Create shared_lock or unique_lock based on a condition

Question

I need to create unique_lock or shared lock based on a parameter in my function. I couldn't define properly in the scope of function. So I need to do something like this.

function (bool check) {
 lock;
 if (check) {
  lock = std::unique_lock<std::shared_mutex>(mutex);
 } else {
  lock = std::shared_lock<std::shared_mutex>(mutex);
 }
 lock.lock();
 doTask....

Answer 1

Like this?

void function (bool check) {
  std::unique_lock<std::shared_mutex> u(mutex, std::defer_lock);
  std::shared_lock<std::shared_mutex> s(mutex, std::defer_lock);
  if (check) {
    u.lock();
  } else {
    s.lock();
  }
  doTask....

Answer 2

Since the caller is passing in the check condition anyway, you could give your function a template parameter to let the caller specify which type of lock to use, eg:

template<typename LockClass>
function () {
 LockClass<std::shared_mutex> lock(mutex);
 doTask....
}

//function (true);
function<std::unique_lock>();

//function (false);
function<std::shared_lock>();

Otherwise, you could split your code into multiple functions:

doTask() {
  ...
}

unique_function () {
  std::unique_lock<std::shared_mutex> lock(mutex);
  doTask();
}

shared_function () {
  std::shared_lock<std::shared_mutex> lock(mutex);
  doTask();
}

Answer 3

Use std::variant :

void function (bool check)
{
 std::variant<std::shared_lock<std::shared_mutex>, std::unique_lock<std::shared_mutex>> lock;
 if (check) {
 lock = std::unique_lock(mut);
 } else {
 lock = std::shared_lock(mut);
 }
 //doTask...
}