Linker error when static member function access static private variable

Question

I am new to c++ and exploring the static feature on class members and functions.

person.h

#include <iostream>

using namespace std;

namespace school 
{

class Person 
{

    public:
        static const int MAX=99;

    private:    
        static int idcount;

    public:
        static void setID(int id) { idcount = id;}
        static int getID() { return idcount;}

    private:
        string name;    

    public:
        Person();
        Person(const Person &other);
        ~Person();  


};

}

Person.cpp

#include "person.h"

namespace school 
{

    //Constructor
    Person::Person()
    {
        this->name = "";
        cout << "object created" << endl;
    }

    //Copy Constructor
    Person::Person(const Person &other)
    {
        this->name = other.name;
    }

    //Destructor
    Person::~Person()
    {
        cout << "Destructor Called" << endl;
    }
}

main.cpp

#include "person.h"

int main()
{


    cout << school::Person::MAX << endl;
    school::Person::setID(5);
    cout << school::Person::getID() << endl;

    return 0;
}

i am getting the below linker error when i compile the above code. but when i change the idcount to public and declare it main ( int Person::idcount;) then i have no issues.

D:\Hari\Project\CPP_Practise\build>mingw32-make
[ 50%] Built target person
Scanning dependencies of target app
[ 75%] Building CXX object CMakeFiles/app.dir/chapter2/classes2.cpp.obj
[100%] Linking CXX executable app.exe
CMakeFiles\app.dir/objects.a(classes2.cpp.obj): In function 
`ZN6school6Person5se
tIDEi':
D:/Hari/Project/CPP_Practise/chapter2/person.h:21: undefined reference to 
`schoo
l::Person::idcount'
CMakeFiles\app.dir/objects.a(classes2.cpp.obj): In function 
`ZN6school6Person5ge
tIDEv':
D:/Hari/Project/CPP_Practise/chapter2/person.h:22: undefined reference to 
`schoo
l::Person::idcount'
collect2.exe: error: ld returned 1 exit status
CMakeFiles\app.dir\build.make:97: recipe for target 'app.exe' failed
mingw32-make[2]: *** [app.exe] Error 1
CMakeFiles\Makefile2:103: recipe for target 'CMakeFiles/app.dir/all' 
failed
mingw32-make[1]: *** [CMakeFiles/app.dir/all] Error 2
Makefile:82: recipe for target 'all' failed
mingw32-make: *** [all] Error 2

What should i do when i am using it as private static variable?

You don't need change it to `private`, just define it as `int Person::idcount;`, better in `Person.cpp`. — songyuanyao, Dec 01 '17 at 05:29
You haven't shown the error, but I'm going to guess it's covered by the bottom of this answer: https://stackoverflow.com/questions/12573816/what-is-an-undefined-reference-unresolved-external-symbol-error-and-how-do-i-fix/12574407#12574407 — chris, Dec 01 '17 at 05:30
yes adding it to the person.cpp - resolves the error, but just curious what is the theory behind it. so every static int, private or public should be declared in class definition? — hariudkmr, Dec 01 '17 at 05:45

score 3 · Accepted Answer · answered Dec 01 '17 at 06:11

3

1.Static variable means it is common for all the objects created for that class. 2.Whatever written in header file acts as a blueprint ie it doesnt allocate memory for it. For this reason all static variables are defined in cpp implementation file. Compiler allocates memory for those defined in cpp files.

answered Dec 01 '17 at 06:11

so it doesnt matter whether it is private or pubic? – hariudkmr Dec 01 '17 at 06:12
It doesnt matter if it is private/public variable. Only static member of that class can access the private variable. You can try a sample app if you want. – Dec 01 '17 at 06:26

score 0 · Answer 2 · answered Dec 01 '17 at 06:02

0

Person::i still needs to be defined (outside the class body): int Person ::i;

Only those statics that aren't assigned a value inside the body need to be defined outside. And only non-volatile constant integrals can have such an in-body assignment.

answered Dec 01 '17 at 06:02

SoronelHaetir

14,104
1
12
23

to extend a bit - except const static variable all others whether private or public should be declared in the file which it is accessed? – hariudkmr Dec 01 '17 at 06:14

Linker error when static member function access static private variable

2 Answers2