Design question LWG 2861: basic_string should require that charT match traits::char_type

Daniel Krügler
Sun Mar 12 13:21:00 GMT 2017

I'm now working on

The new wording state is now equivalent to basic_string_view, whose
current implementation doesn't bother verifying the requirement, so
this code (which as UB) currently compiles just fine:

#include <string>
#include <string_view>

struct MyTraits : std::char_traits<char>
  typedef unsigned char char_type;

int main()
  std::basic_string<char, MyTraits> my_string;
  std::basic_string_view<char, MyTraits> my_string_view;

So the least I could do is just - nothing. But it seems to me that we
could protect users from doing such silly things by adding a
static_assert to both basic_string and basic_string_view, the former
being equivalent to

#if __cplusplus >= 201103L
      static_assert(__are_same<value_type, _CharT>::value,
                    "traits_type::char_type must be equal to _CharT");

and the latter an unconditional

      static_assert(is_same<typename _Traits::char_type, _CharT>::value,
                    "traits_type::char_type must be equal to _CharT");

Would you agree with that course of action?


- Daniel

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