struct allocator_is_nothrow

ABSL_ALLOCATOR_NOTHROW is a build time configuration macro for user to

specify whether the default allocation function can throw or never throws.

If the allocation function never throws, user should define it to a non-zero

value (e.g. via `-DABSL_ALLOCATOR_NOTHROW`).

If the allocation function can throw, user should leave it undefined or

define it to zero.

allocator_is_nothrow

<Alloc

> is a traits class that derives from

Alloc::is_nothrow if present, otherwise std::false_type. It's specialized

for Alloc = std::allocator

<T

> for any type T according to the state of

ABSL_ALLOCATOR_NOTHROW.

default_allocator_is_nothrow is a class that derives from std::true_type

when the default allocator (global operator new) never throws, and

std::false_type when it can throw. It is a convenience shorthand for writing

allocator_is_nothrow

<std

::allocator

<T

>> (T can be any type).

NOTE: allocator_is_nothrow

<std

::allocator

<T

>> is guaranteed to derive from

the same type for all T, because users should specialize neither

allocator_is_nothrow nor std::allocator.