c++ singleton, strange error

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.

About your threadsafe part, read up on C++ guarantees for initialization time and order of statics. — deviantfan, May 16 '16 at 09:50
@πάνταῥεῖ: That is _not_ the most vexing parse. Please do not spread misconceptions!! — Lightness Races in Orbit, May 16 '16 at 10:14
@LiPo MikeMB means you could use `static DataLocation instance{};` Since C++11, this is possible. One of the reasons why it was introduced is the problem you had here; with {} there is no ambiguity anymore. — deviantfan, May 16 '16 at 10:14
@MikeMB: ok, the solution with curly brackets is simply the nicest. — PeptideChain, May 16 '16 at 11:12

score 5 · Accepted Answer · edited May 16 '16 at 10:46

5

Remove the brackets

static DataLocation instance;

To create the instance with the default constructor.

Alternatively, use the braced form of initialisation.

static DataLocation instance {};

edited May 16 '16 at 10:46

Niall

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answered May 16 '16 at 09:47

Sebastian Hoffmann

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@LiPo This is perfectly "logical"; MVP is a well defined language rule. – Bartek Banachewicz May 16 '16 at 09:52
2

well defined is not necessarily logical. one parameter: foo(param), no parameter foo(). that is for me logical. bu we are all different and we have different logics – PeptideChain May 16 '16 at 09:54

score 2 · Answer 2 · answered May 16 '16 at 09:50

2

static DataLocation instance = DataLocation();

or

static DataLocation instance;

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

answered May 16 '16 at 09:50

Teivaz

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Not sure why you would want the first version. Especially as, for a singleton, you usually disable the copy constructor so the first version should not compile. – Galik May 16 '16 at 10:11

score 2 · Answer 3 · answered May 16 '16 at 09:52

You are using brackets () while declaring

static DataLocation instance();

which you intent to create as an object. But is, simply a function declaration. And you cannot create a static function inside some other non-static function. That is what your error refers to.

score 1 · Answer 4 · answered May 16 '16 at 09:48

1

Method getInstance() should be declared as a static.

answered May 16 '16 at 09:48

Radek

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I would also define it static, but I used this [model](http://stackoverflow.com/a/19907903), and the model got +33...and I'm a beginner – PeptideChain May 16 '16 at 09:58
@Niall: There is no ambiguity. Surprise for some? Sure. But by definition there is no ambiguity. BTW, the most vexing parse relates to `A a(B());`. It's caused by the same language rules as a novice's surprise after using the wrong declarator syntax. – Lightness Races in Orbit May 16 '16 at 10:38
1

@LiPo: I did't downvote, but while Radek is right, it doesn't answer the question. – MikeMB May 16 '16 at 10:56

c++ singleton, strange error

4 Answers4