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Documentation update.

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chriseth 2017-03-13 13:29:34 +01:00
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docs/contracts.rst
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@ -428,17 +428,22 @@ change by overriding).
Constant State Variables Constant State Variables
************************ ************************
State variables can be declared as constant. In this case, they have to be State variables can be declared as ``constant``. In this case, they have to be
assigned a value or expression which is a constant at compile time. Expressions assigned from an expression which is a constant at compile time. Any expression
that accesses storage, blockchain data (e.g. ``now``, ``this.balance`` or
``block.number``) or
execution data (``msg.gas``) or make calls to external contracts are disallowed. Expressions
that might have a side-effect on memory allocation are allowed, but those that 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. This makes it possible might have a side-effect on other memory objects are not. The built-in functions
to create constant memory arrays as lookup-tables ``keccak256``, ``sha256``, ``ripemd160``, ``ecrecover``, ``addmod`` and ``mulmod``
(although this is not yet fully implemented). are allowed (ever though they do call external contracts).
Expressions that depend on blockchain data like `now`, `this.balance` or
`block.number` or perform any storage access are disallowed. 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 The compiler does not reserve a storage slot for these variables and every occurrence is
replaced by their constant value (which might be computed by the optimizer). 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 Not all types for constants are implemented at this time. The only supported types are
value types and strings. value types and strings.