是否存在可以无限期编译的C ++代码？

Question

I often hear about "C++ sources take a lot of time and memory to compile".

I also hear about that C++ template is Turing complete, so it may suffer from Halting problem .

I have also built a C++ project that costs 8 GiB of memory and 2 hours of time.

So, the question is: Is there a C++ code that compiles for infinite time?

(Nested includes or nested templates are detectable so they should not count.)

Related question: Is there a C++ code that compiles with infinite memory? (I separated them since I expect different answer.)

Answer 1

theoretically this would compile for 'infinite' time because the template expansion is infinitely recursive:

template <size_t N>
struct eat
{
    static constexpr size_t value = eat<N+1>::value;
};

However, all the compilers I have used defend against this kind of code, emitting an error much like this:

/Users/richardh/Documents/dev/Scratchpad/tryit/tryit/words.cpp:136:37: fatal error: recursive template instantiation exceeded maximum depth of 256
    static constexpr size_t value = eat<N+1>::value;
                                    ^
/Users/richardh/Documents/dev/Scratchpad/tryit/tryit/words.cpp:136:37: note: in instantiation of template class 'eat<257>' requested here
    static constexpr size_t value = eat<N+1>::value;
                                    ^
/Users/richardh/Documents/dev/Scratchpad/tryit/tryit/words.cpp:136:37: note: in instantiation of template class 'eat<256>' requested here
    static constexpr size_t value = eat<N+1>::value;
                                    ^
/Users/richardh/Documents/dev/Scratchpad/tryit/tryit/words.cpp:136:37: note: in instantiation of template class 'eat<255>' requested here
    static constexpr size_t value = eat<N+1>::value;
                                    ^
/Users/richardh/Documents/dev/Scratchpad/tryit/tryit/words.cpp:136:37: note: in instantiation of template class 'eat<254>' requested here
    static constexpr size_t value = eat<N+1>::value;

...etc

EDIT: ok, I think this one really is infinite:

template <class T>
struct eat2
{
    using inner = eat2<eat2<T>>;
    static constexpr int value() {
        return inner::value();
    }
};

int main()
{
    eat2<int> e;
    cout << e.value() << endl;
    return 0;
}