Department of Engineering

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C++: Global warning

Programmers are often warned about using global variables (variables which are outside functions and classes). Here are some reasons why.

System include files

On my machine, the following code doesn't compile.

#include <iostream> int acct; int main() { int i; }

However, the following does -

int acct; int main() { int i; }

and so does

#include <iostream> int main() { int acct; int i; }

Why? The error message when compiling the first program says error: int acct redeclared as different kind of symbol /usr/include/unistd.h:837: error: previous declaration of int acct(const char*)

so it seems that using #include <iostream> causes /usr/include/unistd.h to be read, which happens to mention a routine called acct - in fact the offending line is

    extern int acct (__const char *__name) throw ();

Using global variables risks name clashes like this. The 3rd example compiles because the acct variable in main isn't global.


It's common to have

    using namespace std;

at the top of a file. This makes available the visible names in the std namespace so you can do things like

    cout << "test" << endl;

It's safer not to use using namespace std; and instead type

    std::cout << "test" << std::endl;

By doing this you don't bring into view all the std namespace names and you reduce the risk of name clashes.

More Safety

If you make a mistake when writing code you want to know as soon as possible - at compile time rather than run time. If you have lots of globals you risk creating unnecessary run-time errors. The example below is rather artificial, but in bigger programs I've seen the practical consequences of this approach.

int i1, i2; // I1 and I2 int main () { int il=0; // IL i1=i1+7; // I1 }

In this program, the programmer meant to write "il=il+7" ("IL=IL+7") rather than "i1=i1+7" ("I1=I1+7") . Unfortunately, the typing error doesn't cause a compilation error because "i1" exists as a global.

Again, having as few global names visible as possible increases the chances of producing a successful program.

Naming Rules

Some names are reserved for use by the compiler/libraries. Don't use

  • C++ keywords (for, auto, etc)
  • names that contain a double underscore - you might get away with it, but why take a chance?
  • names that begin with an underscore followed by an uppercase letter

Names that begin with an underscore followed by a lowercase letter are safe to use as long as they're not global.