c++ - Invalid use of non-static data member -

i want create file in can add persons personlist including counter makes sure capacity of personlist not exceeded. commented functions make clear task(s) should perform.

during compilation following error:

invalid use of non-static data member 'personlist::m_capacity' 

unfortunately cannot figure out what's wrong in code:

main.cpp

#include <iostream> #include "person.h" #include "personlist.h"  int main() {      person p1 = person("john", 21);     person p2 = person("elham", 19);     personlist p_list = personlist();      p_list.add(p1);     p_list.add(p2);      std::cout << p_list.get_size() << std::endl;     person p = p_list.get(0);     std::cout << p.get_name() << " " << p.get_age() << std::endl;      return 0; } 

person.h

#ifndef person_h #define person_h  class person {     public:         person();         person(std::string name, int age);          void set_name(std::string name);         void set_age(int age);         std::string get_name();         int get_age();      private:         std::string m_name;         int m_age; };  #endif // person_h 

person.cpp

#include <string> #include "person.h"  person::person(){}  person::person(std::string name, int age){ // pre:  0 <= age // post: m_name, if pre name (is true) age = m_age, //       otherwise age = -1      set_name(name);      if (0 <= age){          set_age(age);     }      else{          set_age(-1);     }  }  void person::set_name(std::string name){ // post: m_name = name      m_name = name; }  void person::set_age(int age){ // pre:  0 <= age // post: if pre age (is true) = m_age, otherwise m_age = -1;      if (0 <= age){          m_age = age;     }      else set_age(-1); }  std::string person::get_name(){ // post: returns name      return m_name; }  int person::get_age(){ // post: returns age      return m_age; } 

personlist.h

#ifndef personlist_h #define personlist_h  class personlist {     public:         personlist();          void add(person p);         void set_size(int size);         int get(int index);         int get_size();      private:         const int m_capacity;         const person m_empty;         person m_data[m_capacity];         int m_size;  };  #endif // personlist_h 

personlist.cpp

#include "person.h" #include "personlist.h"  personlist::personlist() m_capacity(10), m_empty(person()) { // post: has created new personlist-object //      capacity elements , size = 0      set_size(0); }  void personlist::add(person p){ // pre:  size < capacity // post: if pre (is true) p has been stored @ //      first empty location. size incremented.      if (m_size < m_capacity){          m_data[m_size] = p;         set_size(m_size+1);     }  }  void set_size(int size){      m_size = size; }  int personlist::get(int index){ // pre:  0 <= index && index < size // post: if pre returns data[index] otherwise returns empty      if (0 <= index && index < m_size){          return m_data[index];     }      else{          return m_empty;     } }  int personlist::get_size(){ // post returns size;      return m_size;  } 

c++ not support variable length arrays (vlas), have here:

person m_data[m_capacity]; 

for work, m_capacity must compile time constant.

besides that, have syntax error in default constructor, : required indicate initialization list:

personlist::personlist() : m_capacity(10), m_empty(person()) { .... } //                       ^ here!