What are the five advantages and five disadvantages of using global variables?

Answer 1

The only case where a global variable is advantageous is when that global is a constant variable and it represents a truly global concept. The value of PI, for instance, is a truly global concept and it has a constant value. The exchange rate between dollars and Sterling is also a truly global concept, but it is non-constant and should not be declared global.

Global (non-constant) variables are problematic because it can be difficult to track down all the places that interact with a global, especially if the global has external linkage. Even if that is not a problem in itself, a global cannot cater for both single-threaded and multi-threaded applications. If intended purely for single-threaded applications then there will be no synchronisation mechanism, thus it cannot be used in a multi-threaded application as this could lead to a data race. Conversely, if intended purely for multi-threaded applications, it will have a synchronisation mechanism but this could lead to performance issues on single-threaded applications where synchronisation is not required.

In trivial applications, global variables can often be useful because the scope of a global can be well-defined. But in non-trivial applications, it becomes more difficult to limit the scope of a global. One way to limit the scope is to declare the global variable static, thus limiting its scope to the translation unit in which it is declared (static global variables cannot have external linkage). However, by limiting the scope, the variable is no longer a truly global concept.