Sams Teach Yourself C++ in 21 Days

Creating and Using Namespaces 641

18

The first intdefinition,globalScopeInt, on line 1 is visible within the functions

f()and main(). The next definition is found on line 4 within the function f()

and is named localScopeInt. This variable has local scope, meaning that it is visible

only within the block defining it.

The main()function cannot access f()’s localScopeInt. When the f()function returns,

localScopeIntgoes out of scope. The third definition, also named localScopeInt,is

found on line 8 of the main()function. This variable has block scope.

Note that main()’s localScopeIntdoes not conflict with f()’s localScopeInt. The next

two definitions on lines 10 and 11,anotherLocaland localScopeInt, both have block

scope. As soon as the closing brace is reached on line 12, these two variables lose their

visibility.

Notice that this localScopeIntis hiding the localScopeIntdefined just before the

opening brace (the second localScopeIntdefined in the program). When the program

moves past the closing brace, the second localScopeIntdefined resumes visibility. Any

changes made to the localScopeIntdefined within the braces does not affect the con-

tents of the outer localScopeInt.

Linkage ..........................................................................................................

Names can haveinternaland external linkage. These two terms refer to the use or avail-

ability of a name across multiple translation units or within a single translation unit. Any

name having internallinkage can only be referred to within the translation unit in which it

is defined. For example, a variable defined to have internal linkage can be shared by func-

tions within the same translation unit. Names having externallinkage are available to

other translation units. Listings 18.4a and 18.4b demonstrate internal and external linkage.

LISTING18.4A Internal and External Linking

0: // file: first.cpp

1: int externalInt = 5 ;

2: const int j = 10 ;

3: int main()

4: {

5: return 0 ;

6: }

LISTING18.4B Internal and External Linking

0: // file: second.cpp

1: extern int externalInt ;

2: int anExternalInt = 10 ;

3: const int j = 10 ;

ANALYSIS