Unsigned user-defined integer literal

Question

Suppose I want to define an integer literal which also allows for negative values, e.g. -12_km.

I.e., I would like to do

using coord_t = long long;
coord_t operator "" _km(long long int);

However, this is not accepted by my compiler (gcc).

The standard mentions a list of the allowed types for the parameter list of such a literal operator, but no signed integer type is among them.

Why is the standard like that? Why doesn't it allow for user-defined signed integer literals?

Answer 1

Because there are no negative integer literals in C++. -12 is actually the minus operator applying to the positive literal 12. That's the very reason why

• -9'223'372'036'854'775'808LL is unsigned
• we must define INT_MIN as -INT_MAX - 1
• -2147483648 > 0 and 0 < -0x80000000 in some compilers

So you need to overload the unary minus operator to use -12_km

coord_t operator "" _km(unsigned long long);
coord_t operator-();

Answer 2

Because there are no negative integer literals in the grammar. Let's put aside the user-defined nature of your question. When we write -12, it's the literal 12, which has unary - applied to it. The grammatical definition of an integer literal contains no mention of a minus sign.

[lex.icon] (redacted and edited)

integer-literal:
    decimal-literal integer-suffix

decimal-literal:
    nonzero-digit
    decimal-literal digit

nonzero-digit: one of
    1  2  3  4  5  6  7  8  9

digit: one of 
    0 1 2 3 4 5 6 7 8 9

It's right there in the grammar. There are no productions that produce negative integer literal. And that is why user-defined literals follow the same convention. Their grammar production simply reuses the production for an integer literal

[lex.ext]

user-defined-literal:
    user-defined-integer-literal

user-defined-integer-literal:
    decimal-literal ud-suffix

Since negation is always an expression other than a literal, you need to overload the appropriate operator for coord_t. And by the way, the same applies for +12. It's a unary plus applied to 12, not a literal by itself.