#if vs #ifndef vs #ifdef

Question

My problem is first of all, understanding #ifndef and #ifdef . I also want to understand the difference between #if , #ifndef , and #ifdef . I understand that #if is basically an if statement. For example:

#include<iostream>
#define LINUX_GRAPHICS 011x101

int main(){
 long Compare = LINUX_GRAPHICS;
 #if Compare == LINUX_GRAPHICS
   std::cout << "True" << std::endl;
 #endif
}

But the others, although I read about them I can't comprehend. They also seem like very similar terms, but I doubt they work similarly. Help would be greatly appreciated.

Answer 1

#if doesn't have any notion about Compare or the value it contains, so it probably doesn't do what you intend.

Remember the preprocessor does plain text replacement.

The statement will expand as seen from #if as

#if Compare == 011x101

and being expanded as

#if 0 == 011x101

which certainly won't yield true at the preprocessing stage.

---

The #ifdef and #ifndef directives check if a preprocessor symbol was #define 'd at all, either using that (<--) preprocessor directive, or your compilers preprocessor option (most commonly -D<preprocessor-symbol> ).
These don't care if the preprocessor symbol carries an empty value or something. A simple

#define MY_CONDITION

or

-DMY_CONDITION

is enough to satisfy

#ifdef MY_CONDITION

to expand the text coming afterwards (or hide it with #ifndef ).

---

The Compare declaration isn't a preprocessor symbol and can't be used reasonably with #ifdef or #ifndef either.

Answer 2

Macros are expanded by the preprocessor who doesnt know anything about values of variables during runtime. It is only about textual replacement (or comparing symbols known to the preprocessor). Your line

#if Compare == LINUX_GRAPHICS

will expand to

#if Compare == 011x101

and as "Compare" is differnt from "011x101", it evaluates to false. Actually I am not even 100% sure about that, but the point is: you are mixing preprocessor directives with variables that are evaluated at runtime. That is non-sense. Preprocessor directives are not there to replace C++ statements.

For most traditional use cases of macros there are better way nowadays. If you dont really need to use macros, it is better not to use them. It makes it extremly hard to read the code (eg. I dont understand how that macros in your code work and unless I really need it honstely I dont want to know :P) and there are other problems with macros that can lead to very hard to find bugs in your program. Before using macros I would advice you to first consider if there isnt a more natural C++ way of achieving the same.

PS:

#ifdef SYMBOL
    ifdef = "if defined"
    this part of the code is excluded before the compiler even sees it
    if SYMBOL is not defined (via #define)
#endif

#ifndef SYMBOL
    ifndef = "if not defined"
    this part of the code is excluded before the compiler even sees it
    if SYMBOL is defined (via #define)
#endif

I wrote "excluded" on purpose to emphazise the bad impact it has on readability of your code. If you overuse #ifdef or #ifndef inside normal blocks of code, it will be extremely hard to read.

Answer 3

#if is preprocessor if . It can only deal with with preprocessor stuff which is basically preprocessor macros (which are either function like or constant-like) and C tokens with some simple integer-literal arithmetic.

#ifdef SOMETHING is the same as #if defined(SOMETHING) and #ifndef SOMETHING is the same as #if !defined(SOMETHING) . defined is a special preprocessor operator that allows you to test whether SOMETHING is a defined macro. These are basically shortcuts for the most common uses or preprocessor conditionals -- testing whether some macros are defined or not.

You can find a detailed manual (~80 pages) on the gcc preprocessor at https://gcc.gnu.org/onlinedocs/ .

Answer 4

Basicaly, preprocessor does text substitution. Then the compiler compiles program into machine code. And then CPU executes machine instructions. This means you can't use preprocessor #if instead of operator if : one does text substitution, while second generates branching code for CPU.

So preprocessor directives such as #if , #ifdef , #ifndef serve for "semi-automatic mode" of generating (a little) different programs based on some "meta-input". Actually you can always do these substitutions yourself and get working C/C++ program without any preprocessor directives. Also compilers often have a command-line switch which outputs just preprocessed program, ie without any #if directives. Try to play with it, and you should get what these directives do.

#ifdef XXX is just the same as #if defined(XXX) where defined(XXX) is builtin preprocessor-only function which is true when identifier XXX is defined in program text by another preprocessor directive #define . And #ifndef XXX is just #if !defined(XXX) .

Answer 5

Well the preprocessors #ifdef and #ifndef mean the followind: In your example you used #define to set a constant variable named LINUX_GRAPHICS to be equal to 011x101. So later in your program you migth want to check if this variable is defined. Then you use #ifdef, when you want to check if this variable is defined and #ifndef if not. I wish I helped you.