Singleton pattern with MinGW

[Jendker]

Hello everyone,

I am right now switching from MSVC to MinGW and here proposed code for singleton does not work anymore producing following error in line 30 of singleton.h:

singleton.h:30: error: invalid conversion from 'Singleton<Database>::CreateInstanceFunction {aka Database* (*)()}' to 'QBasicAtomicPointer<void>::Type {aka void*}' [-fpermissive]
   Singleton::create.store(create);
   ^~~~~~~~~
qbasicatomic.h:240: note:   initializing argument 1 of 'void QBasicAtomicPointer<X>::store(QBasicAtomicPointer<X>::Type) [with X = void; QBasicAtomicPointer<X>::Type = void*]'
     void store(Type newValue) Q_DECL_NOTHROW { Ops::store(_q_value, newValue); }
          ^~~~~

What would be the way to go around it with MinGW?

EDIT: Maybe I am just creating the instance not properly to make it work with MinGW?
I am calling it like this (creating instance of class Database):

static inline Database* instance() { return singleton<Database>::instance(Database::createInstance); }

this is my "interpretation" because the example in main of the example:

// main.cpp

#include <QTextStream>
#include "myclass.h"

#define MyClassInstance Singleton<MyClass>::instance()

int main(int argc, char* argv[])
{
    QTextStream(stdout) << MyClass::instance()->metaObject()->className() << endl;
    return 0;
}

was not clear to me.

[kshegunov]

The function pointer has be cast to a data pointer, so that should be done manually, which it isn't.
But for the love of god, don't use that $%^@.

If you're so intent on having something globally accessible, just use Q_GLOBAL_STATIC instead.

[Jendker]

@kshegunov Thanks that solved it!

Global is very useful in my case, previously I was passing the Database object from main through all the classes to have access to it whenever possible. This makes it much more reasonable. But I understand, that globals are generally evil :)

[SGaist]

Hi,

If you are talking about a QSqlDatabase object then what you are doing is wrong. There's no need for any global object of that class as show in the details of the class.

[Jendker]

@SGaist Thank you, but this is actually the class where I am creating the tables when database is empty and holding all the models and necessary functions to work on them. So it is fine in this case :)