structs.md

Structs

Structs are similar to classes. They contain field and function members. Unlike classes, structs are value types and by default are allocated on the stack instead of the heap. A variable with a struct type directly contains the data of the struct. A variable of a class type contains a reference to the object which contains the data.

struct
    : access_modifier struct_capability "ref"? struct_kind pseudo_reference "struct" identifier //...
    ;

struct_capability
    : "const"
    | // epsilon
    ;

struct_kind
    : "move"
    | "copy"
    ;

pseudo_reference
    : "ref" "[" identifier "]"

Struct Kinds

There are two main kinds of structs. This kind determines the semantics of assigning values of that type. Additionally, there are enum structs documented in a separate section.

Struct Type	Declaration Syntax	Semantics
Move Structs	move struct	Struct values "move" when assigned so that they can no longer be used.
Copy Structs	copy struct	Struct values are implicitly copied when assigned.

The kind of struct determines the default assignment behavior, but it is often possible to explicitly cause the other behavior.

These kinds are independent of whether the struct is a ref struct (see Ref Structs).

Const Structs

Similar to classes, structs can be declared with the const modifier. This indicates that all references inside of it must be const and that any field bindings are immutable (i.e. let). The compiler can then assume that values of this type will not break the isolation of a reference.

Pseudo-Reference Structs

TODO: Redo section now that structs have reference capabilities

Sometimes it is useful to create a struct that acts as if it were a simple reference to an object on the heap down to even having a reference capability. For example, this is how array slices are implemented efficiently. To allow this, the struct must be distinguished from a regular struct which would not be prefixed by a reference capability, and the reference capability of a given declaration must be available to the code of the struct. These structs are declared with the ref[C] syntax. This should not be confused with a standard ref struct which allows stack reference types to be stored in the struct. Instead, a pseudo-reference struct causes the compiler to treat the struct type as if it were a reference type and give each declaration of the type a reference capability that is made available within the the struct as the capability C.

Pseudo-reference structs may have all the other standard struct modifiers. Most pseudo-references should be copy structs, but a move struct allows for the pseudo-reference to act as if it were a move class and have a destructor or reference a move class. A ref pseudo-reference confines the pseudo-reference to the stack and allows it to contain stack references.