mirror of
https://github.com/ethereum/solidity
synced 2023-10-03 13:03:40 +00:00
b39814a4f8
Constants should conform to style guide at https://solidity.readthedocs.io/en/latest/style-guide.html#constants
36 lines
1.5 KiB
ReStructuredText
36 lines
1.5 KiB
ReStructuredText
.. index:: ! constant
|
|
|
|
************************
|
|
Constant State Variables
|
|
************************
|
|
|
|
State variables can be declared as ``constant``. In this case, they have to be
|
|
assigned from an expression which is a constant at compile time. Any expression
|
|
that accesses storage, blockchain data (e.g. ``now``, ``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.
|
|
|
|
The compiler does not reserve a storage slot for these variables, and every occurrence is
|
|
replaced by the respective constant expression (which might be computed to a single value by the optimizer).
|
|
|
|
Not all types for constants are implemented at this time. The only supported types are
|
|
value types and strings.
|
|
|
|
::
|
|
|
|
pragma solidity >=0.4.0 <0.7.0;
|
|
|
|
contract C {
|
|
uint constant X = 32**22 + 8;
|
|
string constant TEXT = "abc";
|
|
bytes32 constant MY_HASH = keccak256("abc");
|
|
}
|