C++: Classes & Constructors: Using Initialization Lists to Initialize Fields - c++

Question

Full code. Line specified later.
#include <iostream>
#include <string>
using namespace std;
class X
{
private:
int i;
float f;
char c;
public:
X(int first=1, float second=2.0, char third='a') : i(first) , f(second) , c(third) { }
void print() { cout << i << " " << f << " " << c << endl;}
};
int main()
{
X var1;
var1.print();
return 0;
}
What exactly is going on on this line:
X(int first=1, float second=2.0, char third='a') : i(first) , f(second) , c(third) { }
As far as I can understand (could be wrong), we are declaring objects first, second, and third of type (class) X. We are initializing them during declaration. What's going on after the colon? What's going on altogether?

Answer 1

What exactly is going on on this line:
X(int first=1, float second=2.0, char third='a')
: i(first) , f(second) , c(third) {
}
It's a constructor which takes 3 parameters with default values.
This part : i(first) , f(second) , c(third) is called a member initializer list.The member initializer list consists
of a comma-separated list of initializers preceded by a colon. It’s placed after the closing
parenthesis of the argument list and before the opening bracket of the function body.
Only constructors can use this initializer-list syntax. const class members must be initialized in member initializers.

Answer 2

The stuff in the parenthesis are default values for the arguments to the constructor, and the stuff after the colon initializes those items after the colon.
The colon notation is most commonly used to invoke the constructors of other objects the class uses.

Answer 3

You have class X with three fields: i, f, c.
You have defined a constructor with three default parameters - when this constructor invoked fields are initialized with parameters (passed to constructor or defaults).
It's like:
X (int first=1, float second=2.0, char third='a')
{
i = first;
f = second;
c = third;
}
Your line is just an another way to initialize fields, usually they are equal (there are some differences with inheritance).
In your main code you're creating a local variable var1 of type X, so the constructor is called. You don't pass any parameters, so default values are used.
As result, you have one local object of type X initialized with default values listed in constructor.

Answer 4

X(int first=1, float second=2.0, char third='a') : i(first) , f(second) , c(third) { }
This is a constructor for X. There are three agruments, each having a default value so that it can be invoked in different ways
X myX;
X myX(first);
X myX(first, second);
X myX(first, second, third);
The section after the arguments
: i(first) , f(second) , c(third)
are initializers for the i, f and c members. If possible this style of initialization is preferred over initialization in the function body, and if the members are constant types, this style is required.
Then there is an empty body for the constructor {}