4.29.5 reference and shared operations

All operations of the underlying objects are forwarded by reference and shared objects. THis kind of dereferencing is done automatically in most cases:

Example:

system("reference"); system("shared"); int i = 2; reference two = i; shared three = 3; two * three; ==> 6 two ^ three; ==> 8 two ** three; ==> 8 two + two; ==> 4 two - two; ==> 0 ring r = 0, (x,y,z), dp; poly p = x + y + z; reference ref = p; shared zvar =z; subst(ref, x,1, y,2, zvar,3); ==> 6

In some cases references have to be dereferenced explicitly. For instance, this is the case for n-ary function calls not starting with a reference or shared object. You can use the link operator or a type cast to work around this. In contrast, some constructs like left-hand subexpressions prematurely evaluate. You can avoid this by using the def operator or by explicitly type casting to reference.

system("reference"); system("shared"); ring r = 0, (x,y,z), dp; poly p = x + y + z; shared xsh = x; subst(p, xsh,1, y,2, z,3); // fails ==> ? subst(`poly`,`shared`,`int`) failed ==> ? expected subst(`poly`,`poly`,`poly`) ==> ? expected subst(`matrix`,`poly`,`int`) ==> ? error occurred in or before ./examples/reference_and_shared_operatio\ ns_1.sing line 5: `subst(p, xsh,1, y,2, z,3); // fails` subst(p, poly(xsh),1, y,2, z,3); // good ==> 6 subst(p, link(xsh),1, y,2, z,3); // fine ==> 6 list ll = list(xsh, xsh, xsh); ll[1] = y; // replaced only first entry ll; ==> [1]: ==> y ==> [2]: ==> x ==> ==> [3]: ==> x ==> shared(ll[2]) = z; // replaces the others ll; ==> [1]: ==> y ==> [2]: ==> z ==> ==> [3]: ==> z ==> def(ll[2]) = x; // generic alternative ll; ==> [1]: ==> y ==> [2]: ==> x ==> ==> [3]: ==> x ==>

In particular, explicit dereferencing is useful to distinguish between typecasting and nested constructings.

system("reference"); system("shared"); shared shl = list(1); shl; ==> [1]: ==> 1 ==> list(shl); // wraps 'shl' by a list ==> [1]: ==> [1]: ==> 1 ==> link(shl); // extract the list in 'shl' ==> [1]: ==> 1