There's a simple syntax in cxing for creating compound objects and types:
decl Complex := namedtuple() { 're': double, 'im': double };
decl I := Complex() { 're': 0, 'im': 1 };
decl sockaddr := dict() { 'host': "example.net", 'port': 443 };
In the above scenario,
namedtuple() factory function creates such object that is
a type object that creates another type object with 2 members
named "re" and "im", this type is assigned to Complex,
which is then used to create a "complex number"
with the value of the imaginary unit;dict() factory function creates a type object that creates a
dictionary, initializing sockaddr with 2 members - "host" with
the value of "example.net" and "port" with 443.namedtuple, Complex, and dict are "type objects", of which,
with namedtuple being sort of a meta.
A type object contains an method property named __initset__ declared as follow:
[ffi] method [val] __initset__(ref key, ref value);
The __initset__ function may be defined in cxing or in a foreign
language - if the latter, then calling conventions for foreign function
interface must be followed per
12.2. Calling Conventions and Foreign Function Interface.
objdef-start % objdefstart
: objdef-start-comma % comma
| objdef-start-nocomma % nocomma
;
objdef-start-comma % objdefstartcomma
: objdef-start-nocomma "," % genrule
;
objdef-start-nocomma % objdefstartnocomma
: postfix-expr "{" postfix-expr ":" assign-expr % base
| objdef-start-nocomma "," postfix-expr ":" assign-expr % genrule
;
object-notation % objdef
: postfix-expr "{" "}" % empty
| objdef-start "}" % some
;
The postfix-expr MUST NOT be inc or dec. Furthermore, if postfix-expr
is degenerate, then the primary expression MUST NOT be array or const.
On encountering a postfix-expr that is a type object, the key-value pairs
enclosed in the braces delimited by commas are taken and the __initset__
method is called on them in turn. The key is the value of the postfix
expression on the left side of the colon, while the value is that of the
assignment expression on the right side of the colon. After this completes,
the newly created object will receive a property named __proto__,
which will be assigned the value of postfix-expr.
The array production of primary expressions is a syntax sugar that invokes
__initset__ with elements in the expressions-list as value and successive
integer indicies as key, starting with 0.