Thread: Can not access static data member.

Here is the code:

Code:

#include <string>
#include <iostream>
#include <vector>
using namespace std;

class Person {
private:
	static const int _age = 20;
	static vector<int> _elems  ;
		
public:
	static void getAge();
	
};

void Person::getAge() {
	cout << _age << endl;
                //error occure here, _elems.size() can not be resolved.
	cout << _elems.size() << endl;
}

int main() {
	Person p;
	p.getAge();
	
	return 0;
}

The compiler error in Windows is: error LNK2019: unresolved external symbol,
Can any help me why data member "_age" can be accessed but "_elems" failed.
Thanks.

check out:
http://msdn.microsoft.com/library/de...m/class_26.asp

Static member functions have external linkage. These functions do not have this pointers. As a result, the following restrictions apply to such functions:

* They cannot access nonstatic class member data using the member-selection operators (. or >).
* They cannot be declared as virtual.
* They cannot have the same name as a nonstatic function that has the same argument types.

Your problem is that although you have declared the static member _elems, you haven't defined it. Add the statement below after your class definition in order to define _elems:

Code:

vector<int> Person::_elems;

All non-const static data members of a class need to be defined in this manner. Incidently, you shouldn't use identifier names that begin with an underscore as these are reserved for use by the implementation.

>> infact you cannot initilise anything inside class decleration like you have done

Sure you can. static const int _age = 20; is perfectly legal according to the standard, although I know of at least one old compiler that doesn't allow it.

>> Incidently, you shouldn't use identifier names that begin with an underscore as these are reserved for use by the implementation.

Actually, names that begin with an underscore followed by a capital letter are reserved for the implementation, so these names are legal, although some people suggest never using leading underscores to avoid confusion.