Re: what's wrong with the following singleton class???

From:

"Alf P. Steinbach" <alfps@start.no>

Newsgroups:

comp.lang.c++

Date:

Sun, 30 Apr 2006 07:26:21 +0200

Message-ID:

<4bj03vF11o1n0U2@individual.net>

* Alf P. Steinbach:

* ying.gary.zhang@gmail.com:

// T.h:

class T
  {
  public:
    static T* instance();
  private:
    T() {}
    ~T() {}
    static T* smInstance;
  };

// T.cpp:

T* T::instance()
  {
  if (smInstance == NULL)
    smInstance = new T();

  return smInstance;
  }

when I try to compile the above code, there is linker error:
../T.cpp:3: undefined reference to `T::smInstance`

I am using gcc 3.4.6 under gentoo linux, thanks

You have declared but not defined 'smInstance'.

It should be defined in your [T.cpp] file.

But instead, just do

  class T
  {
  private:
      T() {}
      T( T const& );
      ~T() {}
  public:
      static T& instance()
      {
          T theInstance;

Should be

static T theInstance;

of course.

          return theInstance;
      }
  };

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