Component bsls_macroincrement [Package bsls]

Provide a macro to increment preprocessor numbers. More...

Outline

• Purpose
• Classes
• Macros
• Description
  • Macro Summary
  • Usage

Purpose:

Provide a macro to increment preprocessor numbers.

Classes:

Macros:

BSLS_MACROINCREMENT(NUMBER)

expand to evaluated NUMBER + 1

Description:

This component provides a macro, BSLS_MACROINCREMENT(NUMBER), that produces the preprocessor number equal to the number succeeding the supplied argument. This is necessary as the preprocessor performs arithmetic only inside the context of a #if directive. Common use cases for this macro would be #line directives or any attempt to provide a computed value when token-pasting to produce an identifier.

Macro Summary:

This section provides a brief description of the macro defined in this component.

  BSLS_MACROINCREMENT(MACRO_VALUE)
      Return the preprocessor number that is the successor to 'MACRO_VALUE'.
      The behavior is undefined unless 'MACRO_VALUE' is an integer literal,
      or a macro that ultimately expands to an integer literal.

Usage:

Here we illustrate the typical use case, which demonstrates using the BSLS_MACROINCREMENT macro to change the effective name of a file without changing the effective line count.

First, we print out current file name and line number:

  cout << "__LINE__, __FILE__: " << __LINE__ << ", " __FILE__ << "\n";

Now, we provide a new effective file name to compiler by supplying the new file name to #line directive:

#line BSLS_MACROINCREMENT(__LINE__) "some_other_filename.cpp"

Notice that the first argument following #line directive is the designated line number of the next line, so we increment current __LINE__ by calling BSLS_MACROINCREMENT to keep consistent line numbers.

Finally, print out current file name and line number again. Observe that the file name is changed to the one we set in previous #line directive while the line number still works as expected:

  cout << "__LINE__, __FILE__: " << __LINE__ << ", " __FILE__ << "\n";