# typedef struct bar*      Bar;
# typedef struct type_foo* Foo;

# In typemap:
# Bar	T_PTROBJ
# Foo	T_PTROBJ

struct bar*
create_bar(foo_sv = 0)
	SV* foo_sv

CODE:
	struct type_foo* foo = NULL;

	if ( !foo_sv ) {
		/* Use default of NULL. */
	}
	else if ( !SvOK( foo_sv ) ) {
		/* Use default of NULL. */
	}
	else if ( sv_derived_from( foo_sv, 'type_foo' ) ) {
		IV tmp = SvIV( (SV*)SvRV( foo_sv ) );
		foo = INT2PTR( struct type_foo*, tmp );
	}
	else {
		Perl_croak( aTHX_ "\$foo is not of type foo_type" )
	}

	RETVAL = my_c_create_bar(foo);

OUTPUT:
	RETVAL