Static variables and functions usage

Question

I have the following class definition and the main(). Can someone please point me why I am getting the error?

#include <iostream>
#include <list>
using namespace std;

class test
{
protected:
  static list<int> a;
public:
  test()
  {
    a.push_back(150);
  }
  static void send(int c)
  {
    if (c==1)
      cout<<a.front()<<endl;
  }
};

int main()
{
  test c;
  test::send(1);
  return 0;
}

The error that I get is as follows:

/tmp/ccre4um4.o: In function `test::test()':
test_static.cpp:(.text._ZN4testC1Ev[test::test()]+0x1b): undefined reference to `test::a'
/tmp/ccre4um4.o: In function `test::send(int)':
test_static.cpp:(.text._ZN4test4sendEi[test::send(int)]+0x12): undefined reference to `test::a'
collect2: ld returned 1 exit status

The error is same even if I use c.send(1) instead of test::send(1). Thanks in advance for the help.

ildjarn · Accepted Answer · 2011-05-31 21:38:06Z

6

You've declared test::a, but you haven't defined it. Add the definition in namespace scope:

list<int> test::a;

answered May 31, 2011 at 21:38

ildjarn

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3 Comments

Neel Mehta Over a year ago

Sorry. I did not understand. Can you please elaborate?

ildjarn Over a year ago

@Neel : What would you expect to happen if you declared a function void foo(); but called it without defining it? Clearly such a thing cannot work. The same goes for static data members -- declaring them is not sufficient, they must also be defined. My answer shows the syntax for that definition.

Neel Mehta Over a year ago

Thank you. I got it. Even after so many years of working with C++, I never recollect having done anything like this for static variables. Thanks for the help.

frankc · Accepted Answer · 2011-05-31 21:41:03Z

1

a is declared but must still be defined. http://www.parashift.com/c++-faq-lite/ctors.html#faq-10.12

answered May 31, 2011 at 21:41

frankc

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