Learning C++: Variable Scope

Variable scope refers to the ability of a variable to be “seen” in a program. A variable is seen if, in a specific place in a program, the programmer has access to that variable’s value.

Variable scope refers to the ability of a variable to be “seen” in a program. A variable is seen if, in a specific place in a program, the programmer has access to that variable’s value. There are levels of scope in a C++ program, with the level of scope determining how widely a variable can be seen in a program. In this article I’m going to discuss the different levels of variable scope and why they are important.

The Scope Levels

Here are the three levels of scope in a C++ program: global, local, and block. Global is the most expansive scope and means that any variable declared with global scope can be seen in any place in a program. Local scope usually refers to a variable defined in a function and can be seen throughout that function, but not in other functions. Block scope means a variable was defined within a block of code, such as a for loop or an if statement.

With these definitions out of the way, let’s look at how these different scope levels affect a C++ program.

Global Scope

A variable that has global scope can be seen in any other part of a C++ program, from a function definition to a block of code. A variable that is defined outside of the main function is a global variable.

Here is an example that demonstrates how global scope works:

#include <iostream>
using namespace std;

// global space
int number = 1;
void showValue() {
  cout << "Number accessed from a function: " << number << endl;
}
int main ()
{
  showValue();
  cout << "Number accessed from the main function: "
       << number << endl;
  for (int i = 1; i <= 1; i++) {
    cout << "Number accessed from a block: " << number << endl;
  }
  return 0;
}

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