Variables after the colon in a constructor [duplicate]

Question

I am still learning C++ and trying to understand it. I was looking through some code and saw:

point3(float X, float Y, float Z) :
x(X), y(Y), z(Z)  // <----- what is this used for
{
}

What is the meaning of the "x(X), y(Y), z(Z)" sitting beside the constructor's parameters?

Answer 1

It's a way of invoking the constructors of members of the point3 class. if x,y, and z are floats, then this is just a more efficient way of writing this

point3( float X, float Y, float Z): {    x = X;    y = Y;    z = Z; } 

But if x, y & z are classes, then this is the only way to pass parameters into their constructors

Answer 2

In your example point3 is the constructor of the class with the same name (point3), and the stuff to the right of the colon : before the opening bracket { is the initialization list, which in turn constructs (i.e. initializes) point3's member variables (and can also be used to pass arguments to constructors in the base class[es], if any.)