std::allocator_traits<Alloc>::max_size

 

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std::allocator_traits

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allocator_traits::allocate (C++11)

allocator_traits::allocate_at_least (C++23)

allocator_traits::deallocate (C++11)

allocator_traits::construct (C++11)

allocator_traits::destroy (C++11)

allocator_traits::max_size (C++11)

allocator_traits::select_on_container_copy_construction (C++11)

 

Defined in header <memory>
static size_type max_size( const Alloc& a ) noexcept;		(since C++11) (constexpr since C++20)

If possible, obtains the maximum theoretically possible allocation size from the allocator a, by calling a.max_size().

If the above is not possible (e.g. Alloc does not have the member function max_size()), then returns std::numeric_limits<size_type>::max() / sizeof(value_type).

Parameters

a

-

allocator to detect

Return value

Theoretical maximum allocation size.

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2162	C++11	max_size was not requied to be noexcept	required
LWG 2466	C++11	theoretical maximum allocation size in bytes was returned as fallback	size in elements is returned

Example

Run this code

#include <iostream>
#include <memory>
 
int main()
{
    std::allocator<short> b;
    std::allocator<int> d;
 
    const auto p = std::allocator_traits<decltype(b)>::max_size(b);
    const auto q = std::allocator_traits<decltype(d)>::max_size(d);
 
    std::cout.imbue(std::locale("en_US.UTF-8"));
    std::cout << std::uppercase
              << "p = " << std::dec << p << " = 0x" << std::hex << p << '\n'
              << "q = " << std::dec << q << " = 0x" << std::hex << q << '\n';
}

Possible output:

p = 9,223,372,036,854,775,807 = 0x7,FFF,FFF,FFF,FFF,FFF
q = 4,611,686,018,427,387,903 = 0x3,FFF,FFF,FFF,FFF,FFF

See also

max_size (until C++20)

returns the largest supported allocation size (public member function of std::allocator<T>)