1.. title:: clang-tidy - modernize-replace-random-shuffle
2
3modernize-replace-random-shuffle
4================================
5
6This check will find occurrences of ``std::random_shuffle`` and replace it with ``std::shuffle``. In C++17 ``std::random_shuffle`` will no longer be available and thus we need to replace it.
7
8Below are two examples of what kind of occurrences will be found and two examples of what it will be replaced with.
9
10.. code-block:: c++
11
12  std::vector<int> v;
13
14  // First example
15  std::random_shuffle(vec.begin(), vec.end());
16
17  // Second example
18  std::random_shuffle(vec.begin(), vec.end(), randomFunc);
19
20Both of these examples will be replaced with:
21
22.. code-block:: c++
23
24  std::shuffle(vec.begin(), vec.end(), std::mt19937(std::random_device()()));
25
26The second example will also receive a warning that ``randomFunc`` is no longer supported in the same way as before so if the user wants the same functionality, the user will need to change the implementation of the ``randomFunc``.
27
28One thing to be aware of here is that ``std::random_device`` is quite expensive to initialize. So if you are using the code in a performance critical place, you probably want to initialize it elsewhere.
29Another thing is that the seeding quality of the suggested fix is quite poor: ``std::mt19937`` has an internal state of 624 32-bit integers, but is only seeded with a single integer. So if you require
30higher quality randomness, you should consider seeding better, for example:
31
32.. code-block:: c++
33
34  std::shuffle(v.begin(), v.end(), []() {
35    std::mt19937::result_type seeds[std::mt19937::state_size];
36    std::random_device device;
37    std::uniform_int_distribution<typename std::mt19937::result_type> dist;
38    std::generate(std::begin(seeds), std::end(seeds), [&] { return dist(device); });
39    std::seed_seq seq(std::begin(seeds), std::end(seeds));
40    return std::mt19937(seq);
41  }());
42