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

You forgot to declare the constructor in the class definition. Declare it in public section of the class (if you want clients to create instance using it):

class classA
{ 
      public: 
              classA();    // you forgot this!       
      protected:
                void setX(int a);

      private:
              int p;
};

Now you can write its definition outside the class which you've already done.