将QList附加到QList <QList>

Question

I have an interesting problem with Qt Qlist container. Trying to append QList to QList makes my iterator to point to unknown part of memory.

QList<int> listSmall;
QList<QList<int>> listBig;

for(int i = 0; i < 3; ++i)
    listSmall.append(i);

for(auto it = listSmall.begin(); it != listSmall.end(); ++it)
    listBig.append(listSmall);

Condition it != listSmall.end(); always true if i append small list to big. Why does this happen?

PS STL list works just fine.

Answer 1

This is a known limitation of Qt containers when used with stl-like iterators. The documentation explains it.

Implicit sharing has another consequence on STL-style iterators: you should avoid copying a container while iterators are active on that container. The iterators point to an internal structure, and if you copy a container you should be very careful with your iterators.

I am afraid you will have to find a different way of doing what you are trying to do (like using a standard list or iterate differently).

Answer 2

TLDR: use cbegin() and cend() when you don't change values.

Details:

• Compiler calls non-const begin() and end() version just as you asked; it can use const versions if you call cbegin() or begin() through const ref/pointer.
• QList tries to be copy-on-write and shares memory instead when you do listBig.append(listSmall); .
• On next iteration inside non-const end() there is call to detach() which copies data after confirming that local data is referenced more than once (like, it is referenced by listBig content already).

All in all, that is a good example of why copy-on-write is hard to implement correctly.

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