Use constexpr for basic_string

[ebmmy]

This pull request intends to align the use of constexpr with how it is defined for `std::basic_string' in the STL.

[jwellbelove]

I'm pretty sure that this should work for C++14 constexpr, unless you know of any C++17/20 features that are required.

Also, you have not created any unit tests to check that all of the constexpr functions can be used in a constexpr expression.

[ebmmy]

I'm pretty sure that this should work for C++14 constexpr, unless you know of any C++17/20 features that are required.

Right, ETL_CONSTEXPR14 should work fine, I was not sure whether your policy was to conform to the C++ standard or use the first possible version for constexpr.

Also, you have not created any unit tests to check that all of the constexpr functions can be used in a constexpr expression.

What would be the purpose of such tests? This wouldn't really test a functionality, either the function isn't marked as constexpr', the expression wouldn't compile, or it is marked as constexpr' and it can be used inside a `constexpr' expression. What do you think?

[jwellbelove]

The purpose of the constexpr tests is to make sure that the compiler is happy that a function marked as constexpr can actually work in a constexpr expression.

[ebmmy]

The purpose of the constexpr tests is to make sure that the compiler is happy that a function marked as constexpr can actually work in a constexpr expression.

Isn’t that granted by the c++ standard directly? I cannot really think of a case this isn’t true.

[jwellbelove]

Certain compilers will only analyse the template code for what you actually invoke. For example, Microsoft's compiler will not alert that a template function cannot be used in a constexpr unless you actually create code that uses it in a constexpr.

Example:

template <typename T>
constexpr int GetValue(T i)
{
  static int si = 2;

  return i + si;
}

int main()
{
  constexpr int x = GetValue<int>(1); // This is a compile error for C++14 with VS2022

  int y = GetValue<int>(1);           // This is not a compile error for C++14 with VS2022

  return 0;
}