Question

So, I’m wishing to be able to have a static const compile
time struct that holds some value based on a string by using
templates. I only desire up to four characters. I know that
the type of ‘abcd’ is int, and so is ‘ab’,’abc’, and although
‘a’ is of type char, it works out for a template<int v> struct

What I wish to do is take sizes of 2,3,4,5 of some const char, “abcd”
and have the same functionality as if they used ‘abcd’. Note that
I do not mean 1,2,3, or 4 because I expect the null terminator.

cout << typeid("abcd").name() << endl; tells me that the type for
this hard coded string is char const [5], which includes the null
terminator on the end.

I understand that I will need to twiddle the values as characters,
so they are represented as an integer.

I cannot use constexpr since VS10 does not support it (VS11 doesn’t either..)

So, for example with somewhere this template defined, and later the last line

template <int v> struct something {
    static const int value = v;
};

//Eventually in some method
cout << typeid(something<'abcd'>::value).name() << endl;

works just fine.

I’ve tried

template<char v[5]> struct something2 {
    static const int value = v[0];
}

template<char const v[5]> struct something2 {
    static const int value = v[0];
}

template<const char v[5]> struct something2 {
    static const int value = v[0];
}

All of them build individually, though when I throw in my test,

cout << typeid(something2<"abcd">::value).name() << endl;

I get

'something2' : invalid expression as a template argument for 'v'
'something2' : use of class template requires template argument list

---

Is this not feasible or am I misunderstanding something?

Answer 1

14.1 lists the acceptable types of non-type template arguments:

— integral or enumeration type,
— pointer to object or pointer to function,
— lvalue reference to object or lvalue reference to function,
— pointer to member,

Arrays don’t fit under any of these categories.

14.3.2/1 lists categories of what’s permitted as template arguments and 14.3.2/2 goes on to say:

Note: A string literal (2.14.5) does not satisfy the requirements of any of these categories and thus is not an acceptable template-argument.

Therefore you cannot do what you’re trying to do.