What is #if 0

Imtiaz Murtaza

I've seen in many source codes, some statements written in #if 0 block something like: #if 0 printf("Blah Blah"); #endif Please tell me what is the meaning of #if 0 in the code above.

Prakash Nadar

Imtiaz Murtaza wrote:

I've seen in many source codes, some statements written in #if 0 block something like: #if 0 printf("Blah Blah"); #endif Please tell me what is the meaning of #if 0 in the code above

it simply means the code in side the block is ignore for compilation and will not appear in the final build. it is similar to saying #if !defined(SOME_BUILD_FLAG) or #if FALSE #endif since the dev wanted the code to be explicitly removed from compilation he used #if 0 #endif directive

-Prakash

Imtiaz Murtaza

Mr.Prakash wrote:

it simply means the code in side the block is ignore for compilation and will not appear in the final build.

Then what is the purpose of the code inside that block ? For example, if the code is written in #ifdef DEBUG block, it is skipped in the release builds but its purpose is clear and that is: it will be compiled in debug builds. So what is the purpose of writing code in #if 0 when i'll never be present in the executable.

kevincwong

There are many reasons to use #if 0 Actually, I do it all the time. Here is my reason. Let say I re-write some code/method/function and I think it should work. But I would like to keep the old code around. Just in case...... I know I can retrieve the old code from the source control. However, it's more convinence if I have the old code right next to the new code. Kevin

Stephen Hewitt

I hate #if 0:mad:. It's not so bad if the code commented out is small, but when large pieces of code are bracketed with these you can find yourself analyzing code only to find, when you get to the top, that you've been wasting your time and the code isn't even part of the product. This has happened to me many times. If the programmer simply commented it out the syntax highlighting will stop such problems from ever occurring. Steve

toxcct

Stephen Hewitt wrote:

I hate #if 0

me too. i use this purpose instead...

#ifdef DEBUG
#define DPRINT printf
#else
void DPRINT(...) {}
#endif

this way, in debug mode, we could DPRINT() something, which will not been took in account in release mode (and more, the compiler will optimize a call to DPRINT() in release). for instance :

int Foo() {
DPRINT("Foo()\n");
int i = 654;

// modification of i...

DPRINT("Foo output : %d\\n", i);
return i;

}

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[toxcct][VisualCalc 2.20][VisualCalc 3.0]

Prakash Nadar

you can #if 0 the code which needs to be ignored in both debug and release build, i.e. older logic to solve a problem, etc.

-Prakash

Divyang Mithaiwala

[Message Deleted]

kvprasad

Yes steve u r definetly correct. But this is not supposed to be used to large chunks of code. it is more of an debugging approach than just saving old code. Tip: Always it is a good option to try to put a breakpoint to know if this code is compiled or not. a breakpoint cannot be inserted in any code that is not compiled. Hope this helps. kvprasad your future is what you think

Maximilien

:confused:

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David Crow

Stephen Hewitt wrote:

If the programmer simply commented it out...

But nested comments are not allowed. That's where #if 0 is of use.

"The words of God are not like the oak leaf which dies and falls to the earth, but like the pine tree which stays green forever." - Native American Proverb

Stephen Hewitt

True. Steve

Phil J Pearson

I agree that large blocks of #if 0'd code are annoying but VS2005 improves things a lot by greying it out. It even greys out (for example) #ifdef _DEBUG blocks when _DEBUG is not defined.

The opinions expressed in this communication do not necessarily represent those of the author (especially if you find them impolite, discourteous or inflammatory).

Shraddhan

I believe that in Visual C++ 6.0 there is a key combination that will take you to / from #if and #endif but Microsoft in their 'wisdom' removed this very handy functionality from VC++.NET 2001. Yes, thanks Microsoft... Personally, when I need to comment out large chunks of code, I use #if defined dfkagtljarogh #end if // defined dfkagtljarogh where dfkagtljarogh is the result of some random keystrokes. Copy and paste to make sure that two instances match, and then it is not too bad to use Find to jump from beginning to end or vice versa. And as for being unable to nest /* ... */ comments in C++, this is an abomination that should never have been put into the language. Is there any reason whatsoever why C++ (or C even) HAVE TO REFUSE to allow such useful nested comments? Thanks again, Microsoft. Some C++ compilers do allow this. Shraddhan

Prakash Nadar

Shraddhan wrote:

And as for being unable to nest /* ... */ comments in C++, this is an abomination that should never have been put into the language. Is there any reason whatsoever why C++ (or C even) HAVE TO REFUSE to allow such useful nested comments?

yes i agree with you and that is one thing that I hate in VC++ compiler.

-Prakash