const Objects Are Local to a File By Default

When we define a nonconst variable at global scope, it is accessible throughout the program. We can define a nonconst variable in one file andassuming an appropriate declaration has been madecan use that variable in another file:

      // file_1.cc
      int counter;  // definition
      // file_2.cc
      extern int counter; // uses counter from file_1
      ++counter;          // increments counter defined in file_1

Unlike other variables, unless otherwise specified, const variables declared at global scope are local to the file in which the object is defined. The variable exists in that file only and cannot be accessed by other files.

We can make a const object accessible throughout the program by specifying that it is extern:

      // file_1.cc
      // defines and initializes a const that is accessible to other files
      extern const int bufSize = fcn();
      // file_2.cc
      extern const int bufSize; // uses bufSize from file_1
      // uses bufSize defined in file_1
      for (int index = 0; index != bufSize; ++index)
            // ...

In this program, file_1.cc defines and initializes bufSize to the result returned from calling the function named fcn. The definition of bufSize is extern, meaning that bufSize can be used in other files. The declaration in file_2.cc is also made extern. In this case, the extern signifies that bufSize is a declaration and hence no initializer is provided.