Disturbing: const QString &

[magland]

Ok, I see what you are saying. Now I see the danger of passing things by reference (even if declared const). The advantage is that the parameter does not need to be copied, but the tradeoff is that you need to worry about whether the variable is changed part way through the execution of the function - not within the function but elsewhere.

This is probably what was happening to me. But I will never know, because I've restructured my program to avoid the recursion (I'm now using a queuing system to do the same task).

Based on a recent recommendation on this forum I changed my coding habits and use "const QString &" everywhere instead of "QString".... but now I see that I need to be more careful. Do you have a recommendation of what precautions I need to take with this?

Thanks!

This means that we have to take a look at this completely different function. It isn't time for worrying yet.

Probably your code does something similar to this:

#include <iostream>
 
int z = 0;
 
void bar( int y )
{
   z = y;
}
 
void foo( const int & x )
{
   std::cerr << x << std::endl;
   bar( 1 );
   std::cerr << x << std::endl;
}
 
int main()
{
   foo( z );
   return 0;
}

#include <iostream>

int z = 0;

void bar( int y )
{
   z = y;
}

void foo( const int & x )
{
   std::cerr << x << std::endl;
   bar( 1 );
   std::cerr << x << std::endl;
}

int main()
{
   foo( z );
   return 0;
}

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[jacek]

Based on a recent recommendation on this forum I changed my coding habits and use "const QString &" everywhere instead of "QString".... but now I see that I need to be more careful.

It's recommended to pass objects by reference to avoid copying, but on the other hand in Qt it's cheap to copy implicitly shared classes.

Do you have a recommendation of what precautions I need to take with this?

I was thinking about this for few days and I couldn't come up with any rules. You situation is a bit similar to:

QVector v;
...
v.append( v[0] );

QVector v;
...
v.append( v[0] );

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The argument is passed as reference, but it must be copied when QVector needs to reallocate data. You can easily identify such situation in a single data structure, but the problem arises when the data is passed between many classes. But I would simply classify that as one of those Bad Things(tm) that might happen when you use global variables (or you have cycles in call graph).

[magland]

Thanks Jacek. I agree with you.

After finding a number of subtle problems caused by passing by reference (in addition to the one I reported) I have concluded that it is important to NEVER pass arguments by reference UNLESS you can guarantee that the function does not access global variables, or "have cycles in the call graph" that may end up modifying the input variable. It is just not worth the danger! There's nothing worse than problems that occur mysteriously once in every 1000 calls.

Of course, passing by reference is good, if you can ensure such things. I don't use global variables, but I do use a model/view framework which inevitably will have some cycles or recursive calls. Therefore, I need to be careful about it.