I saw this meme on Instagram about some C++ code that should not output anything but does. The code is:

#include <iostream>

int main() {
    while (1)
        ;
}

void unreachable() {
    std::cout << "Hello World!" << std::endl;
}

I compiled it with clang as shown in the meme and got the same result (Ubuntu clang version 14.0.0-1ubuntu1.1) but the same code compiled with gcc does what you expect: nothing (g++ (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0).

I would like to know why clang does things different and how the hack the unreachable function is executed if I never call it from the main function.

>Solution :

In

#include <iostream>

int main() {
    while (1)
        ;
}

void unreachable() {
    std::cout << "Hello World!" << std::endl;
}

The never-ending do-nothing loop breaks the rules. As per [intro.progress]

The implementation may assume that any thread will eventually do one of the following:

• terminate,
• make a call to a library I/O function,
• perform an access through a volatile glvalue, or
• perform a synchronization operation or an atomic operation.

[Note 1: This is intended to allow compiler transformations such as removal of empty loops, even when termination cannot be proven. — end note]

Since the program is invalid, the compiler is allowed to do anything it wants from generate a program that does what you expect, GCC’s response, to literally anything else. So technically clang’s result is valid. Be grateful. The compiler could have opted to produce Skynet.