I'm new to learning c and I have been experiencing a odd error that I can't find the solution for (even on stackoverflow).

Here is the code below.

 struct Date {
 void add_day(int n);
 int d,m,y;
 Date(int d,int m,int y);
};



int main()
{
Date today {3,4,5};
cout << today.d;

}

All I want is to create a simple date object using a constructor however I get a undefined reference, more specifically this error: undefined reference to `Date::Date(int, int, int)'.

If this is relevant, the compiler im using is g version 12.2.0 from MSYS2. I am coding on VSCode.

Im learning via Programming and Principles written by Bjarne Stroup. I am a complete beginner and apologize for asking such a basic question.

Thank you for taking the time to read this question.

CodePudding user response：

The main reason is that compiler does not know where is the function body of the construction function Date::Date(int d,int m,int y);

Solution

You can either implement its function body to replace ";" sign. Or you can implement its function body out side the struct.

Here is a demo with full code:

struct Date {
public:  // public is default for struct
    Date(int _d,int _m,int _y):d(_d){  // did init m is also ok
        y=_y;  // init variable inside the body is also ok
    }
    Date();  // second case, body is outside
void add_day(int n);  // did not implement, ok if not been called.
    int d,m,y;
};

Date::Date():d(0), m(0){
    y=0;
}

int main(){
  Date today(3,4,5);  // this triggers Date(int d,int m,int y)
  Date today2;  // this triggers Date()
  cout << today.d << endl;
  return 0;
}

• Compile and Run

  g   -Wall demo.cpp
  ./a.out
  

• Output

  3
  

p.s. Up-Vote is generous and kind. It also helps and supports the author, if the solution is useful.