Using C++11 range-based for loop correctly in Qt

Question

According to this talk there is a certain pitfall when using C++11 range base for on Qt containers. Consider:

QList<MyStruct> list;  for(const MyStruct &item : list) {     //... } 

The pitfall, according to the talk, comes from the implicit sharing. Under the hood the ranged-based for gets the iterator from the container. But because the container is not const the iterator will be non-const and that is apparently enough for the container to detach.

When you control the lifetime of a container this is easy to fix, one just passes the const reference to the container to force it to use const_iterator and not to detach.

QList<MyStruct> list; const Qlist<MyStruct> &constList = list;  for(const MyStruct &item : constList) {     //... } 

However what about for example containers as return values.

QList<MyStruct> foo() { //... }  void main() {     for(const MyStruct &item : foo())     {     } } 

What does happen here? Is the container still copied? Intuitively I would say it is so to avoid that this might need to be done?

QList<MyStruct> foo() { //... }  main() {      for(const MyStruct &item : const_cast<const QList<MyStruct>>(foo()))     {     } } 

I am not sure. I know it is a bit more verbose but I need this because I use ranged based for loops heavily on huge containers a lot so the talk kind of struck the right string with me.

So far I use a helper function to convert the container to the const reference but if there is a shorter/easier way to achieve the same I would like to hear it.

Answer 1

template<class T> std::remove_reference_t<T> const& as_const(T&&t){return t;} 

might help. An implicitly shared object returned an rvalue can implicitly detect write-shraring (and detatch) due to non-const iteration.

This gives you:

for(auto&&item : as_const(foo())) { } 

which lets you iterate in a const way (and pretty clearly).

If you need reference lifetime extension to work, have 2 overloads:

template<class T> T const as_const(T&&t){return std::forward<T>(t);} template<class T> T const& as_const(T&t){return t;} 

But iterating over const rvalues and caring about it is often a design error: they are throw away copies, why does it matter if you edit them? And if you behave very differently based off const qualification, that will bite you elsewhere.