如何从另一个类参数访问类构造函数参数

Question

STRUGGLING WITH C++ CONSTRUCTOR ARGUMENTS

So, I've just came from TS/JS/Py and trying to understand C++ concepts. But I'm struggling with using the parameter of constructor of the class FOR declaring default value for an argument. Here is the code I'm trying to run:

double Phythagorean_Hypotenuse (int& a, int& b ) {
    return sqrt((a * a) + (b * b));
};

class Triangle {
    public:
      int a;
      int b;
      double c;
      Triangle(int a_param, int b_param, double c_param = Phythagorean_Hypotenuse(a_param, b_param)) {
            a = a_param;
            b = b_param;
            c = c_param;
      }
};

and inside of the main function

Triangle mytri_1(10, 20);
std::cout << mytri_1.a << std:endl;

But when I try to run this code, IDE is throwing me some errors like

[Error] 'a_param' was not declared in this scope

or

[Error] call to 'Triangle::Triangle(int, int, double)' uses the default argument for parameter 3, which is not yet defined

So, please, can someone who can fix this answer the question?

Thanks.

Answer 1

As the compiler is pointing out, the other constructor arguments are not available as default parameters for the c_param argument. Rather than using default values, just overload the constructor, including one that just accepts 2 parameters. This constructor can then invoke the other constructor that accepts all 3:

 // Constructor overload that accepts all 3 parameters
 Triangle(int a_param, int b_param, double c_param):
  a(a_param), b(b_param), c(c_param) {
  }

  // Constructor overload that accepts just a and b, call the other constructor
  // to set all 3 members
  Triangle(int a_param, int b_param):
  Triangle(a_param, b_param, Phythagorean_Hypotenuse(a_param, b_param)) {
  }

Answer 2

Default parameter values cannot reference other parameters. You can define two overloads, one of which delegates to the other, to do what you want:

class Triangle {
public:
    double a;
    double b;
    double c;

    Triangle(double a_param, double b_param, double c_param)
        : a{a_param},
          b{b_param},
          c{c_param}
    {}

    Triangle(double a_param, double b_param)
        : Triangle{a_param, b_param, Phythagorean_Hypotenuse(a_param, b_param)}
    {}
};

Live Demo

---

A few other notes:

• Class constructors do not have a return type. I changed void Triangle(...) to Triangle(...)
• I used constructor initialization lists instead of assignment in the constructor's body. There's likely no difference for small primitive values like int s or double s, but it's a good habit to get into and can make a big difference for more complex types
• int doesn't make sense for the type of c (or a or b for that matter). The sides of a triangle are unlikely to all be integers
• There's no reason to pass parameters to Pythagorean_Hypotenuse by reference. It's simpler and likely faster to pass them by value

Answer 3

There are some issues that prevent your code from compiling, namely:

1. Constructors do not have return type.
2. double c_param = Phythagorean_Hypotenuse(a_param, b_param) is not valid for a parameter, a_param, b_param will not be recognized.

Recommend change:

Since the result of a hypothenuse calculation will most likely be a decimal value, c should be a double .

You can do something like this:

Running sample

#include <iostream>
#include <cmath>

double Phythagorean_Hypotenuse (int& a, int& b ) {
    return sqrt((a * a) + (b * b));
};

class Triangle {
    public:
      int a;
      int b;
      double c; //should be double

      //initializer list is a good practice for member initialization
      Triangle(int a_param, int b_param) 
          : a(a_param), b(b_param), c(Phythagorean_Hypotenuse(a, b)) {} 
};

int main(){

    Triangle mytri_1(10, 20);
    std::cout << mytri_1.a << std::endl;
    std::cout << mytri_1.b << std::endl;
    std::cout << mytri_1.c << std::endl;
}

Output:

10
20
22.3607