User-defined literals without underscore

8
votes

According to cppreference it is possible to define a literal using

CSomeClass operator ""s(const char* literal, size_t size);

Now after reading the paragraph I think it should be also possible to define

CSomeClass operator ""r(const char* literal, size_t size);

(note the r ud-suffix instead of s)

Overloading s just gives the clang warning

warning: user-defined literal suffixes not starting with '_' are reserved [-Wuser-defined-literals]

which I can't really understand as I'm compiling with -std=c++14. Overloading r gives

error: invalid suffix on literal; C++11 requires a space between literal and identifier [-Wreserved-user-defined-literal]
warning: user-defined literal suffixes not starting with '_' are reserved; no literal will invoke this operator [-Wuser-defined-literals]

which seems even less accountable to me.

Why does clang emit these warnings/errors and how can I make the r ud-suffix valid.

1
c++c++11c++14user-defined-literals
What part of the warning don't you understand? It says your literal has to start with _. The reference you link to says the same. You can't declare one called r, but you can declare one called _r. - Mike Seymour
Reading the reference I understood that since c++14 one can overload without the underscore. - Uroc327
OK, I can see how the wording of that reference might imply that the restriction only applied to the form with a space, not the new C++14 form. The standard's quite clear though: "Literal suffix identifiers that do not start with an underscore are reserved for future standardization." - Mike Seymour
@Mike Seymour cppreference edited to stress that point - Cubbi
Are you compiling s with C++14 and r with C++11? I guess I'm confused about the warning on one and the error on the other. I do remember at one point the "" and the identifier were required to touch with no intervening space. Then it was relaxed. I can't remember if that was C++14 or a DR on C++11.. I'll check but I'm pretty sure his was a DR against C++11. - emsr

1 Answers

12
votes

User-defined literals must begin with the underscore _: the suffixes that do not begin with the underscore are reserved for the literal operators provided by the standard library.