Jul 042012
 

One Definition Rule (ODR)

In C++, declaration does not allocate any memory. Its only when an object (or function) is defined that an instance of it is created.

The One definition Rule says that there cannot be more that one definition in a single translation unit of C++.

You may skip the definition of an Object (or function) if it is not used but if it is used ONE AND ONLY ONE definition must be provided.

In some cases there can be multiple definitions of Templates and inline functions can be defined in multiple translation units, but all the definitions must be exactly same.

Achieving ODR

1. Defined all the functions and objects in .cpp file and use header files (.h) only to declare them 
(since multiple declarations never create a problem).

2. Use Include guards:

Include guards are used to avoid multiple inclusion of header files, this cannot be done only by being cautious over not to include the file twice. For example in the below case, you cannot cut short any #include
File: a.h

    /* File: a.h
     */

    int myInt = 20;  // Defining int (Not declaring)

File: b.h

    /* File: b.h
     */

    #include "a.h"
    // This file will have definition of myInt variable

File: mymain.cpp

    /* File: mymain.cpp
     */

    #include "a.h"
    #include "b.h"

    // This file will have two definition of myInt variable. 
    // This will give ERROR

We should use the #include guards. The usage of guards is as follows.

File: a.h

 
    /* File: a.h
     */

    #ifndef _A_H_
    #define _A_H_

    int myInt = 20;  // Defining int (Not declaring)

    #endif

File: b.h

 
    /* File: b.h
     */
    #ifndef _A_H_
    #define _A_H_

    #include “a.h”

    // we have guards in this file also.
    #endif

File: mymain.cpp

    /* File: mymain.cpp
     */
    #include “a.h”
    #include “b.h”

    // This file will have two definition of myInt variable. 
    // This will give ERROR

Here, the first inclusion of “a.h” causes the macro _A_H_ to be defined. Then, when “mymain.cpp” includes “b.h” the second time, the #ifndef test fails, and the preprocessor skips the definition of myInt variable 2nd time, hence the variable is defined only once. Hence, The program compiles correctly.

Ideally, the objects should only be defined in the .cpp (or .c) files and .h files should only have declarations.

Related Links:

https://www.securecoding.cert.org/confluence/display/cplusplus/MSC33-CPP.+Obey+the+One+Definition+Rule
http://en.wikipedia.org/wiki/One_Definition_Rule

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