Is std::string_view trivially copyable?

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For some types, the standard explicitly states that they are trivially copyable. For example, std::chrono::day is such a type.

But, for std::string_view, there is no such statement. Yet, it seems that it fulfills the requirements of being trivially copyable (or maybe I overlooked something?).

The question is, can I count on that std::string_view is trivially copyable?

c++17 language-lawyer c++
1 Answers

I believe it's strongly implied.

[string.view.template] declares the class template as (I'm omitting everything that isn't strictly relevant to this question):

template<class charT, class traits = char_traits<charT>>
class basic_string_view {
public:
  // types
  using const_pointer          = const value_type*;
  using size_type              = size_t;
private:
  const_pointer data_; // exposition only
  size_type size_;     // exposition only
};

And [objects.within.classes] says:

For the sake of exposition, some subclauses provide representative declarations, and semantic requirements, for private members of classes that meet the external specifications of the classes. The declarations for such members are followed by a comment that ends with exposition only, as in:

streambuf* sb;  // exposition only

An implementation may use any technique that provides equivalent observable behavior.

Whether or not std::string_view exactly contains two members of types char const* and size_t or not is unspecified - but the class must have equivalent observable behavior to how such an implementation would behave. And such an implementation would be trivially copyable.

Also, many member functions are declared constexpr, which itself in C++17 required trivial destruction (otherwise, this annotation would be ill-formed NDR).

There is now a proposal, P2251, to go ahead and actually explicitly state that basic_string_view (and span) are trivially copyable.

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