What is this: L"DLGCTRL_"L#dialog_control_id

Question

I've found this in some C++_11 code:

L"DLGCTRL_"L#dialog_control_id

What is a "L#" here? There is no # operator or concatenation so. Yes, there is nothing between the " and the L. How can be this allowed?

I get the error:

error C3688: invalid literal suffix 'L'; literal operator or literal operator template 'operator ""L' not found

I have Visual Studio 2015 and 2017 on Windows 10. It compiles normally on C++98 or 00

Answer 1

I assume this occurs inside a preprocessor macro otherwise the question makes no sense at all, e.g.:

#define X(dialog_control_id)  L"DLGCTRL_"L#dialog_control_id

where it is intended that X(foo) expands to L"DLGCTRL_foo".

---

The code should be:

#define X(dialog_control_id)  L"DLGCTRL_" #dialog_control_id

There are several problems in the original code:

• L# is not allowed in ISO C++, it is an extension of the Microsoft preprocessor to produce a wide string literal.
• In the C++11 preprocessor, a letter directly after a closing quote (without whitespace in between) is tokenized as a user-defined string literal.
• The last L was never necessary anyway, because a wide string literal can be concatenated with a narrow string literal (producing a wide string literal)