Re: Am I or Alexandrescu wrong about singletons?

Ulrich Eckhardt <>
Tue, 16 Mar 2010 04:42:41 CST
DeMarcus wrote:

template<typename T>
class Singleton
    static T& getInstance()
       return *instance_;

    typedef volatile T* SPtr;
    static SPtr instance_;


S::getInstance() = 4711;


But when I compile it with gcc 4.4.1 I get the following error message
at 'return *instance_;'.
"error: invalid initialization of reference of type 'int&' from
  expression of type 'volatile int' "

The compiler is telling you exactly what you do wrong. Imagine you exchanged
the "volatile" against a "const", what would you expect? Similarly, a simple
"const_cast<int&>(*instance_)" will convince the compiler for you.

1. You wouldn't need the template stuff for this example. Makes getting some
things right more complicated.
2. Your instance_ pointer is never initialized.
3. The exact semantics of volatile differ by compiler, the standard only
requires that it behaves similarly to const in above aspect (CV-qualifier).
4. If you had written "T volatile*" instead of "volatile T*", you could use
the same schema (CV-qualifier binding to the left) when creating a volatile
pointer (T* volatile) instead of a pointer to volatile, which I guess is
what you wanted actually.


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