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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.

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    and also sorry guys, for poorish messy code, I'm pretty noob at this, but still learning. Commented Mar 30, 2020 at 22:40

3 Answers 3

Reset to default
2

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
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2 Comments

Thanks, I really appreciate your help, @anastaciu.
@azizbekmasharibov, thank you my friend, I'm glad my answer helped you.
2

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)) { }
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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 ints or doubles, 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
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Wow, thanks for that clear explanation, really appreciate it. In short, thanks so much, @Miles Budnek!

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