** Changed in: zorba
Status: New => Fix Committed
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https://bugs.launchpad.net/bugs/948259
Title:
Store::getNodeReference() should take Item const*
Status in Zorba - The XQuery Processor:
Fix Committed
Bug description:
In the Store class, there is the function that currently has the
signature:
virtual bool getNodeReference(Item_t& result, Item* node) = 0;
Specifically, it takes an Item* (pointer to non-constant Item) because
it presumably mutates the Item object if it has to assign a node
reference.
However, the function signature should really be:
virtual bool getNodeReference(Item_t& result, Item const* node) =
0;
that is, take an Item const* (pointer to constant Item). The operation
is "logically const." An implementation may choose not to alter the
Item at all and instead store a mapping between Items and node
references elsewhere.
However, for an implementation that does store the node reference in
an Item itself, that doesn't change the fact that the operation is
still "logically const." The node reference data member should be made
"mutable."
Having the function take an Item* as it does now means that other
classes elsewhere that return an Item const* for which you want to
obtain its node reference must first const_cast the pointer which is
ugly.
For example, the FTToken class keeps an Item const* pointing to the
Item whence the token came. There is no reason FTToken itself needs
the Item to be non-const, hence it declares its data member as Item
const*. However, in code elsewhere, we want to obtain the node
reference for a token's Item like this:
if ( store::Item const *const token_item = token->item() ) {
store::Item *const temp = const_cast<store::Item*>( token_item );
if ( GENV_STORE.getNodeReference( item, temp ) ) {
// ...
}
}
The introduction of the const_cast and temp is ugly and shouldn't be
necessary. It would be even more ugly to change the FTToken class to
use an Item* because then everything that passes an Item* to FTToken
might have to be changed also, i.e., the non-const-ness would ripple
throughout the code.
Note that changing the function signature to the proposed one using
const is a backwards *compatible* change because increasing "const-
ness" is always so. (The reverse, however, i.e., making something that
is const into non-const, is a backwards incompatible change.)
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