Initializer list syntax of subclass created with QtCreator class "wizard"

[Pl45m4]

Hey guys,

I would say I have a solid C++ knowledge, but I'm not following every change or update to C++.
I've noticed that the QtCreator class wizard (QtC 13 with C17) went from:

MyWidget::MyWidget(QWidget *parent)
    : QWidget(parent){}

as used in earlier versions
to:

MyWidget::MyWidget(QWidget *parent)
    : QWidget{parent}{}

now.

So curly brackets ( {...}) are used instead of parentheses ( (...)).

Why that?
Dis-/Advantages?!
: QWidget(parent) is still valid, but should one continue to use {parent} from now on?

Any explanation appreaciated :)

[kshegunov]

@Pl45m4 said in Initializer list syntax of subclass created with QtCreator class "wizard":

Why that?

The 'feature' (debatable how much of it is really a feature) is called uniform initialization. You may want to take a peek here [1].

: QWidget(parent) is still valid, but should one continue to use {parent} from now on?

Depends on your style and understanding. I don't, as I don't agree that I should mindlessly slap whatever the core guideline guidelines tells me to.

Dis-/Advantages?!

There are cases where that syntax has unexpected behaviour. If you have say:

class Foo
{
    Foo();
    Foo(std::initializer_list<Bar>);
};

You might run in trouble doing:

Foo x{}; //< We call what exactly, is this the default constructor, or the one taking an initializer list?

But I will grant you, the example is contrived. Here's a case where you may hit this in Qt.

QJsonValue a{}; //< Do we mean to call QJsonValue(), or QJsonValue(QJsonObject(std::initializer_list<...>))

As it happens, due to well C++, we call the constructors that take an initializer list in both cases. All in all we shot ourselves in the foot. So, if you ask me - use {} when you mean to use an initializer list, all other cases - simply call the constructor. That is to say:

std::vector x = { 1, 2, 3, 4 }; //< Just fine!
std::vector<int> y(5) //< Creating a vector that has 5 elements!!
int q = 4; //< Can't get more explicit than that. `int q(4);` and `int q{4};` are both valid, but is it clearer to read? I sure don't think so. Looks more like something you'd write to impress your colleagues. 

Warning: Notice in the last code snippet that std::vector<int> y(5) is very, very different from std::vector<int> y{5}

[1]: https://isocpp.org/wiki/faq/cpp11-language#uniform-init

[JonB]

@Pl45m4
Isn't it the same thing as "nowadays" people write int x{16}; in place of int x(16);. {}s for initializers instead of () for function call? Kind of. Now one of the experts will explain, hopefully :)

[kshegunov]

@Pl45m4 said in Initializer list syntax of subclass created with QtCreator class "wizard":

Why that?

The 'feature' (debatable how much of it is really a feature) is called uniform initialization. You may want to take a peek here [1].

: QWidget(parent) is still valid, but should one continue to use {parent} from now on?

Depends on your style and understanding. I don't, as I don't agree that I should mindlessly slap whatever the core guideline guidelines tells me to.

Dis-/Advantages?!

There are cases where that syntax has unexpected behaviour. If you have say:

class Foo
{
    Foo();
    Foo(std::initializer_list<Bar>);
};

You might run in trouble doing:

Foo x{}; //< We call what exactly, is this the default constructor, or the one taking an initializer list?

But I will grant you, the example is contrived. Here's a case where you may hit this in Qt.

QJsonValue a{}; //< Do we mean to call QJsonValue(), or QJsonValue(QJsonObject(std::initializer_list<...>))

As it happens, due to well C++, we call the constructors that take an initializer list in both cases. All in all we shot ourselves in the foot. So, if you ask me - use {} when you mean to use an initializer list, all other cases - simply call the constructor. That is to say:

std::vector x = { 1, 2, 3, 4 }; //< Just fine!
std::vector<int> y(5) //< Creating a vector that has 5 elements!!
int q = 4; //< Can't get more explicit than that. `int q(4);` and `int q{4};` are both valid, but is it clearer to read? I sure don't think so. Looks more like something you'd write to impress your colleagues. 

Warning: Notice in the last code snippet that std::vector<int> y(5) is very, very different from std::vector<int> y{5}

[1]: https://isocpp.org/wiki/faq/cpp11-language#uniform-init

[Pl45m4]

@kshegunov

Thanks a lot :)