explicit & implicit constructors

Question

I've read a number of questions on stackoverflow regarding implicit and explicit constructors, but I'm still having trouble distinguishing between an implicit and explicit constructors.

I was wondering if someone could give me a good definition and some examples or maybe direct me to a book / resources that explains this concept well

Are you thinking about this? [What does the explicit keyword in C++ mean?](https://stackoverflow.com/questions/121162/what-does-the-explicit-keyword-in-c-mean/121163) — Csq, Aug 14 '15 at 09:00
People are just going to post the same information you can already get on "a number of questions on stackoverflow...". — juanchopanza, Aug 14 '15 at 09:20
If what you've read didn't sink in, ask ***specific*** questions about the confusing bits.... — Tony Delroy, Aug 14 '15 at 09:28

Nishant · Answer 1 · 2015-08-14T09:11:43.567

Take this as an example:

class complexNumbers {
      double real, img;
    public:
      complexNumbers() : real(0), img(0) { }
      complexNumbers(const complexNumbers& c) { real = c.real; img = c.img; }
      complexNumbers( double r, double i = 0.0) { real = r; img = i; }
      friend void display(complexNumbers cx);
    };
    void display(complexNumbers cx){
      cout<<&quot;Real Part: &quot;<<cx.real<<&quot; Imag Part: &quot;<<cx.img<<endl;
    }
    int main() {
      complexNumbers one(1);
      display(one);
      display(300);   //This code compiles just fine and produces the ouput Real Part: 300 Imag Part: 0
      return 0;
    }

Since the method display expects an object/instance of the class complexNumbers as the argument, when we pass a decimal value of 300, an implicit conversion happens in-place.

To overcome this situation, we have to force the compiler to create an object using explicit construction only, as given below:

 explicit complexNumbers( double r, double i = 0.0) { real = r; img = i; }  //By Using explicit keyword, we force the compiler to not to do any implicit conversion.

and after this constructoris present in your class, the statement display(300); will give an error.

explicit & implicit constructors

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