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174 lines
5.7 KiB
Markdown
174 lines
5.7 KiB
Markdown
[< back to index](../doc_index.md)
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# Types
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Millfork puts extra limitations on which types can be used in which contexts.
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## Numeric types
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* `byte` – 1-byte value of undefined signedness, defaulting to unsigned
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* `word` – 2-byte value of undefined signedness, defaulting to unsigned
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(alias: `int16`)
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* `int24` – 3-byte value of undefined signedness, defaulting to unsigned
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(alias: `farword`; this alias is deprecated and will be removed in the future)
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* `long` – 4-byte value of undefined signedness, defaulting to unsigned
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(alias: `int32`)
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* `int40`, `int48`,... `int128` – even larger types
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* `sbyte` – signed 1-byte value
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* `ubyte` – unsigned 1-byte value
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* `pointer` – raw pointers; the same as `word`, but variables of this type default to be zero-page-allocated
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and you can index `pointer`-typed expressions.
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You can create pointer values by suffixing `.addr` to the name of a variable, function or array.
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You can access single bytes of variables by using the following notations:
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* for 2-byte-sized variables: `.lo` for the least significant byte and `.hi` for the most significant byte
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* for larger variables: `.b0` for the least significant byte and then `.b1`, `.b2` and so on
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You can also access words that are parts of variables:
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* for 3-byte-sized variables: `.loword` is the word formed from `.b1` and `.b0` and `.hiword` is the word formed from `.b2` and `.b1`
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* for 4-byte-sized variables: `.loword` is the word formed from `.b1` and `.b0` and `.hiword` is the word formed from `.b3` and `.b2`
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Numeric types can be converted automatically:
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* from a smaller type to a bigger type (`byte`→`word`)
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* from a type of undefined signedness to a type of defined signedness (`byte`→`sbyte`)
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* from a type of defined signedness to a type of undefined signedness (`sbyte`→`byte`)
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## Typed pointers
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For every type `T`, there is a pointer type defined called `pointer.T`.
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Unlike raw pointers, they are not subject to arithmetic.
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If the type `T` is of size 1, you can index the pointer like a raw pointer.
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If the type `T` is of size 2, you can index the pointer only with the constant 0.
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Examples:
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pointer.t p
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p.raw // expression of type pointer, pointing to the same location in memory as 'p'
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p.lo // equivalent to 'p.raw.lo'
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p.hi // equivalent to 'p.raw.lo'
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p[0] // valid only if the type 't' is of size 1 or 2, accesses the pointed element
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p[i] // valid only if the type 't' is of size 1, equivalent to 't(p.raw[i])'
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p->x // valid only if the type 't' has a field called 'x', accesses the field 'x' of the pointed element
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p->x.y[0]->z[0][6] // you can stack it
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## `nullptr`
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There is a 2-byte constant `nullptr` that can be assigned to any 2-byte pointer type.
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Its actual value is defined using the feature `NULLPTR`, by default it's 0.
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`nullptr` isn't directly assignable to non-pointer types.
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## Boolean types
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TODO
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## Special types
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* `void` – a unit type containing no information, can be only used as a return type for a function.
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## Enumerations
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Enumeration is a 1-byte type that represents a set of values:
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enum <name> { <variants, separated by commas or newlines> }
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The first variant has value 0. Every next variant has a value increased by 1 compared to a previous one.
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Alternatively, a variant can be given a custom constant value, which will change the sequence.
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If there is at least one variant and no variant is given a custom constant value,
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then the enumeration is considered _plain_. Plain enumeration types can be used as array keys.
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For plain enumerations, a constant `<name>.count` is defined,
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equal to the number of variants in the enumeration.
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Assigment between numeric types and enumerations is not possible without an explicit type cast:
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enum E { EA, EB }
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byte b
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E e
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e = EA // ok
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e = b // won't compile
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b = e // won't compile
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b = byte(e) // ok
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e = E(b) // ok
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array a[E] // E is plain, array has size 2
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a[0] // won't compile
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a[EB] // ok
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Plain enumerations have their variants equal to `byte(0)` to `byte(<name>.count - 1)`.
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Tip: You can use an enumeration with no variants as a strongly checked alternative byte type,
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as there are no checks on values when converting bytes to enumeration values and vice versa.
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## Structs
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Struct is a compound type containing multiple fields of various types:
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struct <name> { <field definitions (type and name), separated by commas or newlines>}
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A struct is represented in memory as a contiguous area of variables laid out one after another.
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Struct can have a maximum size of 255 bytes. Larger structs are not supported.
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You can access a field of a struct with the dot:
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struct point { word x, word y }
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point p
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p.x = 3
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p.y.lo = 4
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Offsets are available as `structname.fieldname.offset`:
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pointer ptr
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ptr = p.addr
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ptr += point.y.offset
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// ptr points now at p.y
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// alternatively:
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ptr = p.y.addr
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You can create constant expressions of struct types using so-called struct constructors, e.g.:
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point(5,6)
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All arguments to the constructor must be constant.
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## Unions
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union <name> { <field definitions (type and name), separated by commas or newlines>}
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Unions are pretty similar to structs, with the difference that all fields of the union
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start at the same point in memory and therefore overlap each other.
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struct point { byte x, byte y }
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union point_or_word { point p, word w }
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point_or_word u
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u.p.x = 0
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u.p.y = 0
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if u.w == 0 { ok() }
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Offset constants are also available, but they're obviously all zero.
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Unions currently do not have an equivalent of struct constructors. This may be improved on in the future.
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