Add a pmr alias for std::stacktrace

Document #: P2301R0
Date: 2021-02-12
Project: Programming Language C++
Audience: LEWG, LWG
Reply-to: Steve Downey
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Abstract: This paper proposes to add an alias in the pmr namespace defaulting the allocator used by the std::basic_stacktrace template to pmr::allocator. No changes to the api of std::stacktrace are necessary.

1 Before / After Table

Before
After 
char buffer[1024];

std::pmr::monotonic_buffer_resource pool{
    std::data(buffer), std::size(buffer)};

std::basic_stacktrace<
    std::pmr::polymorphic_allocator<std::stacktrace_entry>>
    trace{&pool};
 
char buffer[1024];

std::pmr::monotonic_buffer_resource pool{
    std::data(buffer), std::size(buffer)};

std::pmr::stacktrace trace{&pool};

2 Motivation

The type std::basic_stacktrace is a class template with an allocator type parameter, the allocator is fixed to be std::allocator in the std::stacktrace alias, and it models AllocatorAwareContainer. All of the work to enable a pmr using type has been done, except for actually specifying the existence of the template alias std::pmr::stacktrace with a polymorphic_allocator.

In general a template should have an alias in the pmr namespace when the primary template supports the allocator model and the allocator template parameter is defaulted to be std::allocator. Providing the pmr alias is a convenience for changing the default.

3 Proposed Wording

Modify 20.21.2 [stacktrace.syn] as indicated

namespace std {
  // [stacktrace.entry], class stacktrace_­entry
  class stacktrace_entry;

  // [stacktrace.basic], class template basic_­stacktrace
  template<class Allocator>
    class basic_stacktrace;

  // basic_­stacktrace typedef names
  using stacktrace = basic_stacktrace<allocator<stacktrace_entry>>;

  // [stacktrace.basic.nonmem], non-member functions
  template<class Allocator>
    void swap(basic_stacktrace<Allocator>& a, basic_stacktrace<Allocator>& b)
      noexcept(noexcept(a.swap(b)));

  string to_string(const stacktrace_entry& f);

  template<class Allocator>
    string to_string(const basic_stacktrace<Allocator>& st);

  template<class charT, class traits>
    basic_ostream<charT, traits>&
      operator<<(basic_ostream<charT, traits>& os, const stacktrace_entry& f);

  template<class charT, class traits, class Allocator>
    basic_ostream<charT, traits>&
      operator<<(basic_ostream<charT, traits>& os, const basic_stacktrace<Allocator>& st);

  namespace pmr {
    using stacktrace = std::basic_stacktrace<polymorphic_allocator<stacktrace_entry>>;
  }

  // [stacktrace.basic.hash], hash support
  template<class T> struct hash;
  template<> struct hash<stacktrace_entry>;
  template<class Allocator> struct hash<basic_stacktrace<Allocator>>;
}

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