Accessing static variable from a friend function

Question

class Base
{
private:
    static int num; 
public:
    friend void setnum(Base obj); 
};

void setnum(Base obj)
{
    obj.num=4;  /* Error */
}

A friend function is supposed to have access to all the private data of a class. what am i missing here? I cant seem to access the the static variable from the friend function.

Error from codepad--> In function setnum(Base)': undefined reference to Base::num'

Error from visual studio--> error LNK2001: unresolved external symbol "private: static int Base::num"

Answer 1

You only declared the static variable num. You must to define it:

class Base
{
private:
    static int num; 
public:
    friend void setvals(Base obj); 
};

// This must be in a .cpp
int Base::num;

void setvals(Base obj)
{
    obj.num=4;
}

This code works.

Edit:

Actually you can implement the setvals() function as follows:

void setvals()
{
    Base::num=4;
}

And at your Base class:

friend void setvals(); 

Because num is static.