..
    MIT License

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.. meta::
  :description: Understand the libhipcxx namespace hierarchy, including cuda::std::, cuda::, cuda::device::, and their hip:: aliases, and how each tier maps to host and device code.
  :keywords: libhipcxx, ROCm, HIP, namespaces, hip aliasing, cuda, std, device, AMD GPU, heterogeneous

.. _libhipcxx-hip-abstractions:

********************************
Namespace hierarchy in libhipcxx
********************************

libhipcxx organizes its APIs into three tiers, each reflecting where code can run and how closely
it conforms to the C++ Standard. Understanding this hierarchy tells you which header to include
and which namespace to use for any given facility.

.. list-table::
   :widths: 30 30 40
   :header-rows: 1

   * - Namespace
     - Include path
     - Where it runs
   * - ``std::``
     - ``<*>``
     - ``__host__`` only. Your host compiler's Standard Library. libhipcxx does not replace or
       interfere with it.
   * - ``cuda::std::`` / ``hip::std::``
     - ``<cuda/std/*>`` / ``<hip/std/*>``
     - ``__host__`` and ``__device__``. Conforming implementations of Standard Library facilities.
   * - ``cuda::`` / ``hip::``
     - ``<cuda/*>`` / ``<hip/*>``
     - ``__host__`` and ``__device__``. Extensions to the Standard Library with GPU-specific
       semantics.
   * - ``cuda::device::`` / ``hip::device::``
     - ``<cuda/warp>``, ``<cuda/work_stealing>`` (and their ``hip/`` equivalents)
     - ``__device__`` only. Extensions that rely on GPU-only hardware features such as warp
       intrinsics.

The ``cuda::`` and ``hip::`` prefixes, and their include path equivalents, are interchangeable
aliases that resolve to the same headers. Use whichever fits your project's conventions.

The following example shows all three libhipcxx tiers side by side:

.. code-block:: cpp

    // Standard C++, __host__ only.
    #include <atomic>
    std::atomic<int> x;

    // HIP C++, __host__ __device__.
    // Strictly conforming to the C++ Standard.
    #include <cuda/std/atomic>
    cuda::std::atomic<int> x;

    // HIP C++, __host__ __device__.
    // Conforming extensions to the C++ Standard.
    #include <cuda/atomic>
    cuda::atomic<int, cuda::thread_scope_block> x;