Question

I am getting some errors when I try to put declaration and definition of a class into separatte hpp and cpp file. could you help me fix it please.
I am trying to manipulate a singleton like this:

sing.hpp:

class GlobalClass {
    int m_value;
    static GlobalClass *s_instance;
    GlobalClass(int);

  public:
    int get_value();
    void set_value(int v);
    static GlobalClass *instance(); };

sing.cpp:

#include"sing.hpp"
GlobalClass::GlobalClass(int v = 0)
{
    this->m_value = v;
}

int GlobalClass::get_value()
{
    return this->m_value;
}

void GlobalClass::set_value(int v)
{
    this->m_value = v;
}

static GlobalClass GlobalClass::*instance()
{
    if (!s_instance)
        s_instance = new GlobalClass;
    return s_instance;
}

main.cpp:

#include "sing.hpp"
int main()
{
    GlobalClass *s=0;
}

command and errors are:

~/workspace/singleton$ g++  main.cpp sing.cpp 
sing.cpp: In function ‘GlobalClass GlobalClass::* instance()’:
sing.cpp:19:10: error: ‘s_instance’ was not declared in this scope
sing.cpp:2:1: error: ‘GlobalClass::GlobalClass(int)’ is private
sing.cpp:20:23: error: within this context
sing.cpp:21:12: error: ‘s_instance’ was not declared in this scope

Answer 1

Your definition of instance has two errors:

1. The static qualifier is erroneous.

2. The syntax for the return type got scrambled. The pointer belongs to the type, not the name of the function:

   GlobalClass* GlobalClass::instance()
   {
       if (!s_instance)
           s_instance = new GlobalClass;
       return s_instance;
   }
   

Furthermore, you also need to define the static member s_instance as others have noted.

GlobalClass* GlobalClass::s_instance = 0;

But this code has another problem: it leaks memory. Don’t use raw pointers.

Finally, this code isn’t thread safe and this may in some situtaions be a huge problem. Assuming that you can guarantee that your code is never going to run in multi-threaded scenarios, go ahead. Otherwise, you probably want to change it (and who can offer such a strong guarantee anyway?).