global class

[nofx]

Hey, how can i define an instance of a class global?
Like in this example:

Code:

class cBlaat
{
public:
   DoSomething();
};

cBlaat * blaat;

int main()
{
blaat->DoSomething();
return 0;
}

This won't work because i'm trying to define the class cBlaat globally.
Is there anyway to do this??

[KTC]

Code:

#include <iostream>

class cBlaat
{
public:
    void DoSomething()
    {
        std::cout << "Hello\n";
    }
};

cBlaat blaat;

int main()
{
    blaat.DoSomething();
    return 0;
}

[nofx]

Thanks, that did the trick. But why doesn't it work when i declare it with a pointer cBlaat* blaat ..?? How come that only works when i do it inside of a function??
And what are the differences? Except that with one i use '->' to access the methods and with the other i simply use a dot '.'

[charlie]

When you add the * to the declaration, you're creating a pointer. In order to use that pointer, you have to make it point at something. The code you had shouldn't work inside a function either, because you never set blaat to point to any memory.

The fixed version from KTC just creates an object. You can use that object because it has been fully created.

In general, it's better to use the object like KTC does. There are specific instances where a pointer is a good choice, but you should wait until you learn about those before trying to use them.

Also, you should probably put your variable back inside a function. There are specific instances where a global variable might be a good choice, but in general it is better to keep your variables local to the functions you use them in.

[mailsubhra]

Code:

#include <iostream>

class cBlaat
{
public:
    void DoSomething()
    {
        std::cout << "Hello\n";
    }
};

cBlaat * blaat;

int main()
{
    blaat -> DoSomething();
    return 0;
}

Output:

[subchatt@indl120175 C++]$ c++ testPointer1.cpp
[subchatt@indl120175 C++]$ ./a.out
Hello
[subchatt@indl120175 C++]$

On the contrary:

Code:

#include <iostream>

class cBlaat
{
public:
    int i;
    void DoSomething()
    {
        std::cout << "Hello\n";
    }
    void setVal(int arg_i){
        i = arg_i;
    }
    int getVal(){
       return i;
    }
};

cBlaat * blaat = new cBlaat();

int main()
{
    blaat -> DoSomething();
    blaat -> setVal(3);
    std :: cout << blaat -> getVal() << std :: endl;
    return 0;
}

Output:

[subchatt@indl120175 C++]$ c++ testPointer.cpp
[subchatt@indl120175 C++]$ ./a.out
Hello
3
[subchatt@indl120175 C++]$

Changing

Code:

cBlaat * blaat = new cBlaat();

to

Code:

cBlaat * blaat;

will give a segmentation fault because no memory has been allocated.

[mailsubhra]

Please do not read my previous post. It is wrong.
Reply from charlie is absolutely correct. Ideally there must be a memory allocated in order a member function or a member variable to be accessed.

Code:

#include <iostream>

class cBlaat
{
public:
    void DoSomething()
    {
        std::cout << "Hello\n";
    }
};

cBlaat * blaat;

int main()
{
    blaat -> DoSomething();
    reinterpret_cast<cBlaat*>(NULL) -> DoSomething();
    return 0;
}

Output:

[subchatt@indl120175 C++]$ c++ testPointer1.cpp
[subchatt@indl120175 C++]$ ./a.out
Hello
Hello
[subchatt@indl120175 C++]$

[mailsubhra]

The code you had shouldn't work inside a function either, because you never set blaat to point to any memory.

Correct said charlie. Just because you compiler is not reporting an error does not mean that it is the correct usage. It is the sheer power of you compiler that is not making the condition fail. (Say thanks to your compiler and its designers)
Ideally

Code:

reinterpret_cast<cBlaat*>(NULL) -> DoSomething();

should have failed reporting segmentation fault. But it isn't. Rather it is calling and executing the correct function. But it doesn't mean that calling a member function using NULL is the right/appropriate way of coding.
Such cases should always be avoided since they are totally compiler dependent and one never knows how and when they might fail.