C++ FAQ / Section 27 / FAQ 27.15

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Revised Jul 4, 2012

[27.15] What's a good coding standard for using global variables? New!

The names of global variables should start with //.

Here's the ideal way to declare a global variable:

// int xyz;    ←the thing that makes this global ideal is the leading //

Here's the ideal way to use a global variable:

void mycode()
{
  ...
  // do_something_with(xyz);    ←the thing that makes this global ideal is the leading //
  ...
}

Okay, so this is a joke. Sort of. The truth is that there are cases when global variables are less evil than the alternatives — when globals are the lesser of the evils. But they're still evil. So wash your hands after using them. Twice.

Instead of using a global variable, you should seriously consider if there are ways to limit the variable's visibility and/or lifetime, and if multiple threads are involved, either limit the visibility to a single thread or do something else to protect the system against race cases.

Note: if you wish to limit the visibility but neither lifetime nor thread safety, two of the many approaches are to move the variable into a class as a static data member, and to use an unnamed namespace. Here is how to move the variable into a class as a static data member:

class Foo {
  ...
  static int xyz;  // See here and here for idioms about initialization order
  ...
}

Here is how to use an unnamed namespace:

namespace {
  ...
  int xyz;  // See here and here for idioms about initialization order
  ...
}

Repeat: there are three major considerations: visibility, lifetime, and thread safety. The above two examples address only one of those three considerations.