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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.

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);

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.

Constructors

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

Properties

Name	Type	Description
`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.

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.
`toString` ()	Gives the string `"Nullable.null"` if `isNull` is `true`. Otherwise, the result is equivalent to calling `formattedWrite` on the underlying value.

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);

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

Function nullable

Parameters

Example

Example

Example

Struct Nullable

Constructors

Properties

Methods

Example

Example

Struct Nullable

Constructors

Properties

Methods

Parameters

Example

Example

Example

Authors

License

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