Why can the code between ifndef/endif still get run here?

Question

#define cimg_use_jpeg 2

#ifndef cimg_use_jpeg

//code goes here

#endif

I really don't understand...

Answer 1

It's only executed the first time it's included, when cimg_use_jpeg2 it's not already defined. After that it's defined so you can use that code without getting a "symbol already defined" error.

Oooops, I though it was an include guard. Forget that

Answer 2

Either you write

#define yoursymbol

#ifdef yoursymbol

or you write

#define yoursymbol somevalue

#if yousymbol == somevalue

don't mix them

EDIT: since people seem to misunderstand what I meant here's a clarification: when you program it is important to code so that other people understand what your intentions are, so if your intention is only to check whether a symbol is defined it is confusing to specify a numerical value.

Answer 3

This should definately work as you expect. The code in the #ifndef/#endif block will not even be compiled by the compiler, assuming the cimg_use_jpeg is defined as expected.

So the question is, is the cimg_use_jpeg defined as you expect? Or are you simplifying your question and by virtue of the simplification providing a sample that actually works?

Answer 4

It cannot be executed. Maybe you run incorrect executable version. Printing __DATE__ and __TIME__ to trace or log helps to detect such errors.

Answer 5

Every time things similar to this (so-called impossible things) happen me, the reason is: The code I see in my editor is not the code that runs. This can happen in many ways.

• forgetting to make
• forgetting to save
• saving a copy of the code
• running in a different evironment from the one you compiled to
• the new code ran but I opened and am looking at the old output

Answer 6

@Anders K. This works, i.e. the proper things are not executed. The code produces "BAZ is undefined"

#define FOO 2
#define BAR

int main(int argc, char* argv[])
{
#ifndef FOO
  std::cout << "FOO is undefined" << std::endl;
#endif

#ifndef BAR
  std::cout << "BAR is undefined" << std::endl;
#endif

#ifndef BAZ
  std::cout << "BAZ is undefined" << std::endl;
#endif
}