为什么 basic_string_view 不限于字符类型？

Question

Browsing twitter I found this example of C++23 code.

This is my adaptation of it, to make more obvious what I am interested about(I do not care about dangling problems mentioned in replies).

#include <vector>
#include <string_view>
#include <iostream>
#include <type_traits>


int main() {
    std::vector v{84.72};
    std::basic_string_view sv = v;
    static_assert(std::is_same_v<decltype(sv), std::basic_string_view<double>>);
    const auto val = *std::begin(sv);
    std::cout << val;
}

My question is why isn't there some requires/concept constraint on the basic_string_view to make it work only with char ish types, so basic_string_view<double> in this example would not compile?

Answer 1

I suspect that this is a char -like type:

struct Char {
    char c;
    Char() : c{} {}
    Char(char c) : c{c} {}
};

Why shouldn't it work? Indeed it does

std::basic_string<Char> str{'a', 'b'}; // OK
std::cout << str[0].c << std::endl; // prints a
std::cout << str.length() << std::endl; // prints 2

And what makes that class special with respect to, say, this?

struct Char {
    int c;
    Char() : c{} {}
    Char(int c) : c{c} {}
};

Nothing, except our decision that char is a character and int is not. (And that's exactly the reason why I had to write std::cout << str[0].c and couldn't write std::cout << str or std::cout << str[0] , because << is overloaded for char s and maybe something else, but certainly not for my own types.)

So the bottom line, as implied by some comments, is a counter-question:

How would you define a "charish" type?

which I would rephrase as

Can we encode the definiton of "sequence of char -like objects" in a concept?

which leads in turn to another question:

What operations can you do only on a "sequence of char -like objects" that you can't do on all "sequences of non- char -like objects"?

I can't think of one.

So if you wanted to enforce the constraint you mention, you would end up explicitly listing char , wchar , and all the others in some SFINAE thing.

And then you couldn't use it with any other type.