C++ 11- scopes & global variables

Question

How can I reach global variables from inner scopes, given the following code sample, how can I reach the global string X from the main function and from the most inner scope as well, also is the most inner scope is accessible once we quit it to the main scope or other scope?

#include <iostream>
#include <string>
std::string x = "global";
int counter = 1;

int main()
{
    std::cout <<counter ++ << " " << x << std::endl;
    std::string x = "main scope";
    std::cout <<counter ++ << " "  << x << std::endl;
    {
        std::cout <<counter ++ << " " << x << std::endl;
        std::string x = "inner scope";
        std::cout <<counter ++ << " " << x << std::endl;
    }
    std::cout <<counter++ << " "  << x << std::endl;
}

the cout currently is:

1 global
2 main scope
3 main scope
4 inner scope
5 main scope

By using the scoping operator `::`, like e.g. `::x` which refers to the global variable `x`. — Some programmer dude, Feb 06 '15 at 08:46
@Theolodis Sure, am just trying to understand the idea of scopes — A_Matar, Feb 06 '15 at 11:25
The idea of scopes isn't to hide global variables... at least I think so ;) — Theolodis, Feb 06 '15 at 12:33

paxdiablo · Accepted Answer · 2015-02-07T01:40:33.193

Global scope can be reached by using ::x, as per:

#include <iostream>
#include <string>

std::string x = "global";
int counter = 1;

int main()
{
    std::cout << counter++ << " " << x << std::endl;
    std::string x = "main scope";
    std::cout << "  " << ::x << std::endl;
    std::cout << counter++ << " "  << x << std::endl;
    {
        std::cout << "  " << ::x << std::endl;
        std::cout << counter++ << " " << x << std::endl;
        std::string x = "inner scope";
        std::cout << "  " << ::x << std::endl;
        std::cout << counter++ << " " << x << std::endl;
    }
    std::cout << "  " << ::x << std::endl;
    std::cout << counter++ << " "  << x << std::endl;
}

which gives you:

1 global
  global
2 main scope
  global
3 main scope
  global
4 inner scope
  global
5 main scope

The hard bit is actually getting to the intermediate scopes, such as main scope when you're withing the inner scope.

One way to do that is with references:

#include <iostream>
#include <string>

std::string x = "outer";

int main()
{
    std::cout << "1a " << x << "\n\n";

    std::string x = "middle";
    std::cout << "2a " << ::x << '\n';
    std::cout << "2b " << x << "\n\n";

    {
        std::string &midx = x;  // make ref to middle x.
        std::string x = "inner";  // hides middle x.
        std::cout << "3a " << ::x << '\n';
        std::cout << "3b " << midx << '\n';  // get middle x via ref.
        std::cout << "3c " << x << "\n\n";
    }
}

which gives:

1a outer

2a outer
2b middle

3a outer
3b middle
3c inner

But, as good advice, you'll find you won't have anywhere near as many problems if you:

name your variables a little more intelligently so as to avoid clashes; and
avoid global variables like the plague :-)

And, as for the variables in inner scopes, they cease to be available once you leave that scope, even with a reference (you can copy them to a variable with an larger scope but that's not the same as accessing the inner-scoped variable).

sure, I will be using different names for variables, I am just trying to understand scopes. Thank you for the clear answer — A_Matar, Feb 06 '15 at 11:23

C++ 11- scopes & global variables

1 Answers1