mirror of
https://github.com/ethereum/solidity
synced 2023-10-03 13:03:40 +00:00
82 lines
3.5 KiB
ReStructuredText
82 lines
3.5 KiB
ReStructuredText
.. index:: ! constant
|
|
|
|
**************************************
|
|
Constant and Immutable State Variables
|
|
**************************************
|
|
|
|
State variables can be declared as ``constant`` or ``immutable``.
|
|
In both cases, the variables cannot be modified after the contract has been constructed.
|
|
For ``constant`` variables, the value has to be fixed at compile-time, while
|
|
for ``immutable``, it can still be assigned at construction time.
|
|
|
|
The compiler does not reserve a storage slot for these variables, and every occurrence is
|
|
replaced by the respective value.
|
|
|
|
Compared to regular state variables, the gas costs of constant and immutable variables
|
|
are much lower. For a constant variable, the expression assigned to it is copied to
|
|
all the places where it is accessed and also re-evaluated each time. This allows for local
|
|
optimizations. Immutable variables are evaluated once at construction time and their value
|
|
is copied to all the places in the code where they are accessed. For these values,
|
|
32 bytes are reserved, even if they would fit in fewer bytes. Due to this, constant values
|
|
can sometimes be cheaper than immutable values.
|
|
|
|
Not all types for constants and immutables are implemented at this time. The only supported types are
|
|
:ref:`strings <strings>` (only for constants) and :ref:`value types <value-types>`.
|
|
|
|
::
|
|
|
|
// SPDX-License-Identifier: GPL-3.0
|
|
pragma solidity >0.6.99 <0.9.0;
|
|
|
|
contract C {
|
|
uint constant X = 32**22 + 8;
|
|
string constant TEXT = "abc";
|
|
bytes32 constant MY_HASH = keccak256("abc");
|
|
uint immutable decimals;
|
|
uint immutable maxBalance;
|
|
address immutable owner = msg.sender;
|
|
|
|
constructor(uint _decimals, address _reference) {
|
|
decimals = _decimals;
|
|
// Assignments to immutables can even access the environment.
|
|
maxBalance = _reference.balance;
|
|
}
|
|
|
|
function isBalanceTooHigh(address _other) public view returns (bool) {
|
|
return _other.balance > maxBalance;
|
|
}
|
|
}
|
|
|
|
|
|
Constant
|
|
========
|
|
|
|
For ``constant`` variables, the value has to be a constant at compile time and it has to be
|
|
assigned where the variable is declared. Any expression
|
|
that accesses storage, blockchain data (e.g. ``block.timestamp``, ``address(this).balance`` or
|
|
``block.number``) or
|
|
execution data (``msg.value`` or ``gasleft()``) or makes calls to external contracts is disallowed. Expressions
|
|
that might have a side-effect on memory allocation are allowed, but those that
|
|
might have a side-effect on other memory objects are not. The built-in functions
|
|
``keccak256``, ``sha256``, ``ripemd160``, ``ecrecover``, ``addmod`` and ``mulmod``
|
|
are allowed (even though, with the exception of ``keccak256``, they do call external contracts).
|
|
|
|
The reason behind allowing side-effects on the memory allocator is that it
|
|
should be possible to construct complex objects like e.g. lookup-tables.
|
|
This feature is not yet fully usable.
|
|
|
|
Immutable
|
|
=========
|
|
|
|
Variables declared as ``immutable`` are a bit less restricted than those
|
|
declared as ``constant``: Immutable variables can be assigned an arbitrary
|
|
value in the constructor of the contract or at the point of their declaration.
|
|
They cannot be read during construction time and can only be assigned once.
|
|
|
|
The contract creation code generated by the compiler will modify the
|
|
contract's runtime code before it is returned by replacing all references
|
|
to immutables by the values assigned to the them. This is important if
|
|
you are comparing the
|
|
runtime code generated by the compiler with the one actually stored in the
|
|
blockchain.
|