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.

Practically Nullable!T stores a T and a bool.

struct Nullable(T)
struct Nullable (
T
) {
this(T value);
bool opEquals(const(typeof(this)) rhs);
bool opEquals(const(U) rhs);
string toString();
string toString();
void toString(W writer, FormatSpec!char fmt);
void toString(W writer, FormatSpec!char fmt);
bool isNull [@property getter];
void nullify();
void opAssign(T value);
inout(T) get [@property getter];
inout(T) get [@property setter];
inout(U) get [@property setter];
}
struct Nullable(T, T nullValue)
auto nullable(T t)

Constructors

this this(T value): Constructor initializing this with value.

Destructor

A destructor is present on this object, but not explicitly documented in the source.

Alias This

get_

Implicitly converts to T. this must not be in the null state. This feature is deprecated and will be removed after 2.096.

Members

Functions

nullify void nullify(): Forces this to the null state.
opAssign void opAssign(T value): Assigns value to the internally-held state. If the assignment succeeds, this becomes non-null.
opEquals bool opEquals(const(typeof(this)) rhs)
bool opEquals(const(U) 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 string toString()
void toString(W writer, FormatSpec!char fmt): Gives the string "Nullable.null" if isNull is true. Otherwise, the result is equivalent to calling std.format.formattedWrite on the underlying value.

Properties

get inout(T) get [@property getter]
inout(T) get [@property setter]
inout(U) get [@property setter]: 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 bool isNull [@property getter]: Check if this is in the null state.

Examples

1 struct CustomerRecord
2 {
3     string name;
4     string address;
5     int customerNum;
6 }
7 
8 Nullable!CustomerRecord getByName(string name)
9 {
10     //A bunch of hairy stuff
11 
12     return Nullable!CustomerRecord.init;
13 }
14 
15 auto queryResult = getByName("Doe, John");
16 if (!queryResult.isNull)
17 {
18     //Process Mr. Doe's customer record
19     auto address = queryResult.get.address;
20     auto customerNum = queryResult.get.customerNum;
21 
22     //Do some things with this customer's info
23 }
24 else
25 {
26     //Add the customer to the database
27 }

import std.exception : assertThrown;

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

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