Scope (Visual C++)

 

The latest version of this topic can be found at Scope (Visual C++).

C++ names can be used only in certain regions of a program. This area is called the "scope" of the name. Scope determines the "lifetime" of a name that does not denote an object of static extent. Scope also determines the visibility of a name, when class constructors and destructors are called, and when variables local to the scope are initialized. (For more information, see Constructors and Destructors.) There are five kinds of scope:

• Local scope A name declared within a block is accessible only within that block and blocks enclosed by it, and only after the point of declaration. The names of formal arguments to a function in the scope of the outermost block of the function have local scope, as if they had been declared inside the block enclosing the function body. Consider the following code fragment:

  {  
      int i;  
  }  
  

  Because the declaration of i is in a block enclosed by curly braces, i has local scope and is never accessible because no code accesses it before the closing curly brace.

• Function scope Labels are the only names that have function scope. They can be used anywhere within a function, but are not accessible outside that function. Formal arguments (arguments specified in function definitions) to functions are considered to be in the scope of the outermost block of the function body.

• File scope Any name declared outside all blocks or classes has file scope. It is accessible anywhere in the translation unit after its declaration. Names with file scope that do not declare static objects are often called global names.

  In C++, file scope is also known as namespace scope.

• Class scope Names of class members have class scope. Class member functions can be accessed only by using the member-selection operators (. or –>) or pointer-to-member operators (.* or –>*) on an object or pointer to an object of that class; nonstatic class member data is considered local to the object of that class. Consider the following class declaration:

  class Point  
  {  
      int x;  
      int y;  
  };  
  

  The class members x and y are considered to be in the scope of class Point.

• Prototype scope Names declared in a function prototype are visible only until the end of the prototype. The following prototype declares three names (strDestination, numberOfElements, and strSource); these names go out of scope at the end of the prototype:

  errno_t strcpy_s( char *strDestination, size_t numberOfElements, const char *strSource );  
  

Hiding Names

You can hide a name by declaring it in an enclosed block. In the following figure, i is redeclared within the inner block, thereby hiding the variable associated with i in the outer block scope.

Block Scope and Name Hiding

The output from the program shown in the figure is:

i = 0  
i = 7  
j = 9  
i = 0  

Note

The argument szWhat is considered to be in the scope of the function. Therefore, it is treated as if it had been declared in the outermost block of the function.

Hiding class names

You can hide class names by declaring a function, object or variable, or enumerator in the same scope. However, the class name can still be accessed when prefixed by the keyword class.

// hiding_class_names.cpp  
// compile with: /EHsc  
#include <iostream>  
using namespace std;  
  
// Declare class Account at file scope.  
class Account  
{  
public:  
    Account( double InitialBalance )  
        { balance = InitialBalance; }  
    double GetBalance()  
        { return balance; }  
private:  
    double balance;  
};  
  
double Account = 15.37;            // Hides class name Account  
  
int main()  
{  
    class Account Checking( Account ); // Qualifies Account as   
                                       //  class name  
  
    cout << "Opening account with balance of: "  
         << Checking.GetBalance() << "\n";  
}  
//Output: Opening account with balance of: 15.37  

Note

Any place the class name (Account) is called for, the keyword class must be used to differentiate it from the file-scoped variable Account. This rule does not apply when the class name occurs on the left side of the scope-resolution operator (::). Names on the left side of the scope-resolution operator are always considered class names.

The following example demonstrates how to declare a pointer to an object of type Account using the class keyword:

class Account *Checking = new class Account( Account );  

The Account in the initializer (in parentheses) in the preceding statement has file scope; it is of type double.

Note

The reuse of identifier names as shown in this example is considered poor programming style.

For more information about pointers, see Derived Types. For information about declaration and initialization of class objects, see Classes, Structures, and Unions. For information about using the new and delete free-store operators, see Special Member Functions.

Hiding names with file scope

You can hide names with file scope by explicitly declaring the same name in block scope. However, file-scope names can be accessed using the scope-resolution operator (::).

// file_scopes.cpp  
// compile with: /EHsc  
#include <iostream>  
  
int i = 7;   // i has file scope, outside all blocks  
using namespace std;  
  
int main( int argc, char *argv[] ) {  
   int i = 5;   // i has block scope, hides i at file scope  
   cout << "Block-scoped i has the value: " << i << "\n";  
   cout << "File-scoped i has the value: " << ::i << "\n";  
}  

Block-scoped i has the value: 5  
File-scoped i has the value: 7  

See Also

Basic Concepts