Question

Apologies for such a basic question but I can’t figure it out. I know you can initialize a class like this:

QFile file("C:\\example");

But how would you initialize it from a global variable? For example:

QFile file; //QFile class

int main()
{
    file = ?? //need to initialize 'file' with the QFile class
}

Answer 1

1. Straightforward answer

If the class is assignable/copy constructible you can just write

QFile file; //QFile class

int main()
{
    file = QFile("C:\\example");
}

2. Use indirection

If not, you’ll have to resort to other options:

QFile* file = 0;

int main()
{
    file = new QFile("C:\\example");

    //
    delete file;
}

Or use boost::optional<QFile>, std::shared_ptr<QFile>, boost::scoped_ptr<QFile> etc.

3. Use singleton-related patterns:

Due to the Static Initialization Fiasco you could want to write such a function:

static QFile& getFile()
{ 
    static QFile s_file("C:\\example"); // initialized once due to being static
    return s_file;
}

C++11 made such a function-local static initialization thread safe as well (quoting the C++0x draft n3242, §6.7:)