C++ Primer, Fifth Edition

By Default, const Objects Are Local to a File

When a const object is initialized from a compile-time constant, such as in our definition of bufSize:

const int bufSize = 512;    // input buffer size

the compiler will usually replace uses of the variable with its corresponding value during compilation. That is, the compiler will generate code using the value 512 in the places that our code uses bufSize.

To substitute the value for the variable, the compiler has to see the variable’s initializer. When we split a program into multiple files, every file that uses the const must have access to its initializer. In order to see the initializer, the variable must be defined in every file that wants to use the variable’s value (§ 2.2.2, p. 45). To support ...