c++11 range-loop might detach Qt container [duplicate]

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

Qt has an implementation to resolve this, qAsConst (see https://doc.qt.io/qt-5/qtglobal.html#qAsConst). The documentation says that it is Qt's version of C++17's std::as_const().