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
objects can be cached perfectly.
const objects can't be modified either. This
restriction is only valid for the current scope. A
pointer can be created from either a mutable or
immutable object. This means that the object
const for your current scope, but someone
else might modify it in the future. Only with an
can you be certain that an object's value will never
change. It is common for APIs to accept
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
const are transitive which ensures that once
const is applied to a type, it applies recursively to every sub-component of that type.