Constructor syntax in Qt in mainwindow.cpp

Swati777999

Hi All,

I have a doubt about the syntax of the constructor implemented in the mainwindow.cpp file. Below is an example of a constructor defined in mainwindow.cpp file. I would like to be confirmed about this part of the syntax . I feel that it shows the inheritance as MainWindow is inherited from QMainWindow.

Doubts:

Why is the argument i.e parent same for both MainWindow and QMainWindow , for eg. MainWindow(QWidget *parent)
: QMainWindow(parent) . For MainWindow , the argument parent is of type QWidget , does this parent has same type with QMainWindow?
What are name(0), place(0),thing(0), about(0) in the definition ?
: QMainWindow(parent),
name(0),
place(0),
thing(0),
about(0)
```
 MainWindow::MainWindow(QWidget *parent)
 : QMainWindow(parent),
 name(0),
 place(0),
 thing(0),
 about(0)
```

jsulm

@Swati777999 said in Constructor syntax in Qt in mainwindow.cpp:

This is how the variables are initialized.

This is a possibility. But in proper C++ projects this is not the way members are initialised. Such code would not get approved in projects I work on unless there is really a need to do it this way.
Correct way to initialise members is using the syntax you were asking about. So, I suggest you learn and use it. This is also nothing new, exists in C++ for ages already.

jsulm

@Swati777999 Please read https://doc.qt.io/qt-5/objecttrees.html
And also learn about C++ constructors.

Parent is of type QWidget* because any widget can be parent of other widgets. "does this parent has same type with QMainWindow?" - why don't you check documentation (https://doc.qt.io/qt-5/qmainwindow.html#QMainWindow)?
This has nothing to do with Qt. name, place, thing and about seem to be member variables in your MainWindow class which are initialised this way in constructor. This are C++ basics, has nothing to do with Qt.

Swati777999

@jsulm I don't understand how the member variables are represented in the declaration of the constructor with commas.

:QMainWindow(parent),
name(0),
place(0),
thing(0),
about(0)

Generally, in C++ , any function is declared as
void custom_func (arg1,arg2......argn)
{
............
}

Please clarify the above syntax of Qt constructor declaration.

jsulm

@Swati777999 said in Constructor syntax in Qt in mainwindow.cpp:

I don't understand how the member variables are represented in the declaration of the constructor with commas.

Please learn C++! You are asking absolute basics!

class SomeClass
{
public:
    SomeClass():
        a(1),
        b("Some String")
    {}
private:
    int a;
    std::string b;
};

JonB

@Swati777999 said in Constructor syntax in Qt in mainwindow.cpp:

Generally, in C++ , any function is declared as
void custom_func (arg1,arg2......argn)
{

This is a plain C function declaration. The example you are asking about is for a C++ class method, and inheriting from a base class.

It does not declare the variables in the comma-list, it merely initializes them. They must be declared as class members, as @jsulm has shown. In this particular case it is just equivalent to:

 MainWindow::MainWindow(QWidget *parent)
 : QMainWindow(parent)
{
     name = 0;
     place = 0;
     thing = 0;
     about = 0;
    ...
}

KroMignon

@Swati777999 said in Constructor syntax in Qt in mainwindow.cpp:

I don't understand how the member variables are represented in the declaration of the constructor with commas.

As told by @jsulm : this is C++ basic knowledge: https://en.cppreference.com/w/cpp/language/constructor

As Qt is a C++ framework, it will really help you, and save from frustrations, to to first learn C++ basics.

Swati777999

@jsulm I'm familiar with the private variables declaration in your code not with the function declared in the public section.

I've taken up courses of C++ in one of my undergraduate curriculum but have never found the syntax of C++ function like this before. So, put this question.

KroMignon

@Swati777999 said in Constructor syntax in Qt in mainwindow.cpp:

have never found the syntax of C++ function like this before

Because this is not a function but a class constructor declaration.

jsulm

@Swati777999 said in Constructor syntax in Qt in mainwindow.cpp:

syntax of C++ function like this before

This syntax is only valid for C++ constructors, not normal functions/methods.
A C++ course should actually cover this basic thing.
Please follow the link provided by @KroMignon

Swati777999

@JonB Your syntax is what I had expected and seen many times before. This is how the variables are initialized.

jsulm

@Swati777999 said in Constructor syntax in Qt in mainwindow.cpp:

This is how the variables are initialized.

This is a possibility. But in proper C++ projects this is not the way members are initialised. Such code would not get approved in projects I work on unless there is really a need to do it this way.
Correct way to initialise members is using the syntax you were asking about. So, I suggest you learn and use it. This is also nothing new, exists in C++ for ages already.

Swati777999

@KroMignon Precisely, a constructor is a member function of a class.

jsulm

@Swati777999 said in Constructor syntax in Qt in mainwindow.cpp:

a constructor is a member function of a class

It is a special member function of a class

KroMignon

@Swati777999 said in Constructor syntax in Qt in mainwindow.cpp:

Precisely, a constructor is a member function of a class.

I don't understand what your goal is?
You ask a question about very basic C++ syntax, for which you've got answer and pointer to C++ documentation to get more detailed explanation, but you don't seems to accept those responses.
Why?
Do I hurt you in any way?

jsulm

@jsulm Here an explanation why this specual syntax is better:

class A
{
public:
    A() { std::cout << "A" << std::endl; }
    A(int) { std::cout << "A(int)" << std::endl; }
};

class B
{
public:
    B() { a = A(1); }

private:
    A a;
};

B b;

In the above code you will see that both A constructors are called, even though you are creating only one A instance explicetly. Reason is that members are initialised for you if you don't do it explicetly. In this case a is initialised using default constructor first, but then you create a new one using the other constructor and assign it to a.
If you change

B() { a = A(1); }

to

B()
    : a(1)
{  }

you will see that only one constructor is called. So, it is more efficient to use the special constructor syntax.

mpergand

To go along with @jsulm
With const and reference variables, using the initializer list is mandatory.

class B
{
public:
    B() 
        { 
        a = A(1);
        str="hello";  // error
        }

private:
    A a;
    const string str;
};

Funny enough, that way you can initialize a const var two times !

class B
{
public:
    B() : str("hello2") 
        { 
        a = A(1);

        }

private:
    A a;
    const string str="hello";  // since  c++11
};