C++ Impossible nullptr call mystery

So I decided to get rid of singletons in my project and introduce dependency injection. I did all the necessary changes, and I got a little problem: no matter what I did, my NetworkService was called anyway, regardless of the fact it was initialised to nullptr. I started to investigate, and I got an impossible scenario. I'm feeling powerless, and I give up. I don't know how THIS code gets executed without issues:

    auto impossible_response = ((NetworkService*)nullptr)->post(
            format_url("/api/data/fetch/"),
            payload.dump(),
            headers);
    log.crit("How did this succeeded? Please help me, StackOverflow");

Log message

I'm compiling my code on ArcoLinux with G++ (C++20) via Gradle. I've already tryied to rebuild it from scratch without any cache from previous builds.

Solution

Even If I try to dereference it on purpose, it DOES succeed.

No, the program has undefined behavior meaning it is still in error even if it doesn't say so explicitly and "seems to be working". This is due to the use of -> for dereferencing in your program.

Undefined behavior means anything¹ can happen including but not limited to the program giving your expected output. But never rely(or make conclusions based) on the output of a program that has undefined behavior. The program may just crash.

So the output that you're seeing(maybe seeing) is a result of undefined behavior. And as i said don't rely on the output of a program that has UB. The program may just crash.

So the first step to make the program correct would be to remove UB. Then and only then you can start reasoning about the output of the program.

^{¹For a more technically accurate definition of undefined behavior see this, where it is mentioned that: there are no restrictions on the behavior of the program.}