They are vaiables that can be accessed from any function. For example, how many times is function foo() called?
#include <iostream>
int numCalls = 0;
void foo() {
++numCalls;
}
int main() {
foo(); foo(); foo();
std::cout << numCalls << endl; // 3
return 0;
}
numCalls
is a global variable. Where a
variable was declared,
determines where it
can be accessed from.