Answer 1

lambda current class object calling in Java 😑i don't know about cpp

Answer 2

You can specify definition in that way to add to class file, if you put declaration within the class...
Like :

class A {
public :
A();
..
}

A::A() {
cout<<"Constructor definition";
};

Is equal to :

class A {
public :
A() {
cout<<"Constructor definition";
}
..
}
I think, For better clear readability and later implementations, this way is good one..

:: is a scope resolution operator..
hope it helps..

Answer 3

https://www.sololearn.com/learn/CPlusPlus/1955/