Ask for an example of GMP function mpq_class operator"" _mpq (const char *str) that is working in Visual Studio 2015

Below is an excerpt from GMPXX online doc (https://gmplib.org/manual/C_002b_002b-Interface-Rationals.html#C_002b_002b-Interface-Rationals):

I don't quite understand the doc, so I think an example of this function would help. Can you please share an example of this function with me? I have tried mpq_class q("123_mpq"); but the code throws an error in Visual Studio which I think is a C++11 compiler. I am using GMP 5.0.1 and Visual Studio 2015 Update 3 on Windows 10. Thank you.