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At cppref, I find a strange C++ code, which uses a non-ascii character in source code as follows:

template <char...> double operator "" _π(); // OK

However, the code above cannot be cmompiled with clang 6.0. The error message is:

error : source file is not valid UTF-8 1>double operator "" _<A6><D0>() 1> ^

My questions are:

  1. Is this conforming to C++17?
  2. Note that user-defined _π has no argument, then, how to use _π? Just use double var = _π;?
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    Unicode characters in identifiers Commented Apr 24, 2018 at 13:51
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    I think the key element here is that the <char...> would represent the set of characters used with the literal, which essentially takes the place of the argument. So 2.5_π; would call operator "" _π() with the template arguments <'2','.','5'>. Commented Apr 24, 2018 at 14:11
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    The pi character works OK ideone.com/6AGIX0 What environment (os, shell) are you using? A6D0 doesn't look like valid UTF-8 encoding for pi. Commented Apr 24, 2018 at 14:30
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    A6 D0 is not a UTF-8 sequence. Check your editor or IDE to ensure you are saving your source file as UTF-8. Commented Apr 24, 2018 at 15:10

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From the link you provided, it quotes:

If the literal operator is a template, it must have an empty parameter list and can have only one template parameter, which must be a non-type template parameter pack with element type char

template <char...> double operator "" _x();

Let us see what this means,

Notation char... indicates that this template can be instantiated with 0, 1, 2 or more parameters of type char. This means that each time the compiler encounters a literal like 1234_km it should treat it as the following function call:

operator"" _km<'1', '2', '3', '4'>();

The entire string representing the literal is passed (chopped) as template argument. See this and this for usage.

And regarding the range of characters allowed:(See this Annexure E)

00A8, 00AA, 00AD, 00AF, 00B2-00B5, 00B7-00BA, 00BC-00BE, 00C0-00D6, 00D8-00F6, 00F8-00FF 0100-167F, 1681-180D, 180F-1FFF 200B-200D, 202A-202E, 203F-2040, 2054, 2060-206F 2070-218F, 2460-24FF, 2776-2793, 2C00-2DFF, 2E80-2FFF 3004-3007, 3021-302F, 3031-303F 3040-D7FF F900-FD3D, FD40-FDCF, FDF0-FE44, FE47-FFFD 10000-1FFFD, 20000-2FFFD, 30000-3FFFD, 40000-4FFFD, 50000-5FFFD, 60000-6FFFD, 70000-7FFFD, 80000-8FFFD, 90000-9FFFD, A0000-AFFFD, B0000-BFFFD, C0000-CFFFD, D0000-DFFFD, E0000-EFFFD
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