C++ How to Program, 10/e

E.7 Operators # and ##

The # and ## preprocessor operators are available in C++ and ANSI/ISO C. The # operator causes a replacement-text token to be converted to a string surrounded by quotes. Consider the following macro definition:

#define HELLO( x ) cout << "Hello, " #x << endl;

When HELLO(John) appears in a program file, it is expanded to

cout << "Hello, " "John" << endl;

The string "John" replaces #x in the replacement text. Strings separated by white space are concatenated during preprocessing, so the above statement is equivalent to

cout << "Hello, John" << endl;

Note that the # operator must be used in a macro with arguments, because the operand of # refers to an argument of the macro.

The ## operator concatenates two tokens. Consider ...