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.... 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.... 
u.lock(); is a function call. It doesn't construct anything that goes out of scope.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... } 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(); } 
doTask....a function and then call that function in each branch