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

Question

Heres the code:

#include <cstdlib> #include <iostream> using namespace std;  class classA {                          protected:                 void setX(int a);        private:               int p; };  classA:: classA() {                      //error here.  p = 0; }  void classA:: setX(int a) {  p = a;     }  int main() {        system("PAUSE");     return EXIT_SUCCESS; } 

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.

Answer 2

class classA {                          protected:                 classA(); // you were missing an explicit declaration!                 void setX(int a);        private:               int p; };  classA:: classA() {   p = 0; }