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.
There are several type qualifiers in D but the most commonly used ones are
const and immutable.
immutableIn addition to a static type system, D provides type qualifiers (sometimes also
called "type constructors") that enforce additional constraints on certain
objects. For example an immutable object can only be initialized once and
after that 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 with no
synchronization, because they never change by definition. This also implies that
immutable objects can be cached perfectly.
constObjects can't be modified through a const reference.
A mutable reference to the same object can still modify it.
A const pointer can be created from either a mutable or
immutable object. It is common
for APIs to accept const arguments to ensure they don't modify the input as
that allows the same function to process both mutable and immutable data.
void foo(const char[] s)
{
// if not commented out, next line will
// result in error (can't modify const):
// s[0] = 'x';
import std.stdio : writeln;
writeln(s);
}
// thanks to `const`, both calls will compile:
foo("abcd"); // string is an immutable array
foo("abcd".dup); // .dup returns a mutable copy
Both immutable and const are transitive type qualifiers, which ensures that once
const is applied to a type, it applies recursively to every sub-component of that type.
immutable int* p = new int;
p = null; // error
*p = 5; // error
The element type of a pointer (or array) can be qualified separately:
immutable p = new int; // immutable(int*)
immutable(int)* q = p; // OK, mutable pointer
q = null; // OK
*q = 5; // error
The string type is defined as immutable(char)[]:
string s = "hi";
s ~= " world"; // OK, append new characters
s[0] = 'c'; // error, can't change existing data