namespace in static functions

Question

I want to write a little Singleton class that looks like:

#include <vector>

class Interpreter {

private:
    static Interpreter* interInstance;
    Interpreter() {}

public:
    static Interpreter* getInstance();
    ~Interpreter() {}

};

Interpreter* Interpreter::interInstance = 0;

Interpreter* Interpreter::getInstance(){
if (!interInstance)
    interInstance = new Interpreter();

return interInstance;
}

But this will produce this exception:

multiple definition of `Interpreter::getInstance()

This error can be corrected by wrapping the class and the function in one namespace. But I don't really understand why I need a namespace. There is one declaration of getInstance() and one implementation, no?

If you are including this code from several translation units, then there are several implementations of getInstance() and interInstance — K-ballo
– K-ballo, Commented May 15, 2012 at 19:11
Adding on, you should always wrap your headers in a header guard. — Qaz
– Qaz, Commented May 15, 2012 at 19:13
Are you using an anonymous namespace, i.e. namespace { /* stuff */ }? — Niall C.
– Niall C., Commented May 15, 2012 at 19:20

Luchian Grigore · Accepted Answer · 2012-05-15 19:15:53Z

2

Move the definition outside the header, in an implementation file, for both the member initialization and the method:

Interpreter.h

class Interpreter {

private:
    static Interpreter* interInstance;
    Interpreter() {}

public:
    static Interpreter* getInstance();
    ~Interpreter() {}

};

Interpreter.cpp

#include "Interpreter.h"
Interpreter* Interpreter::interInstance = 0;

Interpreter* Interpreter::getInstance(){
if (!interInstance)
    interInstance = new Interpreter();

return interInstance;
}

Inside a class or struct definition, static doesn't give symbols internal linkage as it does outside, so you're breaking the one definition rule.

If multiple translation units include a header that contains non-inline methods or define the same symbol, you'll run into multiple definition.

answered May 15, 2012 at 19:15

Luchian Grigore

260k68 gold badges465 silver badges634 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

abarnert Over a year ago

It might be worth adding a pointer to some larger description of the many inconsistent meanings of "static" in C++, but I think that one-line explanation is as good as anyone's ever going to do in one sentence.

Collectives™ on Stack Overflow

namespace in static functions

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related