There's always a need to return something when your method has a return type. If you don't know what to return at first then return a default value.

Also, always take into account the warnings the compiler throws at you (there is one for functions not returning anything when their signature contains a return type). As the old saying goes: a compiler warning ignored today is a catastrophic crash in production tomorrow.

@Lasith
well std::bad_alloc often comes when out of memory.
My guess is that <List1[0].size() is always true since you add to it for each loop
and hence create an infinite loop.

Did you try VRonin code?

Update:
If you just want list1 in [0] and list2 in[1]
why all the loops then?

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@JNBarchan yeah they are nested for loops!

@Lasith You should open the database only once. Currently you do it each time button is pressed.

Because you didn't ask a question. Use the button to "Ask as question" and then you'll have the "Mark as solved" that should appear.