Does a static_cast make sense on a getInt() function?

Question

Does this statis cast make sense FOR THIS EXPLICIT CASE?

QSqlQuery q;
enum MyEnumType;
obj.setMyEnumType(static_cast<MyEnumType>(q.value(2).toInt()));

or is a static cast for situations where the source type is not sure int?

the set function is

 void setMyEnumType(MyEnumTypetype type) { m_type = type; }

what is the advantage to a simple cast?

obj.setMyEnumType((MyEnumType)q.value(2).toInt());

@slava you are right it accepty enum, but compiles even without static_cast — wutzebaer, Jul 19 '16 at 14:23
without static_cast you mean your example with C style cast? Yes that would compile, but you just replaced one cast with another. — Slava, Jul 19 '16 at 14:27
yes but the c-style writes shorter, has the static_cast any difference to the c-cast in my case? — wutzebaer, Jul 19 '16 at 14:28
static_cast is better as it is easier to catch and it is more restrictive than c-style cast — Slava, Jul 19 '16 at 14:29
yes generally of course, but in my case the toInt() can only return int in every case — wutzebaer, Jul 19 '16 at 14:31
Its better to have proper safe habits rather than write expression couple symbols shorter in particular case. — Slava, Jul 19 '16 at 14:33

score 3 · Answer 1 · edited May 23 '17 at 11:58

Yes the cast makes perfect sense. Let's unpick this a little more:

QSqlQuery q;
enum MyEnumType;
const auto v1 = q.value(2);  // I don't know what the type of v1 will be.
                             // See the QSqlQuery docs.
const auto v2 = v1.toInt();  // v2 is going to be an int or a long or something.

obj.setMyEnumType(v2);       // Error: setMyEnumType doesn't take an int argument.

const auto e = static_cast<MyEnumType>(v2);
obj.setMyEnumType(e);        // OK.  Argument is now the right type.

Edit: Ah-ha! I see now you were asking an entirely different question. The question you were really asking is a duplicate of What is the difference between static_cast<> and C style casting?

Always prefer static_cast because a) a code reviewer will be prompted to think "what happens if the value is out of range?"; b) a code reviewer won't have to think "is this a static_cast, a reinterpret cast, a const cast, or some combination of all three - and is it safe?"

@wutzebaer: I don't understand your comment. I have just split the single expression which evaluated the argument to `setMyEnumType` into multiple expressions, and saved the intermediate values in named variables. — Martin Bonner supports Monica, Jul 19 '16 at 14:28

lorro · Answer 2 · 2016-07-19T15:57:19.990

1

It makes perfect sense to convert from int to an enum and back. You might check for boundary conditions (e.g., int is larger than last element of enum). Note that you're not casting the function here, but the return value.

edited Jul 19 '16 at 15:57

answered Jul 19 '16 at 14:17

lorro

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does static_cast automatically make a boundary check? – wutzebaer Jul 19 '16 at 14:18
@wutzebaer No it does not. – user975989 Jul 19 '16 at 14:19
so whats the advantage of the static_cast in this case? – wutzebaer Jul 19 '16 at 14:20
@wutzebaer there is no implicit conversion from `int` to `enum` – Slava Jul 19 '16 at 14:21
no implicit, but `obj.setMyEnumType((MyEnumType)q.value(2).toInt());` works too – wutzebaer Jul 19 '16 at 14:24
2

@wutzebaer but? this is explicit c-cast – Slava Jul 19 '16 at 14:26

score 1 · Answer 3 · answered Jul 19 '16 at 14:27

Your question is a bit of an XY problem in this scenario, as QVariant provides built-in conversion safety checks and casting.

Use qvariant_cast or QVariant::value<>(), alongside canConvert(), e.g.:

QVariant v = q.value(2);
if (v.canConvert<MyEnumType>()) {
    obj.setEnumType( qvariant_cast<MyEnumType>(v));
    ...
}

Does a static_cast make sense on a getInt() function?

3 Answers3