How can force the user/OS to input an Ascii string

问题:

This is an extended question of this one: Is std::string suppose to have only Ascii characters

I want to build a simple console application that take an input from the user as set of characters. Those characters include 0->9 digits and a->z letters.

I am dealing with input supposing that it is an Ascii. For example, I am using something like : static_cast<unsigned int>(my_char - '0') to get the number as unsigned int.

How can I make this code cross-platform? How can tell that I want the input to be Ascii always? Or I have missed a lot of concepts and static_cast<unsigned int>(my_char - '0') is just a bad way?

P.S. In Ascii (at least) digits have sequenced order. However, in others encoding, I do not know if they have. (I am pretty sure that they are but there is no guarantee, right?)

回答1:

How can force the user/OS to input an Ascii string

You cannot, unless you let the user specify the numeric values of such ASCII input.

It all depends how the terminal implementation used to serve std::cin translates key strokes like 0 to a specific number, and what your toolchain expects to match that number with it's intrinsic translation for '0'.

You simply shouldn't expect ASCII values explicitly (e.g. using magic numbers), but char literals to provide portable code. The assumption that my_char - '0' will result in the actual digits value is true for all character sets. The C++ standard states in [lex.charset]/3 that

The basic execution character set and the basic execution wide-character set shall each contain all the members of the basic source character set, plus control characters representing alert, backspace, and carriage return, plus a null character (respectively, null wide character), whose representation has all zero bits. For each basic execution character set, the values of the members shall be non-negative and distinct from one another. In both the source and execution basic character sets, the value of each character after 0 in the above list of decimal digits shall be one greater than the value of the previous.[...]

^{emphasis mine}

回答2:

You can't force or even verify that beforehand . "Evil user" can always sneak a UTF-8 encoded string into your application, with no characters above U+7F. And such string happens to be also Ascii-encoded.

Also, whatever platform specific measure you take, user can pipe a UTF-16LE encoded file. Or /dev/urandom

Your mistakes string encoding with some magic property of an input stream - and it is not. It is, well, encoding, like JPEG or AVI, and must be handled exactly the same way - read an input, match with format, report errors on parsing failure.

For your case, if you want to accept only ASCII, read input stream byte by byte and throw/exit with error if you ever encounters a byte with the value outside ASCII domain.

However, if later you encounter a terminal providing data with some incompatible encoding, like UTF16LE, you have no choice but to write a detection (based on byte order mark) and a conversion routine.