Question

I am trying to create an instance of a singleton class inside of a wrapper in a multithreaded environment. I am using the wrapper to ease my work and not to write multiple times Lock and Unlock in ManagerSources.

#define ManagerSOURCES() ManagerSources::GetInstance()

// Singleton
class ManagerSources :public Mutex {

protected:

    std::map< std::string , SourcesSPtr > Objects;

    static ManagerSources * Instance; // declared in a cpp file
    ManagerSources() {}
    ManagerSources( const ManagerSources& cpy ) {}
    ManagerSources operator=( ManagerSources& cpy) {}

public:

    static ManagerSources* GetInstance() {
        if ( Instance == NULL )
            Instance = new ManagerSources();
        return Instance;
    }

    ...
};

// This class is a wrapper for ManagerSources in a thread programming environment
template <class T>
class SingletonThreadSafe {

protected:

    T *pointer;

public:

    class proxy {
        T* pointer;

    public:

        proxy(T* _pointer) : pointer(_pointer) {
            // LOCK();            
        }

        ~proxy(){
            // UNLOCK();
        }

        T* operator->() {
            return pointer;

        }
    };


    // Default parameter is needed for containers (eg. insert into a map) where we need
    // a constructor without parameters
    SingletonThreadSafe(T* cpy = NULL ): pointer(cpy) {

    }

    ~SingletonThreadSafe() {
    }

    SingletonThreadSafe(const SingletonThreadSafe & cpy) {

        this->pointer = cpy.pointer;

    }

    SingletonThreadSafe operator=(SingletonThreadSafe cpy) {

            this->pointer = cpy.pointer;

            return *this;

    }

    T operator*() {

        return *pointer;

    }

    proxy operator->() {

        return proxy( pointer );

    }
};

I have the following declaration

typedef SingletonThreadSafe<ManagerSources> aa;

aa( ManagerSources::GetInstance() ); // doesn't work

or 

aa( ManagerSOURCES() ); // the same as above; still not working

The syntax doesn’t work and it gives me the following error “Definition or redeclaration of ‘GetInstance’ not allowed inside a function”. And, I don’t know why. Any ideas of how I can resolve this?

Also, the strange fact for me is if I rewrite the constructor with the default parameter to

SingletonThreadSafe(T* cpy = T::GetInstace() ): pointer(cpy) {

}

the following declaration works

aa()->A_Function(A_Parameter);

And if I declare it still works

aa bb( ManagerSOURCES() ); //  it works

( SmartPtr<ManagerSources>() = ManagerSOURCES() )->A_Function(A_Parameter); // it works; 
// the constructor with the  default parameter is called 

I don’t know why I get the error “Definition or redeclaration of ‘GetInstance’ not allowed inside a function “.

I am using Xcode 4.4.1 and LLVM GCC 4.2 compiler.

Answer 1

This is a variant of the most vexing parse.

aa( ManagerSources::GetInstance() ); // doesn't work

is interpreted as a declaration of a function ManagerSources::GetInstance taking no parameters and returning type aa.

You can work around it with a static_cast:

static_cast<aa>( ManagerSources::GetInstance() );

A cast to void would also work, as would anything else that prevents the expression being parsed as a declaration:

(void) aa( ManagerSources::GetInstance() );