QT : operator after class declaration [duplicate] - c++

Question

This question already has answers here:
What is this weird colon-member (" : ") syntax in the constructor?
(14 answers)
Closed 1 year ago.
Recently I started working on a QT project, but there are more syntax rules than regular C++.
While declaring class and its constructor, in header file, we simply write
class MyObj : public QObject {
Q_OBJECT
public:
explicit MyObj(QObject* parent = nullptr, <params>, ...)
.
.
<some more declarations>
private:
.
.
<some more declarations>
};
In .cpp file, we can define this constructor as
MyObj::MyObj(QObject* parent, <params>) :
QObject(parent),
...
<some more parameter like arguments, that is what I am exactly asking>
{
<regular initializations>
}
I am asking about the parameter like arguments after the : operator in the first line of declaring an initializing the class.
What are they, why QT needs these?
Note: In QT project examples, you can simply find the "Command Line Writer Async" example. In that project, a proper usage of this fact exists. I am using QT 5.15.

Answer 1

That's not an operator. That's a standard c++ feature called member initializer list.
For example
class Test
{
private:
int test_val;
some_type some_test;
public:
Test() :
test_val(7),
some_test()
{}
.
.
//whatever
}
In this example, when an instance of Test is constructed, the members are constructed(or initialized) as shown in the code. test_val is initialized with 7, and some_test is constructed via the call of the default constructor of some_type.
If you written the code above as follows.
class Test
{
private:
int test_val;
some_type some_test;
public:
Test()
{
test_val=7;
some_test=some_type(args...);
}
}
This is equivalent to.
class Test
{
private:
int test_val;
some_type some_test;
public:
Test() :
test_val(),
some_test()
{
test_val=7;
some_test=some_type(args...);
}
}
Which means that every member is constructed(or initialized) by-default, and then is copy-constructed as requested in the body of Test::Test().
The mere purpose for member initialization list is to specify how the members should be initalized
Member initializer list is the place where non-default initialization of these objects can be specified. For bases and non-static data members that cannot be default-initialized, such as members of reference and const-qualified types, member initializers must be specified.