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Struct `std.variant.VariantN`

Back-end type seldom used directly by user code. Two commonly-used types using VariantN are:


						
				struct VariantN(ulong maxDataSize, AllowedTypesParam...)
				;

Algebraic: A closed discriminated union with a limited type universe (e.g., Algebraic!(int, double, string) only accepts these three types and rejects anything else).
Variant: An open discriminated union allowing an unbounded set of types. If any of the types in the Variant are larger than the largest built-in type, they will automatically be boxed. This means that even large types will only be the size of a pointer within the Variant, but this also implies some overhead. Variant can accommodate all primitive types and all user-defined types.

Both Algebraic and Variant share VariantN's interface. (See their respective documentations below.)

VariantN is a discriminated union type parameterized with the largest size of the types stored (maxDataSize) and with the list of allowed types (AllowedTypes). If the list is empty, then any type up of size up to maxDataSize (rounded up for alignment) can be stored in a VariantN object without being boxed (types larger than this will be boxed).

Constructors

Name	Description
`this` (value)	Constructs a `VariantN` value given an argument of a generic type. Statically rejects disallowed types.
`this` (value)	Allows assignment from a subset algebraic type

Properties

Name	Type	Description
`coerce`[get]	`T`	Returns the value stored in the `VariantN` object, explicitly converted (coerced) to the requested type `T`. If `T` is a string type, the value is formatted as a string. If the `VariantN` object is a string, a parse of the string to type `T` is attempted. If a conversion is not possible, throws a `VariantException`.
`convertsTo`[get]	`bool`	Returns `true` if and only if the `VariantN` object holds an object implicitly convertible to type `T`. Implicit convertibility is defined as per AllImplicitConversionTargets.
`get`[get]	`inout(T)`	Returns the value stored in the `VariantN` object, either by specifying the needed type or the index in the list of allowed types. The latter overload only applies to bounded variants (e.g. `Algebraic`).
`hasValue`[get]	`bool`	Returns true if and only if the `VariantN` object holds a valid value (has been initialized with, or assigned from, a valid value).
`length`[get]	`size_t`	If the `VariantN` contains an (associative) array, returns the length of that array. Otherwise, throws an exception.
`peek`[get]	`inout(T)*`	If the `VariantN` object holds a value of the exact type `T`, returns a pointer to that value. Otherwise, returns `null`. In cases where `T` is statically disallowed, `peek` will not compile.
`type`[get]	`TypeInfo`	Returns the `typeid` of the currently held value.

Methods

Name	Description
`opApply` (dg)	If the `VariantN` contains an array, applies `dg` to each element of the array in turn. Otherwise, throws an exception.
`opAssign` (rhs)	Assigns a `VariantN` from a generic argument. Statically rejects disallowed types.
`opBinary` (rhs)	Arithmetic between `VariantN` objects and numeric values. All arithmetic operations return a `VariantN` object typed depending on the types of both values involved. The conversion rules mimic D's built-in rules for arithmetic conversions.
`opBinaryRight` (lhs)	Arithmetic between `VariantN` objects and numeric values. All arithmetic operations return a `VariantN` object typed depending on the types of both values involved. The conversion rules mimic D's built-in rules for arithmetic conversions.
`opCmp` (rhs)	Ordering comparison used by the "<", "<=", ">", and ">=" operators. In case comparison is not sensible between the held value and `rhs`, an exception is thrown.
`opEquals` (rhs)	Comparison for equality used by the "==" and "!=" operators.
`opIndex` (i)	Array and associative array operations. If a `VariantN` contains an (associative) array, it can be indexed into. Otherwise, an exception is thrown.
`opIndexAssign` (value, i)	Array and associative array operations. If a `VariantN` contains an (associative) array, it can be indexed into. Otherwise, an exception is thrown.
`opIndexOpAssign` (value, i)	Array and associative array operations. If a `VariantN` contains an (associative) array, it can be indexed into. Otherwise, an exception is thrown.
`opOpAssign` (rhs)	Arithmetic between `VariantN` objects and numeric values. All arithmetic operations return a `VariantN` object typed depending on the types of both values involved. The conversion rules mimic D's built-in rules for arithmetic conversions.
`toHash` ()	Computes the hash of the held value.
`toString` ()	Formats the stored value as a string.

Aliases

Name	Description
`AllowedTypes`	The list of allowed types. If empty, any type is allowed.

Example

alias Var = VariantN!(maxSize!(int, double, string));

Var a; // Must assign before use, otherwise exception ensues
// Initialize with an integer; make the type int
Var b = 42;
writeln(b.type); // typeid (int)
// Peek at the value
assert(b.peek!(int) !is null && *b.peek!(int) == 42);
// Automatically convert per language rules
auto x = b.get!(real);

// Assign any other type, including other variants
a = b;
a = 3.14;
writeln(a.type); // typeid (double)
// Implicit conversions work just as with built-in types
assert(a < b);
// Check for convertibility
assert(!a.convertsTo!(int)); // double not convertible to int
// Strings and all other arrays are supported
a = "now I'm a string";
writeln(a); // "now I'm a string"

Example

can also assign arrays

alias Var = VariantN!(maxSize!(int[]));

Var a = new int[42];
writeln(a.length); // 42
a[5] = 7;
writeln(a[5]); // 7

Example

Can also assign class values

alias Var = VariantN!(maxSize!(int*)); // classes are pointers
Var a;

class Foo {}
auto foo = new Foo;
a = foo;
assert(*a.peek!(Foo) == foo); // and full type information is preserved

Authors

Andrei Alexandrescu

License

Boost License 1.0.

API Documentation

Struct std.variant.VariantN

Constructors

Properties

Methods

Aliases

Example

Example

Example

Authors

License

Struct `std.variant.VariantN`