They standardized std::unreachable in C++23. The function has a wacky description:
invokes undefined behavior
(Prior to C++23 you can use __builtin_unreachable on Linux)
There are cases when it is absolutely known that the values of the argument in the given function are a limited set, but we are still required to do something on “unreachable” values.
Let’s suppose that the magic_func function accepts only values 1 or 3:
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int magic_func(int value) {
switch (value) {
case 1:
return 100;
case 3:
return 500;
default:
/* ???????????? */
}
}
You need to write useless code - what to do when the value is not equal to 1 or 3. Usually two options are used:
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return 0; // returning a dummy value
throw std::exception(); // throwing a dummy exception
Extra code generates extra instructions: link to godbolt.
The unreachable instruction has no semantics, and is needed to show the compiler that this section of code is “unreachable”. The compiler can somehow optimize this section of code.
undefined behavior means that anything the compiler wants can happen in this section of code.
In our case, if you write unreachable (link to godbolt), the compiler will optimize out an extra check and the code will become equivalent to this:
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int magic_func(int value) {
if (value == 1) {
return 100;
}
return 500;
}