Why is this allocating an unneeded temporary container

Question

I am getting a warning against my foreach loop that I'm "allocating an unneeded temporary container" but I have no idea what that means.

foreach(QString commandName, m_registeredResponseObjects.keys()) {
    delete m_registeredResponseObjects[commandName];
};

Does this means the key() method is called on each iteration of the loop? I don't even see the container the warning is referencing...

foreach is a Qt macro defined as

template <typename T>
class QForeachContainer {
public:
    inline QForeachContainer(const T& t) : c(t), brk(0), i(c.begin()), e(c.end()) { }
    const T c;
    int brk;
    typename T::const_iterator i, e;
};

As I mentioned in a comment to [this question of yours](https://stackoverflow.com/questions/71271359/how-to-pass-an-object-address-created-at-time-of-function-call), please try to make a [mre]. — Ted Lyngmo, Feb 25 '22 at 22:30
Whichever language it turns out to be (the syntax doesn't match `foreach` in C# or `forEach` in Java), the answer is *probably* going to be that you should make `commandName` a *reference* variable. How to do that depends completely on the language. — Adrian Mole, Feb 25 '22 at 22:34
my guess ' foreach' is a macro that does roughly what a ranged for does.(from before ranged for). But we need to see it — pm100, Feb 25 '22 at 22:40
@pm100 Good guess: https://stackoverflow.com/q/10522155/10871073 — Adrian Mole, Feb 25 '22 at 22:41

score 1 · Accepted Answer · answered Feb 26 '22 at 09:00

1

It means that you create a container with this statement: m_registeredResponseObjects.keys() for no good reason. This function iterates over your m_registeredResponseObjects, collects all keys and returns a container where you then iterator over just the get the values from m_registeredResponseObjects by key. This makes no sense at all - why not simply

  for (auto val : qAsConst(m_registeredResponseObjects))
    delete val;

or even simpler with the Qt macro qDeleteAll()

  qDeleteAll(m_registeredResponseObjects);

?

answered Feb 26 '22 at 09:00

chehrlic

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This is new to me....m_registeredResponseObjects is a QMap so will this for loop fill val with the pointer of each map entry? (How does it know not to return the key)? I couldn't find a good explanation of qAsConst used like this – TSG Feb 26 '22 at 12:53
1

Simply try it out - Qt containers do return the value in a range-based for loop. And even if you don't trust it, use Q_FOREACH as in your example... For qAsConst -> see the [documentation](https://doc.qt.io/qt-5/qtglobal.html#qAsConst) – chehrlic Feb 26 '22 at 16:26

Why is this allocating an unneeded temporary container

1 Answers1