#if #endif directives

[n4nature]

1)
Does an #if directive always preceeds a constant expression with a condition? If so, based on the condition, it executes the code upto #endif.

Eg:
int a = -1;
#if a<0
cout << "Negative a" << endl;
#endif
cout << "Non-negative a" << endl;

Why would I use #if #endif directives instead of if-else loop?

2)
Could #if directive preceed a constant number? What would the implicit condition be in that case?

Eg:
#if 0

What is the implicit condition in the case above?

Thanks.

[kbw]

The preprocessor runs before the compiler. It's possible to see the preprocessor output if you want to be clear on what happens.

In example 1, there are two different a. #if a<0 expects a to be defined using #define or on the command line.

int a = -1; is not touched by the preprocessor at all. That a and the a that the preprocessor sees are different.

Expressions evaluate to zero or non-zero and can control #if. In example 2, #if 0 always evaluates to false and code in that block is excluded.

[Bazzy]

#if and similar directives are used for conditional compilation and header guards

http://www.cplusplus.com/doc/tutorial/preprocessor.html

[n4nature]

Thanks for replies.

>>In example 2, #if 0 always evaluates to false

Does it mean that the code with that #if-#endif won't be executed? Then why is the expression needed in the first place?

Which condition is being evaluated to false?

[kbw]

Correct, the code in #if 0 will not be compiled.

This is often used to comment out blocks of code.

[n4nature]

So basically, I'd put it just to remind me of something when I see that code segment, and that's it. Is that correct?

That seems to be a replacement for commenting out code. Ofcourse comments get compiled, as opposed to these, that don't even get compiled.

[Bazzy]

Comments don't get compiled.
#if are used with #defined constants, having #if 0 doesn't make much sense.

This could be a way of using that:

//#ifdef == #if defined
#ifdef __cplusplus
    //Using a C++ compiler
   cout << "Hello";
#else
   //Using a C compiler
   printf("Hello");
#endif 

__cplusplus is defined by the compiler

[Disch]

having #if 0 doesn't make much sense.

It makes lots of sense because it can comment out blocks of code which contain sectional comments:

#if 0

void blah()
{
 /* There is no easier way to comment out this section of code */
 DoStuff();
}

#endif 

[n4nature]

Thanks for the answers!

[seymore15074]

#if 0 makes no sense whatsoever. Unless, I guess, if you have C style comments all through your code...ok, so in C, it makes sense. I spoke too soon.

Now, in C++, I could see something like this where some identifier is used (and DEBUG may be 0):

#if DEBUG
//...
#endif 

[Disch]

you don't ever use "C-style" comments in your code?

I find myself sometimes having paragraphs with function details preceeding a function body. Block comments are a godsend for something like that.

[seymore15074]

I'm sorry, disregard my mindless blabble above... It would be handy for commenting out code in development. Somehow I got my thoughts mixed together...if I saw an #if 0 checked into version control on a project, I would be going "wtf."