Can non-static member function access static member function or data?

Question

I searched the Internet and found that some people said that non-static member function can access static member function or data. Then I wrote a program to verify it.

#include <iostream>
class test
{
public:
    static int a;
    void printa()
    {
        std::cout<<a;
    }
};

int main(int argc, const char * argv[])
{
    test m;
    m.printa();

    return 0;
}

The code generate linker errors!

Undefined symbols for architecture x86_64:
  "test::a", referenced from:
      test::printa() in main.o

Answer 1

Declaring a variable as static inside a class is, well, only a declaration.

You need to define the variable as well, which means adding this line in a single compilation unit :

int test::a = 0;

To be more precise : a compilation unit is basically a .cpp file. You should not put that line directly in a header file, otherwise you will get the opposite error : "multiple definition of...".

This will also, as you have guessed, initialize your variable to 0 once your program starts.

If you put this line under your class declaration, it will fix your problem (in this specific situation : remember not to write this in a header file).

Answer 2

That's because you've only declared test::a, not defined it:

#include <iostream>
class test
{
    ...
};

int test::a = 1; //Needs a definition!

Answer 3

You have only declared the static data member. You have not defined it.

YOu need to do something like int test:: a; to define it.

See this too

Non-static members are allowed to access static data members. The reverse is not allowed because static members do not belong to any object