mirror of
https://github.com/ethereum/solidity
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100 lines
4.3 KiB
ReStructuredText
100 lines
4.3 KiB
ReStructuredText
.. index:: ! constant
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.. _constants:
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**************************************
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Constant and Immutable State Variables
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**************************************
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State variables can be declared as ``constant`` or ``immutable``.
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In both cases, the variables cannot be modified after the contract has been constructed.
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For ``constant`` variables, the value has to be fixed at compile-time, while
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for ``immutable``, it can still be assigned at construction time.
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It is also possible to define ``constant`` variables at the file level.
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The compiler does not reserve a storage slot for these variables, and every occurrence is
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replaced by the respective value.
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Compared to regular state variables, the gas costs of constant and immutable variables
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are much lower. For a constant variable, the expression assigned to it is copied to
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all the places where it is accessed and also re-evaluated each time. This allows for local
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optimizations. Immutable variables are evaluated once at construction time and their value
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is copied to all the places in the code where they are accessed. For these values,
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32 bytes are reserved, even if they would fit in fewer bytes. Due to this, constant values
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can sometimes be cheaper than immutable values.
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Not all types for constants and immutables are implemented at this time. The only supported types are
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:ref:`strings <strings>` (only for constants) and :ref:`value types <value-types>`.
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.. code-block:: solidity
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// SPDX-License-Identifier: GPL-3.0
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pragma solidity >=0.7.4;
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uint constant X = 32**22 + 8;
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contract C {
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string constant TEXT = "abc";
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bytes32 constant MY_HASH = keccak256("abc");
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uint immutable decimals;
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uint immutable maxBalance;
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address immutable owner = msg.sender;
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constructor(uint decimals_, address ref) {
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decimals = decimals_;
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// Assignments to immutables can even access the environment.
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maxBalance = ref.balance;
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}
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function isBalanceTooHigh(address other) public view returns (bool) {
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return other.balance > maxBalance;
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}
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}
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Constant
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========
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For ``constant`` variables, the value has to be a constant at compile time and it has to be
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assigned where the variable is declared. Any expression
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that accesses storage, blockchain data (e.g. ``block.timestamp``, ``address(this).balance`` or
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``block.number``) or
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execution data (``msg.value`` or ``gasleft()``) or makes calls to external contracts is disallowed. Expressions
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that might have a side-effect on memory allocation are allowed, but those that
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might have a side-effect on other memory objects are not. The built-in functions
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``keccak256``, ``sha256``, ``ripemd160``, ``ecrecover``, ``addmod`` and ``mulmod``
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are allowed (even though, with the exception of ``keccak256``, they do call external contracts).
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The reason behind allowing side-effects on the memory allocator is that it
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should be possible to construct complex objects like e.g. lookup-tables.
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This feature is not yet fully usable.
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Immutable
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=========
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Variables declared as ``immutable`` are a bit less restricted than those
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declared as ``constant``: Immutable variables can be assigned an arbitrary
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value in the constructor of the contract or at the point of their declaration.
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They can be assigned only once and can, from that point on, be read even during
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construction time.
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The contract creation code generated by the compiler will modify the
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contract's runtime code before it is returned by replacing all references
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to immutables with the values assigned to them. This is important if
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you are comparing the
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runtime code generated by the compiler with the one actually stored in the
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blockchain. The compiler outputs where these immutables are located in the deployed bytecode
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in the ``immutableReferences`` field of the :ref:`compiler JSON standard output <compiler-api>`.
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.. note::
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Immutables that are assigned at their declaration are only considered
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initialized once the constructor of the contract is executing.
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This means you cannot initialize immutables inline with a value
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that depends on another immutable. You can do this, however,
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inside the constructor of the contract.
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This is a safeguard against different interpretations about the order
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of state variable initialization and constructor execution, especially
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with regards to inheritance.
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