# modernize-replace-random-shuffle¶

This 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.

Below are two examples of what kind of occurrences will be found and two examples of what it will be replaced with.

std::vector<int> v;

// First example
std::random_shuffle(vec.begin(), vec.end());

// Second example
std::random_shuffle(vec.begin(), vec.end(), randomFunc);


Both of these examples will be replaced with:

std::shuffle(vec.begin(), vec.end(), std::mt19937(std::random_device()()));


The 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.

One 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. Another 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 higher quality randomness, you should consider seeding better, for example:

std::shuffle(v.begin(), v.end(), []() {
std::mt19937::result_type seeds[std::mt19937::state_size];
std::random_device device;
std::uniform_int_distribution<typename std::mt19937::result_type> dist;
std::generate(std::begin(seeds), std::end(seeds), [&] { return dist(device); });
std::seed_seq seq(std::begin(seeds), std::end(seeds));
return std::mt19937(seq);
}());