How does the std::is_union implementation work?

Question

I'm currently going through the C++ standard library in some more detail, and I was wondering how the implementation of std::is_union works. In libcxx (LLVM), apart from directly using a possibly built-in __is_union, it is defined as

template <class _Tp> struct __libcpp_union : public false_type {};
template <class _Tp> struct _LIBCPP_TEMPLATE_VIS is_union
    : public __libcpp_union<typename remove_cv<_Tp>::type> {};

Similarly, STLPort, although quite old implements it even more minimalistic:

template <class T>
struct is_union
{ };

This seems to always resolve to std::false_type, or even worse, an empty struct, but it doesn't; how is this achieved? In another question, an answer stated that is_union can't be implemented without compiler hooks, but wouldn't this mean that libcxx, STLPort, and probably all major implementations aren't portable to any compiler that doesn't automagically make it work?

Answer 1

Not all of the std library can be implemented in C++.

You skipped the tests for various intrinsics.

There is no way to implment is_union without intrinsics, essentially.

The std library is not a library that ships with C++, it is part of the language. #include <vector> permits certain code to work; no vector header need exist, just the state of the C++ program has to change after the directive.

In practice (by design) it is written and implemented in C++ as a relatively conventional library with the help from intrinsics, and written with reserved tokens to avoid preprocessor conflicts (the __ and _Ty variable names, for example).