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2466. allocator_traits::max_size() default behavior is incorrect

Section: 16.4.4.6 [allocator.requirements], 20.2.9.3 [allocator.traits.members] Status: C++17 Submitter: Howard Hinnant Opened: 2015-01-17 Last modified: 2017-07-30

Priority: 3

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Discussion:

Table 28 — "Allocator requirements" says that default behavior for a.max_size() is numeric_limits<size_type>::max(). And this is consistent with the matching statement for allocator_traits in 20.2.9.3 [allocator.traits.members]/p7: 

static size_type max_size(const Alloc& a) noexcept;

Returns: a.max_size() if that expression is well-formed; otherwise, numeric_limits<size_type>::max().

However, when allocating memory, an allocator must allocate n*sizeof(value_type) bytes, for example: 

value_type*
allocate(std::size_t n)
{
  return static_cast<value_type*>(::operator new (n * sizeof(value_type)));
}

When n == numeric_limits<size_type>::max(), n * sizeof(value_type) is guaranteed to overflow except when sizeof(value_type) == 1.

A more useful default would be numeric_limits<size_type>::max() / sizeof(value_type).

[2015-05, Lenexa]

Marshall: Is this the right solution?
PJP: I think it's gilding the lily.
STL: I think this is right, and it doesn't interact with the incomplete container stuff because it's in a member function.
Marshall: Objections to this?
STL: Spaces around binary operators.
Hwrd: It's completely wrong without spaces.
Marshall: All in favor of Ready?
Lots.

Proposed resolution:

This wording is relative to N4296.

  1. Change 16.4.4.6 [allocator.requirements], Table 28 — "Allocator requirements", as indicated:

    Table 28 — Allocator requirements
    Expression Return type Assertion/note
    pre-/post-condition    		 Default
    …
    a.max_size() X::size_type the largest value that can
    meaningfully be passed to
    X::allocate()     		numeric_limits<size_type>::max()/sizeof(value_type)
    …

  2. Change 20.2.9.3 [allocator.traits.members]/p7 as indicated:

    static size_type max_size(const Alloc& a) noexcept;

    Returns: a.max_size() if that expression is well-formed; otherwise, numeric_limits<size_type>::max()/sizeof(value_type).