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3304. Allocate functions of std::polymorphic_allocator should require [[nodiscard]]

Section: 20.4.3 [mem.poly.allocator.class] Status: C++20 Submitter: Hiroaki Ando Opened: 2019-10-16 Last modified: 2021-02-25

Priority: 3

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Discussion:

[[nodiscard]] is specified for std::polymorphic_allocator<>::allocate().

But the allocate functions added with P0339R6 doesn't have it.

Isn't [[nodiscard]] necessary for these functions?

[2019-11 Priority to 3 during Monday issue prioritization in Belfast]

[2019-11 After discussion with LEWG, assigning to LEWG]

[2019-11-4; Daniel comments]

This issue is related to LWG 3312.

[2019-11; Friday AM in Belfast. Status changed to "Ready"]

Proposed resolution:

This wording is relative to N4835.

1. Modify 20.4.3 [mem.poly.allocator.class], class template polymorphic_allocator synopsis, as indicated:

   namespace std::pmr {
     template<class Tp = byte> class polymorphic_allocator {
       […]
       // 20.4.3.3 [mem.poly.allocator.mem], member functions
       [[nodiscard]] Tp* allocate(size_t n);
       void deallocate(Tp* p, size_t n);
   
       [[nodiscard]] void* allocate_bytes(size_t nbytes, size_t alignment = alignof(max_align_t));
       void deallocate_bytes(void* p, size_t nbytes, size_t alignment = alignof(max_align_t));
       template<class T> [[nodiscard]] T* allocate_object(size_t n = 1);
       template<class T> void deallocate_object(T* p, size_t n = 1);
       template<class T, class... CtorArgs> [[nodiscard]] T* new_object(CtorArgs&&... ctor_args);
       template<class T> void delete_object(T* p);  
       […]
     };  
   }
   

2. Modify 20.4.3.3 [mem.poly.allocator.mem] as indicated:

   [[nodiscard]] void* allocate_bytes(size_t nbytes, size_t alignment = alignof(max_align_t));
   

   -5- Effects: Equivalent to: return memory_rsrc->allocate(nbytes, alignment);

   […]

   […]

   template<class T>
     [[nodiscard]] T* allocate_object(size_t n = 1);
   

   -8- Effects: Allocates memory suitable for holding an array of n objects of type T, as follows:

   1. (8.1) — if SIZE_MAX / sizeof(T) < n, throws length_error,

   2. (8.2) — otherwise equivalent to:

      return static_cast<T*>(allocate_bytes(n*sizeof(T), alignof(T)));
      

   […]

   template<class T, class CtorArgs...>
     [[nodiscard]] T* new_object(CtorArgs&&... ctor_args);
   

   -11- Effects: Allocates and constructs an object of type T, as follows. Equivalent to:

   T* p = allocate_object<T>();
   try {
     construct(p, std::forward<CtorArgs>(ctor_args)...);
   } catch (...) {
     deallocate_object(p);
     throw;
   }
   return p;
   

   […]