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Oracle Solaris Studio 12.3: C++ User's Guide     Oracle Solaris Studio 12.3 Information Library
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Document Information


Part I C++ Compiler

1.  The C++ Compiler

2.  Using the C++ Compiler

3.  Using the C++ Compiler Options

Part II Writing C++ Programs

4.  Language Extensions

5.  Program Organization

5.1 Header Files

5.1.1 Language-Adaptable Header Files

5.1.2 Idempotent Header Files

5.2 Template Definitions

5.2.1 Template Definitions Included

5.2.2 Template Definitions Separate

6.  Creating and Using Templates

7.  Compiling Templates

8.  Exception Handling

9.  Improving Program Performance

10.  Building Multithreaded Programs

Part III Libraries

11.  Using Libraries

12.  Using the C++ Standard Library

13.  Using the Classic iostream Library

14.  Building Libraries

Part IV Appendixes

A.  C++ Compiler Options

B.  Pragmas



5.1 Header Files

Creating an effective header file can be difficult. Often your header file must adapt to different versions of both C and C++. To accommodate templates, make sure your header file is tolerant of multiple inclusions (idempotent).

5.1.1 Language-Adaptable Header Files

You might need to develop header files for inclusion in both C and C++ programs. However, Kernighan and Ritchie C (K&R C), also known as “classic C,” ANSI C, Annotated Reference Manual C++ (ARM C++), and ISO C++ sometimes require different declarations or definitions for the same program element within a single header file. (See the C++ Migration Guide for additional information on the variations between languages and versions.) To make header files acceptable to all these standards, you might need to use conditional compilation based on the existence or value of the preprocessor macros __STDC__ and __cplusplus.

The macro __STDC__ is not defined in K&R C, but is defined in both ANSI C and C++. Use this macro to separate K&R C code from ANSI C or C++ code. This macro is most useful for separating prototyped from nonprototyped function definitions.

#ifdef __STDC__
int function(char*,...);      // C++ & ANSI C declaration
int function();               // K&R C

The macro __cplusplus is not defined in C, but is defined in C++.

Note - Early versions of C++ defined the macro c_plusplus instead of __ cplusplus. The macro c_plusplus is no longer defined.

Use the definition of the __cplusplus macro to separate C and C++. This macro is most useful in guarding the specification of an extern “C” interface for function declarations, as shown in the following example. To prevent inconsistent specification of extern “C”, never place an #include directive within the scope of an extern “C” linkage specification.

#include “header.h”
...                     // ... other include files...
#if defined(__cplusplus)
extern “C” {
  int g1();
  int g2();
  int g3()
#if defined(__cplusplus)

In ARM C++, the __cplusplus macro has a value of 1. In ISO C++, the macro has the value 199711L (the year and month of the standard expressed as a long constant). Use the value of this macro to separate ARM C++ from ISO C++. The macro value is most useful for guarding changes in template syntax.

// template function specialization
#if __cplusplus < 199711L
int power(int,int);                       // ARM C++
template <> int power(int,int);           // ISO C++

5.1.2 Idempotent Header Files

Your header files should be idempotent, that is, the effect of including a header file many times should be exactly the same as including the header file only once. This property is especially important for templates. You can best accomplish idempotency by setting preprocessor conditions that prevent the body of your header file from appearing more than once.

#ifndef HEADER_H
#define HEADER_H
/* contents of header file */