c++ singleton, strange error [duplicate]

Question

I'm implementing a thread-safe singleton. But this aspect (singleton & thread-safe) is not part of my question.

Compare the two codes. Code 1:

#include <iostream>
using namespace std;
class DataLocation {
private:
  DataLocation(std::string) {
  }
public:
  DataLocation& getInstance() {
    std::string s = " ";
    static DataLocation instance(s);
    return instance;
  }
};
int main() {
}

and code 2:

#include <iostream>
using namespace std;
class DataLocation {
private:
  DataLocation() {
  }
public:
  DataLocation& getInstance() {
    static DataLocation instance();    
    return instance;
  }
};
int main() {
}

Code 1 compiles fine. Code 2 gives the following error:

15_singleton.cpp: In member function ‘DataLocation& DataLocation::getInstance()’:
15_singleton.cpp:15:34: error: cannot declare static function inside another function
     static DataLocation instance();    
                                  ^
15_singleton.cpp:16:12: error: invalid initialization of non-const reference of type ‘DataLocation&’ from an rvalue of type ‘DataLocation (*)()’
     return instance;
            ^

From my point of view the only difference is that the private constructor has one, respectively zero parameters.

How can I help the compiler to understand that I'm not defining anything new, but I'm just calling the constructor? The compiler is able to understand it, when there is one parameter.

Answer 1

Remove the brackets

static DataLocation instance; 

To create the instance with the default constructor.

Alternatively, use the braced form of initialisation.

static DataLocation instance {};

Answer 2

static DataLocation instance = DataLocation(); 

or

static DataLocation instance;

And you would probably want to declare DataLocation& getInstance(); as a static method.