string_view constructor isn't constexpr

[akrzemi1]

boost::string_view could be a good choice for compilers that do not support constexpr string_view yet. But with boost::string_view one cannot create constexpr globals either because the converting constructor isn't constexpr. It calls char_traits::length, and the latter is not declared constexpr. Could the definition of the constructor be fixed to use a custom implementation of length, at least for type char?

See the example of what I describe: https://wandbox.org/permlink/LwJnxVrsHyY1V156

[mclow]

You can use your own char_traits and declare the length method constexpr.

[akrzemi1]

Looking at the declaration of operator<<:

template<class charT, class traits>
  inline std::basic_ostream<charT, traits>&
  operator<<(std::basic_ostream<charT, traits>& os,
             const basic_string_view<charT,traits>& str);

I would also have to provide custom ostream specializations, and could not use my string_ref with std::cout.

[mclow]

I would really rather not do this, because the user expects traits::length to be called.
You could write (untested code):

constexpr string_view make_string_view(const char *s)
{ return string_view(s, my_length(s)); }

and call that instead of using the constructor directly.

[pdimov]

The user can't expect std::char_traits<char>::length to be called because it's unobservable.

[fsmoke]

besides std::string_view ctors are constexpr and it'seems len traits also constexpr
cos i can write like this

constexpr std::string_view sv = "Hello";

without any compiler errors

but in this expression
constexpr boost::string_view sv = "Hello";
i've got compiler errors

[Lastique]

boost::string_view constructor is constexpr now, and as long as std::char_traits::length() is constexpr, it works fine.