2.16. Struct¶

Daslang uses a structure mechanism similar to languages like C/C++, Java, C#, etc. However, there are some important difference. Structures are first class objects like integers or strings and can be stored in table slots, other structures, local variables, arrays, tuples, variants, etc., and passed as function parameters.

2.16.1. Struct Declaration¶

A structure object is created through the keyword struct:

struct Foo
    x, y: int
    xf: float

Sturctures can be private or public:

struct private Foo
    x, y: int

struct public Bar
    xf: float

If not specified, structures inherit module publicity (i.e. in public modules structures are public, and in private modules structures are private).

Structure instances are created through a ‘new expression’ or a variable declaration statement:

let foo: Foo
let foo: Foo? = new Foo

There are intentionally no member functions. There are only data members, since it is a data type itself. Structures can handle members with a function type as data (meaning it’s a function pointer that can be changed during execution). There are initializers that simplify writing complex structure initialization. Basically, a function with same name as the structure itself works as an initializer. The compiler will generate a ‘default’ initializer if there are any members with an initializer:

struct Foo
    x: int = 1
    y: int = 2

Structure fields are initialized to zero by default, regardless of ‘initializers’ for members, unless you specifically call the initializer:

let fZero : Foo     // no initializer is called, x, y = 0
let fInited = Foo() // initializer is called, x = 1, y = 2

Structure field types are inferred, where possible:

struct Foo
    x = 1    // inferred as int
    y = 2.0    // inferred as float

Explicit structure initialization during creation leaves all uninitialized members zeroed:

let fExplicit = [[Foo x=13]]  // x = 13, y = 0

The previous code example is syntactic sugar for:

let fExplicit: Foo
fExplicit.x = 13

Post-construction initialization only needs to specify overwritten fields:

let fPostConstruction = [[Foo() x=13]]  // x = 13, y = 2

The previous code example is syntactic sugar for:

let fPostConstruction: Foo
fPostConstruction.x = 13
fPostConstruction.y = 2

The “Clone initializer” is useful pattern for creating a clone of an existing structure when both structures are on the heap:

def Foo ( p : Foo? )                // "clone initializer" takes pointer to existing structure
    var self := *p
    return <- self
...
let a = new [[Foo x=1, y=2.]]       // create new instance of Foo on the heap, initialize it
let b = new Foo(a)                  // clone of b is created here

2.16.2. Structure Function Members¶

Daslang doesn’t have embedded structure member functions, virtual (that can be overridden in inherited structures) or non-virtual. Those features are implemented for classes. For ease of Objected Oriented Programming, non-virtual member functions can be easily emulated with the pipe operator |>:

struct Foo
    x, y: int = 0

def setXY(var thisFoo: Foo; X, Y: int)
    with thisFoo
        x = X
        y = Y

var foo: Foo
foo |> setXY(10, 11)   // this is syntactic sugar for setXY(foo, 10, 11)
setXY(foo, 10, 11)     // exactly same thing as the line above

Since function pointers are a thing, one can emulate ‘virtual’ functions by storing function pointers as members:

struct Foo
    x, y: int = 0
    set = @@setXY

def setXY(var thisFoo: Foo; X, Y: int)
    with thisFoo
        x = X
        y = Y
...
var foo: Foo = Foo()
foo->set(1, 2)  // this one can call something else, if overridden in derived class.
                // It is also just syntactic sugar for function pointer call
invoke(foo.set, foo, 1, 2)  // exactly same thing as above

This makes the difference between virtual and non-virtual calls in the OOP paradigm explicit. In fact, Daslang classes implement virtual functions in exactly this manner.

2.16.3. Inheritance¶

Daslang’s structures support single inheritance by adding a ‘ : ‘, followed by the parent structure’s name in the structure declaration. The syntax for a derived struct is the following:

struct Bar: Foo
    yf: float

When a derived structure is declared, Daslang first copies all base’s members to the new structure and then proceeds with evaluating the rest of the declaration.

A derived structure has all members of its base structure. It is just syntactic sugar for copying all the members manually first.

2.16.4. Alignment¶

Structure size and alignment are similar to that of C++:

individual members are aligned individually
overall structure alignment is that of the largest member’s alignment

Inherited structure alignment can be controlled via the [cpp_layout] annotation:

[cpp_layout (pod=false)]
struct CppS1
    vtable : void?              // we are simulating C++ class
    b : int64 = 2l
    c : int = 3

[cpp_layout (pod=false)]
struct CppS2 : CppS1            // d will be aligned on the class bounds
    d : int = 4

2.16.5. OOP¶

There is sufficient amount of infrastructure to support basic OOP on top of the structures. However, it is already available in form of classes with some fixed memory overhead (see Classes).

It’s possible to override the method of the base class with override syntax. Here an example:

struct Foo
    x, y: int = 0
    set = @@Foo_setXY

def Foo_setXY(var this: Foo; x, y: int)
    this.x = x
    this.y = y

struct Foo3D: Foo
    z: int = 3
    override set = cast<auto> @@Foo3D_setXY

def Foo3D_setXY(var thisFoo: Foo3D; x, y: int)
    thisFoo.x = x
    thisFoo.y = y
    thisFoo.z = -1

It is safe to use the cast keyword to cast a derived structure instance into its parent type:

var f3d: Foo3D = Foo3D()
(cast<Foo> f3d).y = 5

It is unsafe to cast a base struct to it’s derived child type:

var f3d: Foo3D = Foo3D()
def foo(var foo: Foo)
    (cast<Foo3D> foo).z = 5  // error, won't compile

If needed, the upcast can be used with the unsafe keyword:

struct Foo
    x: int

struct Foo2:Foo
    y: int

def setY(var foo: Foo; y: int)  // Warning! Can make awful things to your app if its not really Foo2
    unsafe
        (upcast<Foo2> foo).y = y

As the example above is very dangerous, and in order to make it safer, you can modify it to following:

struct Foo
    x: int
    typeTag: uint = hash("Foo")

struct Foo2:Foo
    y: int
    override typeTag: uint = hash("Foo2")

def setY(var foo: Foo; y: int)  // this won't do anything really bad, but will panic on wrong reference
    unsafe
        if foo.typeTag == hash("Foo2")
            (upcast<Foo2> foo).y = y
            print("Foo2 type references was passed\n")
        else
            assert(false, "Not Foo2 type references was passed\n")