How to use C++11 range-based for with QJsonArray

Question

With old-style loops, I can delve into a QJsonArray and, in the example below, add element "foo" with contents of existing element "bar" for each array item. How can I do this using C++11 range-based for?

// QJsonArray oldArray contains an array of which one element is "bar"
QJsonArray newArray;
int i, b = oldArray.count();
for (i=0; i<n; ++i) {
    QJsonObject element = oldArray.at(i).toObject();
    element["foo"] = element["bar"];
    newArray.append(element);
}

I have tried the following (admittedly as trial and error):

auto&

for (auto& element : oldArray) {
    element["foo"] = element["bar];
    newArray.append(element);
}

I get the error

non-const lvalue reference to type 'QJsonValueRef' cannot bind to a temporary of type 'QJsonValueRef'

const auto&

for (const auto& element : oldArray) {
...

I get a warning

loop variable 'element' is always a copy because the range of type 'QJsonArray' does not return a reference

const auto

for (const auto element : oldArray) {
    element["foo"] = element["bar];
    ...

I get the error

no viable overloaded operator[] for type 'const QJsonValueRef'

relating to element["bar"]

Answer 1

The problem is that the iterator for QJsonArray returns a temporary QJsonValueRef object, and lvalue references can not bind to temporaries. We can either hold that temporary by value:

// QJsonArray oldArray contains an array of which one element is "bar"
QJsonArray newArray;
for (auto v : oldArray) {
    QJsonObject element = v.toObject();
    element["foo"] = element["bar"];
    newArray.append(element);
}

In this case v is a QJsonValueRef object (similar to what oldArray.at(i) gives in the old-style loop). After that, we convert that QJsonValueRef to a QJsonObject using .toObject().

Or we can use forwarding references since they can bind to rvalues:

for (auto&& v : oldArray) {
    ...
}

In this case v is deduced to be an rvalue reference to a QJsonValueRef.

Both solutions are identical in terms of the number of objects being created/destructed (since in the former, the copy is elided under C++17 guaranteed copy elision rules or even in pre-C++17 when using any decent compiler. In the latter, the reference is bound to the temporary and this prolongs its lifetime to match the whole iteration).

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Notes

1. This problem is similar to what happens when using range-based-for with a std::vector<bool>; since both std::vector<bool> and QJsonArrayhave iterators that return proxy objects.
2. If using the Clang code model, the second solution generates the warning "loop variable 'v' is always a copy because the range of type 'QJsonArray' does not return a reference". See this question for an explanation.