Initializing global variable class

问题:

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
}

回答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:)

回答2:

The same way:

// file.cpp

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

int main()
{
  // use `file`
}

The constructors of all global objects execute before main() is invoked (and inversely for destructors).

回答3:

#include <iostream>
struct QFile
{
    char const* name;
    QFile(char const* name) : name(name) {}
};

QFile file("test.dat"); // global object

int main()
{
    std::cout << file.name << std::endl;
}

By the way, you wrote:

I know you can initialize a class

You can not initialize a class. You create and initialize object, not a class.

回答4:

It is totally up to you to decide whether you use a global variable or some thing else.

For a global variable you can define it in globally in the file and then initialize it in function as below, make sure that you write copy constructor properly.

QFile file; //class object

int main()
{
    file = QFile("C:\\example"); //create a temporary object and copy it to `file`
}

But, it is better if you use global pointer,

QFile *file;

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

Note, if you are using this global variable only within the file, you can make it a static variable. Then it is limited to file itself. So you can minimize the bad of global variables.