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Structures

6   Structures

They are defined with struct and opt_struct keywords.

        struct s {
                int a;
                string b;
        }
is roughly equivalent of C's:

        typedef struct {
                int a;
                char *b;
        } *s;
So, name s can be used (only) without any struct or opt_struct. For example, this piece of code uses structure definition above:
        string f(s foo)
        {
                foo.a = 5;
                return foo.b;
        }
You should note, that structures are passed by pointer (or by reference if you prefer C++ naming style), therefore changes made to struct inside function are reflected in state of it on the caller side, hence:
        s x;
        ...
        x.a = 10;
        f(x);
        // x.a is 5 here
Fields of structure can be accessed with `.' operator, there is no `->' operator.

6.1   What's opt_struct?

struct value is always valid, i.e. it has to be initialized with object instance, before it is used, and you cannot assign null pointer to it.

opt_struct can be null. You still have to initialize it before, it is used, but you can do it with null keyword. You can also assign null to it later on.

This involves runtime check on each access to opt_struct value. When you try to access opt_struct value, that is null, Null_access exception is raised. This behavior can be controlled with compiler switch.

6.2   Assignment to structures

If you do:
        void f(s1 x)
        {
                s1 y;
                y = x;
                y.fld = 0;  // here you also modify x.fld
        }
In general assigning values other then int's, bool's and float's copies pointer, not content, i.e. makes an alias of an object.

6.3   Structure initializers

When initializing structures one has to spell field name along with expression initializing it. For example:

        struct foo {
                int bar;
                string baz;
        }

        void f()
        {
                foo qux = ({ bar = 1, baz = "quxx" });
        }
({ ... }) is also normal expression, for instance:

        void f() 
        {
                foo(({ bar = 1, baz = "qux" }));
        }
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Structures