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Explain gas implications of constant and immutable variables.

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chriseth 2020-07-27 10:40:33 +02:00
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docs/contracts/constant-state-variables.rst
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@ -12,6 +12,14 @@ for ``immutable``, it can still be assigned at construction time.
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 the respective value. replaced by the respective value.
Compared to regular state variables, the gas costs of constant and immutable variables
are much lower. For a constant variable, the expression assigned to it is copied to
all the places where it is accessed and also re-evaluated each time. This allows for local
optimizations. Immutable variables are evaluated once at construction time and their value
is copied to all the places in the code where they are accessed. For these values,
32 bytes are reserved, even if they would fit in fewer bytes. Due to this, constant values
can sometimes be cheaper than immutable values.
Not all types for constants and immutables are implemented at this time. The only supported types are Not all types for constants and immutables are implemented at this time. The only supported types are
`strings <strings>`_ (only for constants) and `value types <value-types>`_. `strings <strings>`_ (only for constants) and `value types <value-types>`_.