Should we maybe provide the fields as string constants in the relevant job APIs (or elsewhere)?

Coding style: Type &var instead of Type& var

IMO should be a QStringList to allow usage like

job->setFields({Field1, Field2, Field3});

Same for the getter. We can concatenate the fields just before serializing them into the query ourselves, but the API will be nicer for users.

Extra Am ?

Use QVector instead of a QList, please. You can drop the space between > >, too. C++11 fixed the grammar, so it's not misparsed as a streaming operator anymore.

Also, create a custom struct with properly named members (query, value, maybe?) instead of using a QPair. I know the QUrlQuery API uses that, but let's not be lazy just because Qt is :-)

Pass complex types as const &. It doesn't make much difference since Qt containers are implicitly shared, but it's a convention in Qt-based code.

prettyPrint doesn't need to be (shouldn't be!) a reference.

Do you maybe need to adjust /all/ tests, now to reflect this?

I'm not sure how I feel about this barrage of members, if nothing else this will pollute auto-completer in the IDEs :-)

What do you think about this?

class About {
    ....
    struct Fields {
        static const QString AdditionalRoleInfo;
       ...
    };
};

Then the usage would be About::Fields::AdditionalRoleInfo, which feels cleaner and won't pollute About itself that much.

Coding style: spaces around = (here and everywhere below)