Hi all - here comes a tricky one:
(good bugs are usually tricky - not everything tricky is necessarily a bug)
Imagine an arbitrary PropertyList config file like this:
<PropertyList>
<foo type="double">3.14</foo>
</PropertyList>
and a code fragment like
class MyClass {
public:
double getter() const { return foo; }
void setter( double d ) { foo = d; }
void dosomethingfancy();
private:
double foo;
};
void MyClass::dosomethingfancy()
{
someBaseNode->tie( "foo", this, MyClass:getter, MyClass::setter );
}
Ignore the fact, that it doesn't make much sense here - it's just an example.
One would expect that foo in MyClass (and the value of the thereafter tied
property) gets initialized to the value of <foo> from the config file which is
3.14. This actually works fine.
Now, modify the xml file to look like this:
<PropertyList>
<foo type="double" write="n">3.14</foo>
</PropertyList>
(note: foo is now read-only)
The value of the foo-property now remains at an unchangeable value of zero, it
no longer gets initialized to the provided value in <foo>. That is because the
property is declared read-only and the setter method never gets called.
In simgear/props/props.hxx in SGPropertyNode::tie() some code exist to set the
value of a property after being tied, to the default value using setValue().
However, setValue() is smart enough to check for a read-only property and
refuses to change a property value in that case.
A fix might be to temporarily set the WRITE-flag for the property in the tie()
method like this:
(fragment from props.hxx, SGPropertyNode::tie(const SGRawValue<T> &rawValue,
bool useDefault), line 1789ff
if (useDefault) {
int save_attributes = getAttributes(); // save current write-flag
setAttribute( WRITE, true ); // write-enable the property
setValue(old_val); // set the old value
setAttributes( save_attributes ); // restore write-flag
}
instead of
if (useDefault) {
setValue(old_val); // noop for read-only property
}
Comments?
Torsten
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