Storage classes

D is a statically typed language: once a variable has been declared, its type can't be changed from that point onwards. This allows the compiler to prevent bugs early and enforce restrictions at compile time. Good type-safety provides the support one needs to make large programs safer and more maintainable.


In addition to a static type system, D provides storage classes that enforce additional constraints on certain objects. For example, an immutable object can only be initialized once and then isn't allowed to change.

immutable int err = 5;
// or: immutable err = 5 and int is inferred.
err = 5; // won't compile

immutable objects can thus be safely shared among different threads because they never change by design. This implies that immutable objects can be cached perfectly.


const objects can't be modified either. This restriction is only valid for the current scope. A const pointer can be created from either a mutable or immutable object. This means that the object is const for your current scope, but someone else might modify it in the future. Only with an immutable can you be certain that an object's value will never change. It is common for APIs to accept const objects to ensure they don't modify the input.

immutable a = 10;
int b = 5;
const int* pa = &a;
const int* pb = &b;
*pa = 7; // disallowed

Both immutable and const are transitive which ensures that once const is applied to a type, it applies recursively to every sub-component of that type.


Basic references

Advanced references

rdmd playground.d