Is _ (single underscore) a valid C++ variable name?

Question

With gcc 4.7.2 this compiles just fine for me:

int main()
{
  int _ = 1;
  return 0;
}

Can I expect this to compile in general? I've read the answers about underscores as prefixes. But what if the underscore isn't prefixing anything?

Answer 1

Yes, from The C++ Programming Language, 4th Edition:

A name (identifier) consists of a sequence of letters and digits. The first character must be a letter. The underscore character, _, is considered a letter.

Answer 2

According to Stroustrup (3rd edition, section 4.9.3), an identifier consists of a sequence of letters and digits. The first character must be a letter. The underscore character is considered a letter.

So, yes, it should be portable.

Answer 3

Yes, _ is a valid identifier, since it meets the syntax requirements. Basically, an identifier consists of an underscore or letter, followed by zero or more other characters that can be underscores, letters or digits. (As of C++ 2011, identifiers can also contain universal character names and other implementation-defined characters, but that's not relevant to your question.)

But it's probably not one that you should use in your own code, unless you're very careful.

As this answer says, quoting the 2003 C++ standard:

Each name that begins with an underscore is reserved to the implementation for use as a name in the global namespace.

Your _ variable isn't in the global namespace, so you're safe in this case, but it's a good idea to avoid defining any identifiers starting with an underscore.

Also, I believe GNU gettext (which provides support for localized messages) uses the name _ for its own purposes. (It was arguably a bad idea for GNU gettext do to this, but it was a convenient choice.)