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For example, an object that uses memoization to cache the results of expensive computations could still be considered an immutable object. Strings and other concrete objects are typically expressed as immutable objects to improve readability and runtime efficiency in object-oriented programming.
The immutable keyword denotes data that cannot be modified through any reference. The const keyword denotes a non-mutable view of mutable data. Unlike C++ const, D const and immutable are "deep" or transitive, and anything reachable through a const or immutable object is const or immutable respectively. Example of const vs. immutable in D
In C#, a class is a reference type while a struct (concept derived from the struct in C language) is a value type. [5] Hence an instance derived from a class definition is an object while an instance derived from a struct definition is said to be a value object (to be precise a struct can be made immutable to represent a value object declaring attributes as readonly [6]).
An object being immutable confers some advantages in program design. For instance, it may be "copied" simply by copying its pointer or reference, avoiding a time-consuming copy operation and conserving memory. Object-oriented languages such as C++ extend constantness even further.
The main difference between an arbitrary data structure and a purely functional one is that the latter is (strongly) immutable. This restriction ensures the data structure possesses the advantages of immutable objects: (full) persistency, quick copy of objects, and thread safety.
There are multiple ways to implement the flyweight pattern. One example is mutability: whether the objects storing extrinsic flyweight state can change. Immutable objects are easily shared, but require creating new extrinsic objects whenever a change in state occurs. In contrast, mutable objects can share state.
In object-oriented programming, "immutable interface" is a pattern for designing an immutable object. [1] The immutable interface pattern involves defining a type which does not provide any methods which mutate state. Objects which are referenced by that type are not seen to have any mutable state, and appear immutable.
For example, strings and arrays are passed by reference, but when modified, they are duplicated if they have non-zero reference counts. This allows them to act as value types without the performance problems of copying on assignment or making them immutable. [8] In the Qt framework, many types are copy-on-write ("implicitly shared" in Qt's terms).