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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

NameDescription
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

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.

struct Nullable(T) ;

Practically Nullable!T stores a T and a bool.

Constructors

NameDescription
this (value) Constructor initializing this with value.

Properties

NameTypeDescription
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] boolCheck if this is in the null state.

Methods

NameDescription
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 std.format.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

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.

struct Nullable(T, T nullValue) ;

Constructors

NameDescription
this (value) Constructor initializing this with value.

Properties

NameTypeDescription
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] boolCheck if this is in the null state.

Methods

NameDescription
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

NameDescription
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.