Can you start a class name with a numeric digit?
c++, class, naming
Solution
Rules for identifier names in C++ are:
- It can not start with a number
- Can be composed of letters, numbers, underscore, universal character names1 and implementation defined characters
- Can not be a keyword.
The sections in the C++ draft standard that cover this are `2.11` Identifiers which includes the following grammar:
identifier:
identifier-nondigit <- Can only start with a non-digit
identifier identifier-nondigit <- Next two rules allows for subsequent
identifier digit <- characters to be those outlined in 2 above
identifier-nondigit:
nondigit <- a-z, A-Z and _
universal-character-name
other implementation-defined characters
[...]
and `2.12` Keywords explains all the identifier reserved for use as keywords.
Finally, the following names are also reserved:
- Names that contain a double underscore `__`, or start with either an underscore followed by an uppercase letter (like `_Apple`) in any scope,
- Names that start with an underscore in the global namespace (like `_apple` in the global namespace) are reserved.
The section that covers this in the draft standard is `17.6.4.3.2`. We can find a rationale for why these are reserved from Rationale for International Standard—Programming Languages—C which says:
[...]This gives a name space for writing the numerous behind-the-scenes non-external macros and functions a library needs to do its job properly[...]
In C++ this also applies to name mangling as this example shows.
Footnotes
- 1. Allowed universal characters
The universal characters that are allowed is covered in `Annex E.1`:
E.1 Ranges of characters allowed [charname.allowed]
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
Problem
In C++, is it possible to start a class name with a digit? For example, ``` template <class T> class 2DArray { public: // 1D ARRAY CLASS class 1DArray { public: 1DArray() { Create(); } 1DArray(iterator arr) : array1d_(arr) { } explicit 1DArray(size_type cols, const T& t = T()) { Create(cols, t); } 1DArray(const 1DArray& arr) { Create(arr.begin(), arr.end()); } 1DArray& operator=(const 2DArray&); ~1DArray() { Uncreate(); } T& operator[](size_type n) { return array1d_[n]; } const T& operator[](size_type n) const { return array1d_[n]; } } ```