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(C++) scope

 

The scope of a variable is the area where its name is valid. 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 'without scope' is called a global variable. Avoid using global data [1-5]. When the compiler can choose between a global and local variable with the same name, the local will be used, as shown in the example below:

 

#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] . Don't use the same name in both a scope and an enclosing scope [7].

 

 

 

 

 

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'.

 

 

 

 

 

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