😃 (and other Unicode characters) in identifiers not allowed by g++

Question

I am 😞 to find that I cannot use 😃 as a valid identifier with g++ 4.7, even with the -fextended-identifiers option enabled:

int main(int argc, const char* argv[])
{
  const char* 😃 = "I'm very happy";
  return 0;
}

main.cpp:3:3: error: stray ‘\360’ in program
main.cpp:3:3: error: stray ‘\237’ in program
main.cpp:3:3: error: stray ‘\230’ in program
main.cpp:3:3: error: stray ‘\203’ in program

After some googling, I discovered that UTF-8 characters are not yet supported in identifiers, but a universal-character-name should work. So I convert my source to:

int main(int argc, const char* argv[])
{
  const char* \U0001F603 = "I'm very happy";
  return 0;
}

main.cpp:3:15: error: universal character \U0001F603 is not valid in an identifier

So apparently 😃 isn't a valid identifier character. However, the standard specifically allows characters from the range 10000-1FFFD in Annex E.1 and doesn't disallow it as an initial character in E.2.

My next effort was to see if any other allowed Unicode characters worked - but none that I tried did. Not even the ever important PILE OF POO (💩) character.

So, for the sake of meaningful and descriptive variable names, what gives? Does -fextended-identifiers do as it advertises or not? Is it only supported in the very latest build? And what kind of support do other compilers have?

Comintern

Read this. – Harp

@Harp Unfortunately that page doesn't mention that an identifier can contain a universal-character-name, so whatever advice they give on naming conventions doesn't take into account the importance of using smiley faces as variable names. See §2.11 of ISO/IEC 14882:2011(E). – Comintern

Hmm it seems the program static const char* x😃 = "I'm very happy"; crashes clang 3.1... – Forejudge

See this example. – Harp

clang supports this since 3.3 with no special options but gcc 4.8.1 still doesn't. Related: #26660680 – Fasciation

You can, but you need C++11 also your source should be encoded as unicode. – Ardellearden

Answer 1

As of 4.8, GCC does not support characters outside of the BMP used as identifiers. It seems to be an unnecessary restriction. Also, GCC only supports a very restricted set of character described in ucnid.tab, based on C99 and C++98 (it is not updated to C11 and C++11 yet, it seems).

As described in the manual, -fextended-identifiers is experimental, so it has a higher chance won't work as expected.

---

GCC supported the C11 character set starting from 4.9.0 (SVN r204886 to be precise). So OP's second piece of code using \U0001F603 does work. I still can't get the actual code using 😃 to work even with -finput-charset=UTF-8 with GCC 8.2 on https://gcc.godbolt.org though (You may want to follow this bug report, provided by @DanielWolf).

Meanwhile, both pieces of code work on Clang 3.3 without any options other than -std=c++11.

Forejudge

How about main.cpp:3:15: error: universal character \u00a8 is not valid in an identifier? This is with 4.7, though. – Comintern

Answer 2

This was a known bug in GCC 9 and before. This has been fixed in GCC 10.

The official changelog for GCC 10 contains this section:

Extended characters in identifiers may now be specified directly in the input encoding (UTF-8, by default), in addition to the UCN syntax (\uNNNN or \UNNNNNNNN) that is already supported:

static const int π = 3;
int get_naïve_pi() {
  return π;
}

Photocomposition

There isn't a GCC 10 (10.0), only GCC 10.1(?). – Kovrov

@PeterMortensen I'm assuming that GCC follows some form of SemVer, so a statement about features in GCC 10 should hold for any 10.x version. – Photocomposition

Answer 3

However, the standard specifically allows characters from the range 10000-1FFFD in Annex E.1 and doesn't disallow it as an initial character in E.2.

One thing to keep in mind is that just because the C++ standard allows (or disallows) some feature, does not necessarily mean that your compiler supports (or doesn't support) that feature.

Hugohugon

Yes, allowing the full set of Unicode characters specified by the standard is one that that, as far as I know, no compilers support yet, either literally or with UCNs. – Rum

Of course! I only meant to find some documentation or source that shows they don't support this feature. – Comintern

@sftrabbit Okay, maybe my answer is pointing out the obvious. KennyTM gave the link re: gcc. – Hugohugon