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## Struct template uses_allocator

boost::container::uses_allocator

## Synopsis

    // In header: <boost/container/uses_allocator.hpp>

    template<typename T, typename AllocArg>
    struct uses_allocator {
    };

## Description

Remark: Automatically detects whether `T` has a nested `allocator_type` that is convertible from `AllocArg`. Meets the BinaryTypeTrait requirements ([meta.rqmts] 20.4.1). This trait is used to signal if type `T` supports uses-allocator construction.

uses-allocator construction protocol specifies three conventions of passing an allocator argument `alloc_arg` of type `AllocArg` to a constructor of some type T in addition to an arbitrary number of arguments (specified as `args...` in this explanation):

- If `T` does not use a compatible allocator (`uses_allocator<T, AllocArg>::value` is false), then `alloc_arg` is ignored and `T` is constructed as `T(args...)`

- Otherwise, if `uses_allocator<T, AllocArg>::value` is true and `T` is constructible as `T(std::allocator_arg, alloc_arg, args...)` then uses-allocator construction uses this form.

- Otherwise, if `T` is constructible as `T(args..., alloc_arg)`, then uses-allocator construction uses this form.

- Otherwise, as an non-standard extension provided by Boost.Container, `alloc_arg` is ignored and `T(args...)` is called. (Note that this extension is provided to enhance backwards compatibility for types created without uses-allocator construction in mind that declare an `allocator_type` type but are not prepared to be built with an additional allocator argument)

Result: `uses_allocator<T, AllocArg>::value == true` if a type `T::allocator_type` exists and either `is_convertible<AllocArg, T::allocator_type>::value == true` or `T::allocator_type` is an alias of `erased_type`. False otherwise.

Note: A program may specialize this type to define `uses_allocator<X>::value` as true for a `T` of user-defined type if `T` does not have a nested `allocator_type` but is nonetheless constructible using the specified `AllocArg` where either: the first argument of a constructor has type `allocator_arg_t` and the second argument has type `AllocArg` or the last argument of a constructor has type `AllocArg`.