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Similarly, a n agent that receives a reference to data from an outside source must assume the worst-that the data is shared-unless an alternative arrangement is known, such as only ever being called by an agent that guarantees the data is isolated Again, the lack of type system and verifi cation support makes this convention notoriously tricky to implement and manage in real programs, particularly when multiple developers are involved
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Immutabi lity As noted earlier, read / read "hazards" are not really hazardous at all Many threads can safely read from some shared memory location concurrently without cause for concern Therefore, if some piece of shared state is guar anteed to be immutable-that is, read-only-then accessing it from many threads inside a concurrent system will be safe Proving that a piece of complex data is immutable is not terribly difficult with some discipline Both C++ and NET offer constructs to help make immutable types If each of an object's fields never changes during its life time, it is shallow immutable If the object's fields also only refer to objects whose state does not change over time, the object is deeply immutable An entire object graph can be transitively immutable if all objects within it are themselves deeply immutable In the case that data transitions between private and shared throughout its lifetime, as discussed above in the context of isolation, it is sometimes useful to have a conditionally-immutable type, in which it remains immutable so long as it is shared but can be mutated while private So, for example, a thread may remove a piece of shared state from public view, making it temporarily private, mutate it, and then later share the state again to public view
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A popular technique for enforcing the immutability of data is to use single assignment variables Many programming systems offer static verification that certain data is indeed assigned a value only once, leading to the term static single assignment, or SSA The CLR offers limited support for single assignment variables in its common type system through the i n i tonly field modifier, surfaced in C#
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through the r e a d o n l y keyword And C++ offers the c o n st modifier to achieve a similar effect, though it is far more powerful: pointers may be marked as being con st, ensuring (statically) that the instance referred to is not modified by the user of such a pointer (though unlike readonly C++ pro grammers can explicitly "cast away the con st-ness" of a reference, bypass ing the safety guarantees) Using these constructs can be tremendously useful because it avoids having to depend on brittle and subtle program ming convention and rules Let's look at each briefly
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CLR ini ton Ly Fields (aka C# readon Ly Fields) When you mark a field as readonly in C#, the compiler emits a field with the i n i t o n ly modifier in the resulting IL The only writes to such variables that will pass the type system's verification process are those that occur inside that type's constructor or field initializers This ensures that the value of such a field cannot change after the object has been constructed While it is not a true single assignment variable, as it can be written multiple times during initialization, it is similar in spirit Another subtle issue can arise if a reference to an object with r e a d o n l y fields escapes from its constructor Fields are not guaranteed to have been initialized with the permanent immutable values until after the constructor has finished running and could be assigned multiple values during the con struction process If an object's constructor shares itself before finishing ini tialization, then other concurrent threads in the system cannot safely depend on the r e a d o n l y nature of the fields Letting the object's t h i s reference escape before the object is fully constructed like this is a bad practice any way, and is easily avoided When a field is marked r e a d o n ly, it simply means the field's value cannot change In other words, a type with only readonly fields is shallow immutable but not necessarily deeply immutable If an object depends on the state of the objects it references, then those objects should be immutable also Unfortunately, the CLR offers no type system support for building deeply immutable types Of course they may be immutable by convention, r e a d o n l y fields, or a combination of both There are some cases where the mutability of referenced objects does not matter For example, if we had an immutable pair class that refers to two mutable objects but never accesses the state of those objects, then is the pair itself immutable
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