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What does IFNDEF do and why do I care to use it?  Please explain.

Posted on 2011-02-12
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Last Modified: 2013-12-14
Please see code...

What is the point of IFNDEF?  

And FUNCTIONS_H_INCLUDED?

And define FUNCTIONS_H_INCLUDED?
#ifndef FUNCTIONS_H_INCLUDED
#define FUNCTIONS_H_INCLUDED

class MathFunc
{
public:
     static int add(int operand1, int operand2);
};

#endif

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Question by:shaolinfunk
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by:jkr
jkr earned 250 total points
ID: 34879787
The construct above is a so called "header guard" (http://en.wikipedia.org/wiki/Include_guard), which is used to ensure that a header file and the declarations it holds are only seen once by the compiler, thus avoiding "symbolmultiply defined" errors. See also http://faculty.cs.niu.edu/~mcmahon/CS241/c241man/node90.html ("Include guard")
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AndyAinscow earned 250 total points
ID: 34879788
The # indicates something the compiler should be checking.
#ifndef   - if not defined
#ifdef    - if defined
both work with the #else and #endif, like the if...else... statements in code.

The FUNCTIONS_H_INCLUDED is just something (unique) you made up.


#ifndef FUNCTIONS_H_INCLUDED
#define FUNCTIONS_H_INCLUDED

class MathFunc
{
public:
     static int add(int operand1, int operand2);
};

#endif

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basicaly that tells the compiler that if FUNCTIONS_H_INCLUDED has not yet been defined that it should enter this code section, assign a value to FUNCTIONS_H_INCLUDED (so it won't be entred again) then compile that block of code
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Expert Comment

by:jkr
ID: 34879795
BTW, to elaborate on your code example:
#ifndef FUNCTIONS_H_INCLUDED // precompiler checks if that symbol has already been set. If so, the entire code up to '#endif' will be skipped
#define FUNCTIONS_H_INCLUDED // symbol not encountered earlier, define it. A second encounter will trigger the effect menioned above

class MathFunc
{
public:
     static int add(int operand1, int operand2);
};

#endif // Kinda the 'closing brace' for the conditional compilation

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by:jkr
ID: 34879800
And a final 'PS:' on the whole concept of conditional compilation: http://en.wikipedia.org/wiki/C_preprocessor#Conditional_compilation
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Author Closing Comment

by:shaolinfunk
ID: 34879803
Very illuminating.  Thank you, kind sirs!
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Author Comment

by:shaolinfunk
ID: 34879806
Thank you for that additional post jkr.  That helped!
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