Converting Between Pointer Types with the reinterpret_cast Operator

Unfortunately, most pointers that we pass to function write as the first argument are not of type const char *. To output objects of other types, we must convert the pointers to those objects to type const char *; otherwise, the compiler will not compile calls to function write. C++ provides the reinterpret_cast operator for cases like this in which a pointer of one type must be cast to an unrelated pointer type. Without a reinterpret_cast, the write statement that outputs the integer number will not compile because the compiler does not allow a pointer of type int * (the type returned by the expression &number) to be passed to a function that expects an argument of type const char *—as far as the compiler is concerned, these types are inconsistent.

A reinterpret_cast is performed at compile time and does not change the value of the object to which its operand points. Instead, it requests that the compiler reinterpret the operand as the target type (specified in the angle brackets following the keyword reinterpret_cast). In Fig. 14.11, we use reinterpret_cast to convert a ClientData pointer to a const char *, which reinterprets a ClientData object as bytes to be output to a file. Random-access file-processing programs rarely write a single field to a file. Typically, they write one object of a class at a time, as we show in the following examples.