Question

Here is the code,

class A {
public:
    static A *get_a()
    {
        if(_pa == 0)
            _pa = new A;
        return _pa;
    }

private:
    static A *_pa = 0;  //cannot compile
};

In the above code, if I move _pa‘s definition outof the class,

A * A::_pa = 0;  //can compile

My problem is, static A *_pa = 0 inside the class body is just a declaration, not a definition, right?

Moreover, is it valid to assign a value to a static data member inside the class?

Answer 1

Unless it’s a const integral type (char, short, int, …) you have to define the static member in a .cpp-File in addition to the declaration in the header.

header:
class XYZ {
  static XYZ * instance;
};

//cpp:
XYZ * XYZ::instance;