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std.typecons.Rebindable/rebindable - multiple declarations

Function rebindable

This function simply returns the Rebindable object passed in. It's useful in generic programming cases when a given object may be either a regular class or a Rebindable.

Rebindable!T rebindable(T) (
  Rebindable!T obj
);

Parameters

NameDescription
obj An instance of Rebindable!T.

Returns

obj without any modification.

Example

class C
{
    int payload;
    this(int p) { payload = p; }
}
const c = new C(1);

auto c2 = c.rebindable;
writeln(c2.payload); // 1
// passing Rebindable to rebindable
c2 = c2.rebindable;
writeln(c2.payload); // 1

Function rebindable

Convenience function for creating a Rebindable using automatic type inference.

Rebindable!T rebindable(T) (
  T obj
)
if (is(T == class) || is(T == interface) || isDynamicArray!T || isAssociativeArray!T);

Rebindable!T rebindable(T) (
  T value
)
if (!is(T == class) && !is(T == interface) && !isDynamicArray!T && !isAssociativeArray!T && !is(T : Rebindable!U, U));

Parameters

NameDescription
obj A reference to a value to initialize the Rebindable with.

Returns

A newly constructed Rebindable initialized with the given reference.

Example

class C
{
    int payload;
    this(int p) { payload = p; }
}
const c = new C(1);

auto c2 = c.rebindable;
writeln(c2.payload); // 1
// passing Rebindable to rebindable
c2 = c2.rebindable;

c2 = new C(2);
writeln(c2.payload); // 2

const c3 = c2.get;
writeln(c3.payload); // 2

Example

immutable struct S
{
    int[] array;
}
auto s1 = [3].idup.rebindable;
s1 = [4].idup.rebindable;
writeln(s1); // [4]

Alias/Struct Rebindable

Rebindable!(T) is a simple, efficient wrapper that behaves just like an object of type T, except that you can reassign it to refer to another object. For completeness, Rebindable!(T) aliases itself away to T if T is a non-const object type.

alias Rebindable(T) = const(ElementEncodingType!T)[];
struct Rebindable(T)
  
if (!is(T == class) && !is(T == interface) && !isDynamicArray!T && !isAssociativeArray!T);

You may want to use Rebindable when you want to have mutable storage referring to const objects, for example an array of references that must be sorted in place. Rebindable does not break the soundness of D's type system and does not incur any of the risks usually associated with cast.

Alias Rebindable

Struct Rebindable

Constructors

NameDescription
this (value) Constructs a Rebindable from a given value.

Properties

NameTypeDescription
get[get] TReturns the value currently stored in the Rebindable.

Methods

NameDescription
opAssign (value) Overwrites the currently stored value with value.

Parameters

NameDescription
T Any type.

Example

Regular const object references cannot be reassigned. 

class Widget { int x; int y() @safe const { return x; } }
const a = new Widget;
// Fine
a.y();
// error! can't modify const a
// a.x = 5;
// error! can't modify const a
// a = new Widget;

Example

However, Rebindable!(Widget) does allow reassignment, while otherwise behaving exactly like a const Widget. 

class Widget { int x; int y() const @safe { return x; } }
auto a = Rebindable!(const Widget)(new Widget);
// Fine
a.y();
// error! can't modify const a
// a.x = 5;
// Fine
a = new Widget;

Example

Using Rebindable in a generic algorithm: 

import std.range.primitives : front, popFront;

// simple version of std.algorithm.searching.maxElement
typeof(R.init.front) maxElement(R)(R r)
{
    auto max = rebindable(r.front);
    r.popFront;
    foreach (e; r)
        if (e > max)
            max = e; // Rebindable allows const-correct reassignment
    return max;
}
struct S
{
    char[] arr;
    alias arr this; // for comparison
}
// can't convert to mutable
const S cs;
static assert(!__traits(compiles, { S s = cs; }));

alias CS = const S;
CS[] arr = [CS("harp"), CS("apple"), CS("pot")];
CS ms = maxElement(arr);
writeln(ms.arr); // "pot"

Example

static struct S
{
    int* ptr;
}
S s = S(new int);

const cs = s;
// Can't assign s.ptr to cs.ptr
static assert(!__traits(compiles, {s = cs;}));

Rebindable!(const S) rs = s;
assert(rs.ptr is s.ptr);
// rs.ptr is const
static assert(!__traits(compiles, {rs.ptr = null;}));

// Can't assign s.ptr to rs.ptr
static assert(!__traits(compiles, {s = rs;}));

const S cs2 = rs;
// Rebind rs
rs = cs2;
rs = S();
assert(rs.ptr is null);

Authors

Andrei Alexandrescu, Bartosz Milewski, Don Clugston, Shin Fujishiro, Kenji Hara

License

Boost License 1.0.