``x >> uint(256 - y)``. * Repeat the ``using A for B`` statements in all derived contracts if needed. * Remove the ``public`` keyword from every constructor. * Remove the ``internal`` keyword from every constructor and add ``abstract`` to the contract (if not already present). * Change ``_slot`` and ``_offset`` suffixes in inline assembly to ``.slot`` and ``.offset``, respectively.