C++ read-only struct that can be set from within a class?

Question

I have a class Foo that has defined a public struct Data inside of it, with a QVector<Data> allData nesting multiple objects of the Data struct inside of it:

class Foo
{
    public:
        struct Data {
            uchar valueA;
            uchar valueB;
            uchar valueC;
        };
    private:
        QVector<Data> allData;
};

I also have two other classes B and C that need to read allData multiple times a second. They shall not be able to write into allData at all, this shall be handled by the Foo class only.

With performance in mind, I'm looking for a way to do this without creating a new QVector everytime. As far as I understand, a method like this:

QVector<Data> getAllData() {
    return allData;
}

would result in a new object being created everytime this method gets called.

If I gave the other classes something like a QVector<Data> allDataCopy that is just being handed over to Foo to be filled with the values inside of allData that would just result in having to copy all values everytime, which I imagine would not be very perfomant aswell.

Is there any efficient way to solve this?

`QVector` is [cheap to copy](http://doc.qt.io/qt-5/implicit-sharing.html). That said, you can make it `const QVector& getAllData()` — Igor Tandetnik, Jan 18 '18 at 15:27
You could return by const reference, since the value is a member variable, but I am unsure if this is safe or not. — Arnav Borborah, Jan 18 '18 at 15:28
"multiple times a second": please give the order of "mutliple" we're talking. Units? Tens? Hundreds? Millions? — Angew is no longer proud of SO, Jan 18 '18 at 15:32
@manni66: I was thinking that could be the basis for a question on its own. Turns out [there is one](https://stackoverflow.com/q/1668259/10077). — Fred Larson, Jan 18 '18 at 15:49

Vittorio Romeo · Answer 1 · 2018-01-18T16:43:49.123

4

Your "getters" should not return by value unless you want to force a copy on the user. Provide two overloads that will respectively return an lvalue reference and a const lvalue reference (or only the const one if you don't want users of Foo to mutate allData):

QVector<Data>& getAllData() {
    return allData;
}

const QVector<Data>& getAllData() const {
    return allData;
}

This will still allow your user to make a copy (if wanted), but also enable your user to just observe/mutate allData through a reference, without any copy.

In C++11 and above, if QVector supports move semantics, you might want to overload on the && ref qualifier as well:

QVector<Data>& getAllData()       &      { return allData; }
const QVector<Data>& getAllData() const& { return allData; }
QVector<Data> getAllData()        &&     { return std::move(allData); }

This allows users to conveniently move allData from an rvalue of type Foo.

edited Jan 18 '18 at 16:43

answered Jan 18 '18 at 15:34

Vittorio Romeo

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1

Good advice in general. Though I would like to add t hat `QVector` are implicitly shared. The copy constructor is always constant time and, instead, the elements are copied when any reference writes to a shared instance. – François Andrieux Jan 18 '18 at 15:39
QVector [has a move constructor since Qt 5.2](http://doc.qt.io/qt-5/qvector.html#QVector-4). But having a getter that moves a member variable seems like an awful idea. Am I wrong? – Fred Larson Jan 18 '18 at 15:42
3

@FredLarson Notice that particular getter is reference qualified with `&&` meaning it only participates if you are calling it from an rvalue. For example `std::move(foo).getAllData()` would move out `allData` but `foo.getAllData()` would not. – François Andrieux Jan 18 '18 at 15:43
@FrançoisAndrieux: Thanks. I'm not accustomed to those qualifiers yet. – Fred Larson Jan 18 '18 at 15:45
1

The OP doesn't want the first overload, the second is sufficient. – Martin Bonner supports Monica Jan 18 '18 at 16:09
@Vittorio Romeo, would that still work with allData being `private` as in question code? – Mohammad Kanan Jan 18 '18 at 16:10
@MohammadKanan: sure, why not. – Vittorio Romeo Jan 18 '18 at 16:15
@VittorioRomeo, because in question, it will be called from two _**other**_ classes B, C. – Mohammad Kanan Jan 18 '18 at 16:27
If you let the caller get a non-const reference to a private member, why not simply let that member be public? – YSC Jan 18 '18 at 16:31
Made it call by reference, thank you very much. TIL! – ExtraGoofy Feb 06 '18 at 11:32

score 3 · Answer 2 · answered Jan 18 '18 at 16:40

QVector is cheap to copy; it uses smart reference copy on write semantics.

If your code has any danger of multiple thread access, QVector const& is dangerous, as references are not thread safe. The same kind of danger can occur if you are reentrant, and cause the QVector to be modified while someone else is holding onto the QVector const&.

Simply return a copy of QVector. Fancy alternatives involving references can cause potential bugs, do not consider them until you have evidence that the small cost of "copying" a QVector is causing performance issues.

C++ read-only struct that can be set from within a class?

2 Answers2