How can I use an initialization list to select which constructor to use?

Question

I just asked this question and the good answers mentioned using an initialization list. So I looked it up in many various places. It was often said that one can use an initialization list to select which constructor to use.

class First
   {private: 
         int a, b, c;
  public:
First(int x);
First(int x, int y);
}

First::First(int x, int y, int z = 0)
{ /* this is a constructor that will take two or three int arguements. */ }

First::First(int x, int y = 0, int z = 0)
{ /* and this will be called if one arguement is given */ }

I thought all assignments should be avoided, so how would I write the initializer lists for these two constructors?

Answer 1

Ctor initialization list isn't for the purpose of choosing which ver of ctor will be chosen.

Answer 2

I'm not quite sure I follow. As it stands, by providing an argument to x and y (and therefore z), both constructors will be available to call, resulting in ambiguity.

I think what you're looking for is:

class First
{
public:
  First(int x);
  First(int x, int y, int z = 0);
};    

// invoked as First f(1);
First::First(int x) :
a(x), b(0), c(0)
{}

// invoked as First f(1, 2); or First f(1, 2, 3);
First::First(int x, int y, int z) :
a(x), b(y), c(z)
{}

Answer 3

This is not how things work. If you want to use default arguments, use only one constructof, declare it with default arguments, and define it (without redefining them).

class First
{
  private: 
         int a, b, c;
  public:
First(int x, int y = 0, int z = 0);
};

First::First(int x, int y, int z)
{   /*...*/ }

Considering your question, i am not sure you know what an initialisation list is...

Answer 4

I think you mean this:

class First
{
private: 
    int a, b, c;
public:
    First(int x);
    First(int x, int y);
}

class Second
{
private:
Second(int x) : First(x) { }
Second(int x, int y) : First(x, y) { }
}    

Maybe?