doc/LPC/structs

CONCEPT
        structs

INTRODUCTION
        structs are, next to arrays and mappings, a way to group a
        collection of value together.

        A struct holds a fixed number of values, called 'members', and
        allows to access them by their given name. The name is resolved
        when the LPC code is compiled, making struct member access as fast
        as array member access.

        structs are passed by reference.


DEFINITION
        A new struct type has to be defined at the top level of an
        object. For example

            struct Foo {
              int        one, *two;
              struct Bar three;
            };

        defines the new struct 'Foo' with three members: integer 'one',
        integer array 'two', and struct Bar 'three'

        It is possible to 'inherit' structs from each other. Given above
        definition of struct Foo, the following definition

            struct Quux (Foo) {
              int four;
            };

        is equivalent to the definition

            struct Quux {
              int        one, *two;
              struct Bar three;
              int four;
            };


        The usual visibility modifiers apply, e.g.

            protected struct Bang {...};


        struct definitions are promoted through inheritance like functions,
        with the difference that all structs live in the same flat namespace.
        This means: a struct defined in a program is visible in _all_
        inherited programs, regardless of how deep the inheritance is
        nested.  This also means that in one program there must not be
        two structs, inherited or not, with the same name.


        To declare a struct without defining it, write:

            struct Quux;

        This notation is useful if you have two structs referencing
        each other:

            struct Quux;

            struct Bar {
              struct Quux quux;
            };
            struct Quux {
              struct Bar bar;
            };


USAGE
        To use a struct, its definition must be visible - either because it
        is defined in the object compiled, or it has been inherited.
        (Note: #include'ing structs does not what you think it does: in
        LPC it constructs a new struct type whereever it is included).


        A variable to hold a struct is defined like this:

            struct Foo var;

        and similar for function arguments:

            void fun (struct Foo arg)


        Just writing 'struct Foo var' however does not _create_ a struct,
        it just creates a variable capable of holding one. To assign a value
        to the variable upon creation, assign it with a struct value, either
        from another variable or from a literal struct:

            struct Foo var = (<Foo>);


        Literal structs are written using (<>) as delimiters:

            (<Foo>)
                creates an empty instance of struct Foo

            (<Foo> 1, ({ 2 }), bar)
                creates an instance of struct Foo, and assigns 1 to member
                'one', ({ 2 }) to member 'two', and the content of variable
                bar to member 'three'.

            (<Foo> two: ({ 2 }) )
               creates an instance of struct Foo which is all empty except
               for member 'two' which is assigned the value ({ 2 }).

        It is not possible to use both named and unnamed initializers
        in the same literal.


        A struct member is accessed using the -> operator:

            struct Foo var = ...;

            var->one = 1;


        It is possible to compute struct lookups at runtime:

            struct Foo bar = ...;
            string member = "one";

            bar->(member) = 1; // sets bar->one to 1
            bar->(0) = 1;      // sets bar->one to 1


        When using struct values held in variables/expressions of type
        'mixed', the 'mixed' value should to be casted to the struct
        value. The cast can be omitted if the looked-up member exists
        in only one struct (and its children) known to the compiler:

            struct Foo { int one; };
            struct Bar { int two; };
            struct Baz { int two; };
            mixed var;

                        var->one  // looks up Foo->one
            (struct Foo)var->one  // looks up Foo->one
                        var->two  // ERROR: ambiguous lookup
            (struct Bar)var->one  // looks up Bar->one


USAGE IN CLOSURES
        The #'(< operator can be used in lambda closures to create a
        struct; the type of the struct is given by the 'template'
        struct passed as first argument. The content of the template
        struct is irrelevant, so an empty struct suffices. For
        example, to create an instance of struct Foo:
        
            ({ #'(<, (<Foo>), 1, ({ 2 }), (<Bar>) })

        The order of the member values is the order in which they
        appear in the struct definition.

        To access a struct member in a lambda closure, use the #'->
        operator with the name of the member as double-quoted symbol
        or literal string:

            ({ #'->, struct-expression, ''one })
            ({ #'->, struct-expression, "one" })


MISCELLANEOUS
        Internally structs can be identified by the ID string
        returned from get_type_info(). This string contains the name
        of the struct, the name of the program its type was defined in,
        and the ID number of the program. However, do not rely on
        a particular format of this string!

        Support for structs is signaled by the macro __LPC_STRUCTS__.

        Since structs are tied to the program they are defined in,
        re-compiling a program creates new struct types which are
        in principle incompatible to the old ones. However, the LPC
        compiler checks if the newly compiled structs have the same
        structure as their older counterparts of the same name
        (and defining program). If the structures conform, the existing
        older struct types are used instead of the new ones. This way
        an accidental of for example /std/types.c doesn't break
        the whole mud.


EXAMPLES
        Suppose we have two objects: a monster, and a monster
        coordinate tracker, and we want to use a struct to store the
        coordinate:

          -- monster_coordinate.c --
            struct Coordinate { int x; int y; };

          -- monster_tracker.c --
            inherit "monster_coordinate";

            void track (struct Coordinate coord) { ... }

          -- monster.c --
            inherit "monster_coordinate";

            int move (..) {
              ...
              "monster_tracker"->track( (<Coordinate> my_x, my_y) );
            }

        Note that using '#include "monster_coordinate.c"' instead of inherit
        won't work. While the objects would compile, the first call to
        track() would cause a runtime error of the type 

          Illegal type to struct->(): struct Coordinate (/monster.c #234),
                                      expected struct Coordinate
                                      (/monster_tracker.c #552)


HISTORY
        structs were fully implemented first in LDMud 3.3.246.
        The implementation was revised in LDMud 3.3.344.
        The reactivation of unchanged structs in object updates was
        implemented in LDMud 3.3.417.


SEE ALSO
        mappings(LPC), get_type_info(E), structp(E), to_mapping(E),
        to_struct(E), struct_info(E), baseof(E)