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36 lines
1.5 KiB
ReStructuredText
36 lines
1.5 KiB
ReStructuredText
.. index:: ! constant
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************************
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Constant State Variables
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************************
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State variables can be declared as ``constant``. In this case, they have to be
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assigned from an expression which is a constant at compile time. Any expression
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that accesses storage, blockchain data (e.g. ``now``, ``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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The compiler does not reserve a storage slot for these variables, and every occurrence is
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replaced by the respective constant expression (which might be computed to a single value by the optimizer).
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Not all types for constants are implemented at this time. The only supported types are
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value types and strings.
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::
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pragma solidity >=0.4.0 <0.7.0;
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contract C {
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uint constant x = 32**22 + 8;
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string constant text = "abc";
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bytes32 constant myHash = keccak256("abc");
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}
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