CppScope.md

(C++) scope

The scope of a variable is the area where its name is valid.

There are multiple types of scope [10]:

• local scope
• class scope
• namespace scope
• global scope
• statement scope
• function scope

A variable declared inside a function is only valid inside that function. Thus its scope is inside the function.

A scope is defined by accolades, as shown in the commented example below:

#include <iostream>

int main()
{
  //Start of main() scope
  int a = 0;
  {
    //Start of scope inside of main() scope
    int b = 0;
  } //b goes out of scope
  for (int i=0; i!=10; ++i)
  {
    //Start of for-loop scope in main() scope
    //Note: i is scoped to here
    std::cout << i << ": Hello world\n";
  }
}
``` 

A [variable](CppVariable.md) 'without scope' is called a
[global](CppGlobal.md) [variable](CppVariable.md). Avoid using
[global](CppGlobal.md) data \[1-5,8-9\]. When the
[compiler](CppCompiler.md) can choose between a [global](CppGlobal.md)
and [local](CppLocal.md) [variable](CppVariable.md) with the same
name, the [local](CppLocal.md) will be used, as shown in the example
below:

```c++
#include <cassert>
#include <iostream>

int a = 1; //Global 'a'

int main()
{
  int a = 2; //Local 'a'
  assert(a==2);
  assert(::a==1); //Use scope operator to get global 'a'
}

Keep scopes small [6,11] . Don't use the same name in both a scope and an enclosing scope [7,12].

References

• 1. Herb Sutter, Andrei Alexandrescu. C++ coding standards: 101 rules, guidelines, and best practices. ISBN: 0-32-111358-6. Item 10: 'Minimize global and shared data'.
• 2. Herb Sutter, Andrei Alexandrescu . C++ coding standards: 101 rules, guidelines, and best practices. ISBN: 0-32-111358-6. Item 18: 'Declare variables as locally as possible'.
• 3. Bjarne Stroustrup. The C++ Programming Language (3rd edition).ISBN: 0-201-88954-4. Chapter 1.8.2.a: 'Don't use global data (use members)'
• 4. Jarrod Hollingworth , Bob Swart, Mark Cashman, Paul Gustavson. Sams C++ Builder 6 Developer's Guide. ISBN: 0-672-32480-6. Chapter 3: 'Avoid using global variables'
• 5. Jesse Liberty . Sams teach yourself C++ in 24 hours. ISBN: 0-672-32224-2. Hour 5, paragraph 'Global variables': 'In C++, global variables are avoided because they can create very confusing code that is hard to maintain.'
• 6. Bjarne Stroustrup. The C++ Programming Language (3rd edition). 1997. ISBN: 0-201-88954-4. Item 4.10.1: 'Keep scopes small'.
• 7. Bjarne Stroustrup . The C++ Programming Language (3rd edition). 1997. ISBN: 0-201-88954-4. Item 4.10.2: 'Don't use the same name in both a scope and an enclosing scope'.
• 8. Stephen C. Dewhurst. C++ Gotchas. 2003. ISBN: 0-321-12518-5. Gotcha #3: 'Avoid global variables'.
• 9. C++ FAQ Lite: 'The names of global variables should start with //' and 'Instead of using a global variable, you should seriously consider if there are ways to limit the variable's visibility and/or lifetime'.
• 10. Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 6.3.4. Scope, page 157
• 11. Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 6.6. Advice. page 169: '[17] Keep scopes small'
• 12. Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 6.6. Advice. page 169: '[18] Don't use the same name in both a scope and its enclosing scope'