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4.26.5 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
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In some cases references have to be dereferenced explicitely. 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
==>
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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
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