Programmatically determine via ifdef if a label is defined within a Translation Unit

I have the following bit of code, I expect that given cstdio is included that the first line will be printed, however the second line is printed.

What am I doing wrong? is it possible to know if labels such as printf or strncmp or memcpy have been defined in the current translation unit at compile time?

#include <iostream>
#include <cstdio>

int main()
{
   #ifdef printf
      std::cout << "printf is defined.\n";
   #else
      std::cout << "printf NOT defined!\n";
   #endif
   return 0;
}

Is the reason because the preprocessor is run before variables and labels are introduced into the scope/TU?

In short is the following code bogus? :

http://code.google.com/p/cmockery/source/browse/trunk/src/example/calculator.c#35

Solution

#ifdef only applies to preprocessor macros, defined with #define, not to symbols like function names and variables. You can imagine the preprocessor as an actual separate preliminary step, like running your code through a perl script, that occurs before the "real" compiler gets a crack at it.

So there is no programmatic way to check whether symbols like printf are defined in the current scope. If you use one and it's not defined, you'll get a compiler error. The normal thing to do is to #include a header file with the required definition in the source file where you reference it, not to write a source file that will adapt itself to different possible sets of headers.

As a hack, and depending on your environment and specific problem, the header file that does define printf (or whatever function you care about) may also contain some preprocessor #defines that you could check for.