Immutable data, once initialized, is never changed. This has many uses:
- Access to immutable data need not be synchronized when multiple threads read it.
- Data races, tearing, sequential consistency, and cache consistency are all non-issues when working with immutable data.
- Pure functions can only accept immutable parameters.
- When doing a deep copy of a data structure, the immutable portions need not be copied.
- Invariance allows a large chunk of data to be treated as a value type even if it is passed around by reference (strings are the most common case of this).
- Immutable type provides more self-documenting information to the programmer.
- Immutable data can be placed in hardware protected read-only memory, or even in ROMs.
- If immutable data does change, it is a sure sign of a memory corruption bug, and it is possible to automatically check for such data integrity.
- Immutable types provide for many program optimization opportunities.
- const acts as a bridge between the mutable and immutable worlds, so a single function can be used to accept both types of arguments.
