Default value for parameter of class where value is another class-CodePudding

Title may be a little bit confusing but basically I have a class 'Quaternion' which has 2 parameters, the first being another instance of a class Vector3 and the other being a float.

Vector3 takes 3 floats as parameters and assigns them to x, y, and z.

I want to set default parameters for the Quaternion class but I am unsure how to set default parameters with a class as a parameter.

Vector3

class Vector3 {
  public:
    float x, y, z;

    Vector3(float uX, float uY, float uZ) {
      this->x = uX;
      this->y = uY;
      this->z = uZ; 
    }
};

Quaternion

class Quaternion {
  public:
    Vector3 axis;
    float scalar;

    Quaternion(Vector3 uAxis, float uScalar = 0) {
      axis = uAxis;
      scalar = uScalar;
    };
};

I would like to have the default parameter for uAxis to be a Vector3 with x, y, and z set to 1, 0, 0 respectively, but i am unsure how i can do this.

CodePudding user response：

I think this is what you were looking for:

class Quaternion {
  public:
    Vector3 axis;
    float scalar;

    Quaternion(Vector3 uAxis = Vector3(1.0, 0.0, 0.0), float uScalar = 0) {
      axis = uAxis;
      scalar = uScalar;
    };
};

It is possible to call a constructor of a class to set a default parameter. Here is the corresponding cpp reference default arguments.

CodePudding user response：

This is what you want. Always use the initialisation list on the constructor unless you have to do something with the parameters docs. Also I am passing the uAxis as const reference which is always a good thing to do for ADT (Abstract Data Types) Unless you have special requirements.

 class Quaternion 
 {
    Vector3 axis;
    float scalar;

 public:
    Quaternion(const Vector3 & uAxis = Vector3(1.0, 0.0, 0.0), float uScalar = 0) :
      axis(uAxis),
      scalar(uScalar)
    {
    }
 };

CodePudding user response：

Another option would be to use default member initialization for axis within Quaternion. That way, just creating a Quaternion q{} would set axis to whatever you want. [Demo]

class Quaternion {
public:
    Vector3 axis{1.0, 0.0, 0.0};
    float scalar{};

    Quaternion() = default;
    ...
};

For the Quaternion(Vector3 uAxis, float uScalar) constructor, it would be preferable to use a member initializer list:

Quaternion(Vector3 uAxis, float uScalar) : axis{uAxis}, scalar{uScalar} {};

Also, you can remove Vector3's constructor, making Vector3 an aggregate:

struct Vector3 {
    float x{};
    float y{};
    float z{};
};

Vector3 v{1.1, 2.2, 3.3};