Can a global variable be incremented in a class in c++?

Question

I want to increment a global variable inside a c++ class but can not configure out how to do so

int line = 0;
int ivar = 0;
int cvar = 0;
int svar = 0;

class file {
protected:
    char name[50];
    string read;
    string write;
    ifstream readfile;
    ofstream writefile;
    int i;
    int j;
};

I want to increment the cvar variable inside the class file. Something like

class file {
protected:
    char name[50];  cvar++;
    string read;
    string write;
    ifstream readfile;
    ofstream writefile;
    int i;
    int j;
};

But the compiler gives error if done this way. Can someone devise an alternative method to do so?

You can increment it inside of class method. Also global variables are not good. — user7860670, Jul 10 '17 at 11:23
You cannot put arbitrary code in a class declaration. As for proposing alternatives, we would have to know what you're trying to achieve. (P.S: `ivar`, `cvar` and `svar` are bad variable names) — Borgleader, Jul 10 '17 at 11:25
_But the compiler gives error if done this way._ Did you try.. I don't know.. Reading the error? That may have given you a clue on what you were doing wrong. — Algirdas Preidžius, Jul 10 '17 at 11:25
@AlgirdasPreidžius the error says says that "this declaration has no class or type specifier". — Zainzam, Jul 10 '17 at 11:30
@Borgleader I am trying to add a counter to my program that counts the number of character type variables declared in the complete program. — Zainzam, Jul 10 '17 at 11:32
@Zainzam Hence, you should've concluded that compiler was expecting a declaration. `cvar++` is not a declaration. You should read a [good C++ book](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) if you are having these kinds of problems. — Algirdas Preidžius, Jul 10 '17 at 11:35
@Zainzam you can't do this. BTW why do you want to do this ? It doesn't make much sense. Sounds like an [XY Problem](http://xyproblem.info/) — Jabberwocky, Jul 10 '17 at 11:36

score 2 · Answer 1 · answered Jul 10 '17 at 11:27

You can, but the incrementation needs to be in a method of the class.

Using the default constructor would work, and this has the bonus that the incrementation happens when you create a new instance of your class:

file::file() /*ToDo - declare this in the class definition*/
{
    cvar++;
}

Finally, it would be a good idea to use std::atomic<int> as the type for cvar, in order to prevent concurrency issues (namely simultaneous read and writes on cvar).

score 1 · Answer 2 · answered Jul 10 '17 at 11:27

You can't put lines of code to be executed in a class definition. They need to be in a function.

Perhaps in your case, a constructor:

class file {
public:
    file() {
        cvar++;
    }

protected:
    char name[50];  
    string read;
    string write;
    ifstream readfile;
    ofstream writefile;
    int i;
    int j;
};

There are several better approaches than global variables - which depend on what you are trying to achieve.

score 0 · Answer 3 · answered Jul 10 '17 at 12:10

From your comment:

I am trying to add a counter to my program that counts the number of character type variables declared in the complete program.

Global variables are not nice. Also "manually" incrementing a counter whenever you use a char[] isnt nice either. I would probably use something similar to this:

struct MyString {
    std::string theString;
    static int counter;
    MyString() { counter++;}
};

int MyString::counter = 0;

and then

struct file {
    MyString name;
    /*...*/
};

The static variable acts more or less like a global, but it doest pollute your namespace, because it is where it belongs (MyString::counter). Further, if you use MyString you have no chance to forget incrementing the counter each time you need a string. Last but not least I changed from char[] to std::string, because unless there is a reason std::string should be prefered over char[].

Can a global variable be incremented in a class in c++?

3 Answers3