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`std.typecons.Nullable/nullable` - multiple declarations

Function nullable

Defines a value paired with a distinctive "null" state that denotes the absence of a value. If default constructed, a Nullable!T object starts in the null state. Assigning it renders it non-null. Calling nullify can nullify it again.


						
				auto nullable(T)
				(
				

				  T t
				

				);

Practically Nullable!T stores a T and a bool.

See also: apply, an alternative way to use the payload.

Example

struct CustomerRecord
{
    string name;
    string address;
    int customerNum;
}

Nullable!CustomerRecord getByName(string name)
{
    //A bunch of hairy stuff

    return Nullable!CustomerRecord.init;
}

auto queryResult = getByName("Doe, John");
if (!queryResult.isNull)
{
    //Process Mr. Doe's customer record
    auto address = queryResult.get.address;
    auto customerNum = queryResult.get.customerNum;

    //Do some things with this customer's info
}
else
{
    //Add the customer to the database
}

Example

import std.exception : assertThrown;

auto a = 42.nullable;
assert(!a.isNull);
writeln(a.get); // 42

a.nullify();
assert(a.isNull);
assertThrown!Throwable(a.get);

Example

import std.algorithm.iteration : each, joiner;
Nullable!int a = 42;
Nullable!int b;
// Add each value to an array
int[] arr;
a.each!((n) => arr ~= n);
writeln(arr); // [42]
b.each!((n) => arr ~= n);
writeln(arr); // [42]
// Take first value from an array of Nullables
Nullable!int[] c = new Nullable!int[](10);
c[7] = Nullable!int(42);
writeln(c.joiner.front); // 42

Function nullable

Just like Nullable!T, except that the null state is defined as a particular value. For example, Nullable!(uint, uint.max) is an uint that sets aside the value uint.max to denote a null state. Nullable!(T, nullValue) is more storage-efficient than Nullable!T because it does not need to store an extra bool.


						
				auto nullable(alias nullValue, T)
				(
				

				  T t
				

				)
				

				if (is(typeof(nullValue) == T));

Parameters

Name	Description
T	The wrapped type for which Nullable provides a null value.
nullValue	The null value which denotes the null state of this `Nullable`. Must be of type `T`.

Example

Nullable!(size_t, size_t.max) indexOf(string[] haystack, string needle)
{
    //Find the needle, returning -1 if not found

    return Nullable!(size_t, size_t.max).init;
}

void sendLunchInvite(string name)
{
}

//It's safer than C...
auto coworkers = ["Jane", "Jim", "Marry", "Fred"];
auto pos = indexOf(coworkers, "Bob");
if (!pos.isNull)
{
    //Send Bob an invitation to lunch
    sendLunchInvite(coworkers[pos]);
}
else
{
    //Bob not found; report the error
}

//And there's no overhead
static assert(Nullable!(size_t, size_t.max).sizeof == size_t.sizeof);

Example

import std.exception : assertThrown;

Nullable!(int, int.min) a;
assert(a.isNull);
assertThrown!Throwable(a.get);
a = 5;
assert(!a.isNull);
writeln(a); // 5
static assert(a.sizeof == int.sizeof);

Example

auto a = nullable!(int.min)(8);
writeln(a); // 8
a.nullify();
assert(a.isNull);

Struct Nullable


						
				struct Nullable(T)
				;

Practically Nullable!T stores a T and a bool.

See also: apply, an alternative way to use the payload.

Constructors

Name	Description
`this` (value)	Constructor initializing `this` with `value`.

Properties

Name	Type	Description
`front`[get]	`inout(T)`	Range interface functions.
`get`[get, set]	`inout(T)`	Gets the value if not null. If `this` is in the null state, and the optional parameter `fallback` was provided, it will be returned. Without `fallback`, calling `get` with a null state is invalid.
`isNull`[get]	`bool`	Check if `this` is in the null state.
`length`[get]	`size_t`	Range interface functions.
`save`[get]	`inout(typeof(this))`	Range interface functions.

Methods

Name	Description
`nullify` ()	Forces `this` to the null state.
`opAssign` (value)	Assigns `value` to the internally-held state. If the assignment succeeds, `this` becomes non-null.
`opEquals` (rhs)	If they are both null, then they are equal. If one is null and the other is not, then they are not equal. If they are both non-null, then they are equal if their values are equal.
`opIndex` ()	Range interface functions.
`opSlice` (from, to)	Range interface functions.
`toString` ()	Gives the string `"Nullable.null"` if `isNull` is `true`. Otherwise, the result is equivalent to calling `std.format.formattedWrite` on the underlying value.

Aliases

Name	Description
`back`	Range interface functions.
`empty`	Range interface functions.
`opDollar`	Range interface functions.
`popBack`	Range interface functions.
`popFront`	Range interface functions.

Example

struct CustomerRecord
{
    string name;
    string address;
    int customerNum;
}

Nullable!CustomerRecord getByName(string name)
{
    //A bunch of hairy stuff

    return Nullable!CustomerRecord.init;
}

auto queryResult = getByName("Doe, John");
if (!queryResult.isNull)
{
    //Process Mr. Doe's customer record
    auto address = queryResult.get.address;
    auto customerNum = queryResult.get.customerNum;

    //Do some things with this customer's info
}
else
{
    //Add the customer to the database
}

Example

import std.exception : assertThrown;

auto a = 42.nullable;
assert(!a.isNull);
writeln(a.get); // 42

a.nullify();
assert(a.isNull);
assertThrown!Throwable(a.get);

Example

import std.algorithm.iteration : each, joiner;
Nullable!int a = 42;
Nullable!int b;
// Add each value to an array
int[] arr;
a.each!((n) => arr ~= n);
writeln(arr); // [42]
b.each!((n) => arr ~= n);
writeln(arr); // [42]
// Take first value from an array of Nullables
Nullable!int[] c = new Nullable!int[](10);
c[7] = Nullable!int(42);
writeln(c.joiner.front); // 42

Struct Nullable


						
				struct Nullable(T, T nullValue)
				;

Constructors

Name	Description
`this` (value)	Constructor initializing `this` with `value`.

Properties

Name	Type	Description
`get`[get]	`inout(T)`	Gets the value. `this` must not be in the null state. This function is also called for the implicit conversion to `T`.
`isNull`[get]	`bool`	Check if `this` is in the null state.

Methods

Name	Description
`nullify` ()	Forces `this` to the null state.
`opAssign` (value)	Assigns `value` to the internally-held state. If the assignment succeeds, `this` becomes non-null. No null checks are made. Note that the assignment may leave `this` in the null state.

Parameters

Name	Description
T	The wrapped type for which Nullable provides a null value.
nullValue	The null value which denotes the null state of this `Nullable`. Must be of type `T`.

Example

Nullable!(size_t, size_t.max) indexOf(string[] haystack, string needle)
{
    //Find the needle, returning -1 if not found

    return Nullable!(size_t, size_t.max).init;
}

void sendLunchInvite(string name)
{
}

//It's safer than C...
auto coworkers = ["Jane", "Jim", "Marry", "Fred"];
auto pos = indexOf(coworkers, "Bob");
if (!pos.isNull)
{
    //Send Bob an invitation to lunch
    sendLunchInvite(coworkers[pos]);
}
else
{
    //Bob not found; report the error
}

//And there's no overhead
static assert(Nullable!(size_t, size_t.max).sizeof == size_t.sizeof);

Example

import std.exception : assertThrown;

Nullable!(int, int.min) a;
assert(a.isNull);
assertThrown!Throwable(a.get);
a = 5;
assert(!a.isNull);
writeln(a); // 5
static assert(a.sizeof == int.sizeof);

Example

auto a = nullable!(int.min)(8);
writeln(a); // 8
a.nullify();
assert(a.isNull);

Authors

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

License

Boost License 1.0.

API Documentation

std.typecons.Nullable/nullable - multiple declarations

Function nullable

Example

Example

Example

Function nullable

Parameters

Example

Example

Example

Struct Nullable

Constructors

Properties

Methods

Aliases

Example

Example

Example

Struct Nullable

Constructors

Properties

Methods

Parameters

Example

Example

Example

Authors

License

`std.typecons.Nullable/nullable` - multiple declarations