How to trigger a warning when a certain function is called (not compiled)?

Question

If we write:

void foo() {
#warning "Danger Will Robinson!"
}

we'll get a warning when compiling foo(). Now, What I want to do is for the compilation not to emit a warning, but for a warning to be emitted if foo() is called.

Is that possible, using g++ and/or clang++?

Note: This question is somewhat similar to this one, about marking functions as "deprecated" (a-la C++14's [[deprecated]] attribute). However, it's a different semantic. I do not want people to get the notion that my function is deprecated, but rather get a custom warning.

I am afraid that's not possible the way. I'd rather go for a [`deprecated` attribute](https://stackoverflow.com/questions/295120/c-mark-as-deprecated) in that case. — πάντα ῥεῖ, Feb 27 '21 at 09:40
How could the compiler tell if a function is called or not? Consider `if (run_time_function() > 40173) foo();`. — David Schwartz, Feb 27 '21 at 09:43
@DavidSchwartz should handle it the same way it handles the deprecated attribute. — Aykhan Hagverdili, Feb 27 '21 at 09:44
@πάνταῥεῖ: I think that would be an answer. "No" is a valid answer, no need to mark as a dupe. Plus, in my case, the semantic is not "deprecated"; but I'll clear that up. — einpoklum, Feb 27 '21 at 10:24
@einpoklum You can write that as answe yourself. The duplicate already answered in a way that it's doing exactly what you asked for. The semantics can be changed in any way you want giving an additional message. — πάντα ῥεῖ, Feb 27 '21 at 10:27
How is the behavior you ask for different from `[[nodiscard]]`? — HolyBlackCat, Feb 27 '21 at 10:30
@πάνταῥεῖ: It's not doing what I asked for, because it's saying that the function is deprecated, which is not the case. — einpoklum, Feb 27 '21 at 10:59
@HolyBlackCat: There's no return value, for example. Other than that, yes, it's similar to `[[nodiscard]]` and `[[deprecated]]`. — einpoklum, Feb 27 '21 at 10:59
@πάνταῥεῖ: I could write that as an answer myself, had the question not been closed again... — einpoklum, Feb 27 '21 at 11:00
@einpoklum could you add what kind of warning you want to add? It's a bit weird to get a warning when calling a non-deprecated function. Do you want it to warn about freeing the return value, or something like that? — Aykhan Hagverdili, Feb 27 '21 at 13:55
@AyxanHaqverdili: Arbitrary warning. It could be: "Calling this function costs the company money1" It could be: "Calling this function may cause a nuclear explosion". It could be: "You are calling an untested function". etc. — einpoklum, Feb 27 '21 at 17:56
@einpoklum hmm. Interesting. I am voting to reopen this question as it's absolutely not a duplicate. — Aykhan Hagverdili, Feb 27 '21 at 18:01
@einpoklum How about something like [this](https://gcc.godbolt.org/z/T4M354)? A bit hacky, but gets the job done — Aykhan Hagverdili, Feb 27 '21 at 18:15
If GNU is on the table, then there is `__attribute__ ((warning ("message")))` - Though I'm not sure it's exactly what you are after. Dead code elimination can remove the warning should the call be on the eliminated branch. — StoryTeller - Unslander Monica, Feb 27 '21 at 18:43
@StoryTeller-UnslanderMonica that's quite a bit more elegant than what I was proposing. The only problem being it doesn't work with clang. — Aykhan Hagverdili, Feb 27 '21 at 18:52
@StoryTeller-UnslanderMonica: This is good enough, as I said I was willing to be compiler-specific. Is this avaialable in clang as well? — einpoklum, Feb 27 '21 at 20:39
@AyxanHaqverdili: clang-only, but does seem to be more robust than one might expect... Nice :-) Too bad the question is closed. — einpoklum, Feb 27 '21 at 20:43
@einpoklum mine works with GCC too! https://gcc.godbolt.org/z/1h7zsT (I still suggest you go with the other option) — Aykhan Hagverdili, Feb 27 '21 at 20:48
@StoryTeller-UnslanderMonica: Reopened. Would you like to make your comment into an answer? — einpoklum, Feb 27 '21 at 23:54

Aykhan Hagverdili · Answer 1 · 2021-02-28T08:11:18.770

C++11 added _Pragma, which is like #pragma but macros can expand to it. So, you can make a macro, that expands to this warning and the function:

#include <cstdio>

void foo() {
  std::puts("foo is called!");
}

#define foo(...) _Pragma("message \"Danger Will Robinson!\"") \
  foo(__VA_ARGS__)

int main() { 
  foo(); 
}

The main issue with this approach is that it will not respect overloads and warn on any function with the name foo.

score 0 · Answer 2 · answered Feb 27 '21 at 23:53

A suggestion from @StoryTeller in the comments, which works - partially - in GCC:

 __attribute__ ((warning ("Danger Will Robinson!"))) 
void foo() { }

int bar() {
    const int x = 123;
    foo();
    return x;
}

Why "partially"? As @StoryTeller says:

Dead code elimination can remove the warning should the call be on the eliminated branch

Have look at This GodBolt example: When you enable optimizations so that the function is inlined - the warning attribute is ignored.

How to trigger a warning when a certain function is called (not compiled)?

2 Answers2