C++ Programming Guide

Chapter 2

Program Organization

The file organization of a C++ program requires more care than is typical for a C program. This chapter describes how to set up your header files, inline function definitions, and template definitions.

2.1 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), and is self-contained.

2.1.1 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 #else int function(); // K&R C #endif

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

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Note – Early versions of C++ defined the macro c_plusplus instead of _ _cplusplus. The macro c_plusplus is no longer defined.

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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" { #endif int g1(); int g2(); int g3() #if defined(_ _cplusplus) } #endif

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++ #else template <> int power(int,int); // ISO C++ #endif

2.1.2 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 */ #endif

2.1.3 Self-Contained Header Files

Your header files should include all the definitions that they need to be fully compilable. Make your header file self-contained by including within it all header files that contain needed definitions.

#include "another.h" /* definitions that depend on another.h */

In general, your header files should be both idempotent and self-contained.

#ifndef HEADER_H #define HEADER_H #include "another.h" /* definitions that depend on another.h */ #endif

2.1.4 Unnecessary Header File Inclusion

Programs written in C++ typically include many more declarations than do C programs, resulting in longer compilation times. You can reduce the number of declarations through judicious use of several techniques.

One technique is to conditionally include the header file itself, using the macro defined to make it idempotent. This approach introduces an additional interfile dependence.

#ifndef HEADER_H #include "header.h" #endif

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Note – System header files often include guards of the form _Xxxx, where X is an uppercase letter. These identifiers are reserved and should not be used as a model for constructing macro guard identifiers.

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Another way to reduce compilation time is to use incomplete class and structure declarations rather than including a header file that contains the definitions. This technique is applicable only if the complete definition is not needed, and if the identifier is actually a class or structure, and not a typedef or template. (The standard library has many typedefs that are actually templates and not classes.) For example, rather than writing:

#include "class.h" a_class* a_ptr;

write:

class a_class; a_class* a_ptr;

(If a_class is really a typedef, the technique does not work.)

One other technique is to use interface classes and factories, as described in the book Design Patterns: Elements of Reusable Object-Oriented Software, by Erich Gamma (Addison-Wesley, 1994).

2.2 Inline Function Definitions

You can organize your inline function definitions in two ways: with definitions inline and with definitions included. Each approach has advantages and disadvantages.

2.2.1 Function Definitions Inline

You can use the definitions-inline organization only with member functions. Place the body of the function directly following the function declaration within the class definition.

class Class { int method() { return 3; } };

This organization avoids repeating the prototype of the function, reduces the bulk of source files and the chance for inconsistencies. However, this organization can introduce implementation details into what would otherwise be read as an interface. You would have to do significant editing if the function became non-inline.

Use this organization only when the body of the function is trivial (that is, empty braces) or the function will always be inline.

2.2.2 Function Definitions Included

You can use the definitions-included organization for all inline functions. Place the body of the function together with a repeat (if necessary) of the prototype. The function definition may appear directly within the source file or be included with the source file

class Class { int method(); }; inline int Class::method() { return 3; }

.

This organization separates interface and implementation. You can move definitions easily from header files to source files when the function is no longer implemented inline. The disadvantage is that this organization repeats the prototype of the class, which increases the bulk of source files and the chance for inconsistencies.

2.3 Template Definitions

You can organize your template definitions in two ways: with definitions included and with definitions separated. The definitions-included organization allows greater control over template compilation.

2.3.1 Template Definitions Included

When you put the declarations and definitions for a template within the file that uses the template, the organization is definitions-included. For example:

main.cc

template <class Number> Number twice( Number original ); template <class Number> Number twice( Number original ) { return original + original; } int main( ) { return twice<int>( -3 ); }

When a file using a template includes a file that contains both the template's declaration and the template's definition, the file that uses the template also has the definitions-included organization. For example:

twice.h	#ifndef TWICE_H #define TWICE_H template <class Number> Number twice( Number original ); template <class Number> Number twice( Number original ) { return original + original; } #endif
main.cc	#include "twice.h" int main( ) { return twice( -3 ); }

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Note – It is very important to make your template headers idempotent. (See Section 2.1.2 "Idempotent Header Files"".)

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2.3.2 Template Definitions Separate

Another way to organize template definitions is to keep the definitions in template definition files, as shown in the following example.

twice.h	template <class Number> Number twice( Number original );
twice.cc	template <class Number> Number twice( Number original ) { return original + original; }
main.cc	#include "twice.h" int main( ) { return twice<int>( -3 ); }

Template definition files must not include any non-idempotent header files and often need not include any header files at all. (See Section 2.1.2 "Idempotent Header Files"".)

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Note – Although it is common to use source-file extensions for template definition files (.c, .C, .cc, .cpp, .cxx), template definition files are header files. The compiler includes them automatically if necessary. Template definition files should not be compiled independently.

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If you place template declarations in one file and template definitions in another file, you have to be very careful how you construct the definition file, what you name it, and where you put it. You might also need to identify explicitly to the compiler the location of the definitions. Refer to C++ User's Guide for information about the template definition search rules.