How to compose stringification with user defined literal (UDL) in Macro?

Question

How to use literal suffix for identifier transformed into literal string in MACRO by #identifier?

struct SomeType;
SomeType operator "" _udl(const char* self);

#define STRINGIFY_AS_UDL(id) /* #id _udl doesn't work */ /* How to have "id"_udl */ 

STRINGIFY_AS_UDL(foo) // -> "foo"_udl
STRINGIFY_AS_UDL(bar) // -> "bar"_udl
STRINGIFY_AS_UDL(42)  // -> "42"_udl

Answer 1

UDL operators are also "regular" functions, so you can call them instead:

#define STRINGIFY_AS_UDL(id) operator ""_udl(#id)

but you can use the token-pasting operator ##:

#define STRINGIFY_AS_UDL(id) #id ## _udl

or concatenation of adjacent strings:

#define STRINGIFY_AS_UDL(id) #id ""_udl

Note that any of the concatenation method would be required for template UDL for string (extension of gcc/clang):

// gcc/clang extension
template<typename Char, Char... Cs>
/*constexpr*/ SomeType operator"" _udl();

// Usage
// "some text"_udl