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Split Constant State Variables doc

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Chris Ward 2019-01-07 14:14:26 +01:00
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36
docs/contracts.rst
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@ -441,41 +441,7 @@ all symbols visible from the function are visible in the modifier. Symbols
introduced in the modifier are not visible in the function (as they might introduced in the modifier are not visible in the function (as they might
change by overriding). change by overriding).
.. index:: ! constant .. include:: contracts/constant-state-variables.rst
************************
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 <0.6.0;
contract C {
uint constant x = 32**22 + 8;
string constant text = "abc";
bytes32 constant myHash = keccak256("abc");
}
.. index:: ! functions .. index:: ! functions

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docs/contracts/constant-state-variables.rst Normal file
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.. 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 <0.6.0;
contract C {
uint constant x = 32**22 + 8;
string constant text = "abc";
bytes32 constant myHash = keccak256("abc");
}