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

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.