Can anyone help me understand this error? “definition of implicitly-declared ‘classA::classA()’”

Question

Heres the code:

#include 
#include 
using namespace std;

class classA
{                   
      protected:
                v         


        

Answer 1

class classA
{                   
      protected:
                classA(); // you were missing an explicit declaration!
                void setX(int a);

      private:
              int p;
};

classA:: classA()
{ 
 p = 0;
}