diff options
Diffstat (limited to 'docs')
| -rw-r--r-- | docs/contracts.rst | 26 |
1 files changed, 20 insertions, 6 deletions
diff --git a/docs/contracts.rst b/docs/contracts.rst index 1516ba0b..9145f016 100644 --- a/docs/contracts.rst +++ b/docs/contracts.rst @@ -428,8 +428,25 @@ change by overriding). Constant State Variables ************************ -State variables can be declared as constant (this is not yet implemented -for array and struct types and not possible for mapping types). +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``, ``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 +might have a side-effect on other memory objects are not. The built-in functions +``keccak256``, ``sha256``, ``ripemd160``, ``ecrecover``, ``addmod`` and ``mulmod`` +are allowed (ever though 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. :: @@ -438,12 +455,9 @@ for array and struct types and not possible for mapping types). contract C { uint constant x = 32**22 + 8; string constant text = "abc"; + bytes32 constant myHash = keccak256("abc"); } -This has the effect that the compiler does not reserve a storage slot -for these variables and every occurrence is replaced by their constant value. - -The value expression can only contain integer arithmetics. ****************** Constant Functions |