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6.4 — Introduction to global variables

In lesson 6.3 -- Local variables [1], we covered that local variables are variables defined inside a function (or function parameters). Local variables have block scope (are only visible within the block they are declared in), and have automatic duration (they are created at the point of definition and destroyed when the block is exited).

In C++, variables can also be declared outside of a function. Such variables are called global variables.

Declaring and naming global variables

By convention, global variables are declared at the top of a file, below the includes, but above any code. Here’s an example of a global variable being defined:

The above example prints:

3
3
5

By convention, many developers prefix global variable identifiers with “g” or “g_” to indicate that they are global.

Best practice

Consider using a “g” or “g_” prefix for global variables to help differentiate them from local variables.

Global variables have file scope and static duration

Global variables have file scope (also informally called global scope or global namespace scope), which means they are visible from the point of declaration until the end of the file in which they are declared. Once declared, a global variable can be used anywhere in the file from that point onward! In the above example, global variable g_x is used in both functions doSomething() and main().

Because they are defined outside of a function, global variables are considered to be part of the global namespace (hence the term “global namespace scope”).

Global variables are created when the program starts, and destroyed when it ends. This is called static duration. Variables with static duration are sometimes called static variables.

Unlike local variables, which are uninitialized by default, static variables are zero-initialized by default.

Global variable initialization

Non-constant global variables can be optionally initialized:

Constant global variables

Just like local variables, global variables can be constant. As with all constants, constant global variables must be initialized.

Related content

We discuss global constants in more detail in lesson 6.9 -- Sharing global constants across multiple files (using inline variables) [2].

A word of caution about (non-constant) global variables

New programmers are often tempted to use lots of global variables, because they can be used without having to explicitly pass them to every function that needs them. However, use of non-constant global variables should generally be avoided altogether! We’ll discuss why in upcoming lesson 6.8 -- Why (non-const) global variables are evil [3].

Quick Summary


6.5 -- Variable shadowing (name hiding) [4]
Index [5]
6.3 -- Local variables [1]