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| Member functions | ||||
Defined in header <mutex> | ||
template<class Mutex> class lock_guard; | (since C++11) | |
The classlock_guard is a mutex wrapper that provides a convenientRAII-style mechanism for owning a mutex for the duration of a scoped block.
When alock_guard object is created, it attempts to take ownership of the mutex it is given. When control leaves the scope in which thelock_guard object was created, thelock_guard is destructed and the mutex is released.
Thelock_guard class is non-copyable.
Contents |
| Mutex | - | the type of the mutex to lock. The type must meet theBasicLockable requirements |
| Member type | Definition |
mutex_type | Mutex |
constructs alock_guard, optionally locking the given mutex(public member function)[edit] | |
destructs thelock_guard object, unlocks the underlying mutex(public member function)[edit] | |
operator= [deleted] | not copy-assignable (public member function)[edit] |
A common beginner error is to forget to give alock_guard variable a name, such as bystd::lock_guard{mtx}. This constructs a prvalue object that is immediately destroyed, thereby not actually constructing a lock that holds a mutex for the rest of the scope.
std::scoped_lock offers an alternative for | (since C++17) |
Demonstrates safe and unsafe increments of a volatile variable by two threads.
#include <iostream>#include <mutex>#include <string_view>#include <syncstream>#include <thread> volatileint g_i=0;std::mutex g_i_mutex;// protects g_i void safe_increment(int iterations){const std::lock_guard<std::mutex> lock(g_i_mutex);while(iterations-->0) g_i= g_i+1;std::cout<<"thread #"<<std::this_thread::get_id()<<", g_i: "<< g_i<<'\n'; // g_i_mutex is automatically released when lock goes out of scope} void unsafe_increment(int iterations){while(iterations-->0) g_i= g_i+1;std::osyncstream(std::cout)<<"thread #"<<std::this_thread::get_id()<<", g_i: "<< g_i<<'\n';} int main(){auto test=[](std::string_view fun_name,auto fun){ g_i=0;std::cout<< fun_name<<":\nbefore, g_i: "<< g_i<<'\n';{std::jthread t1(fun,1'000'000);std::jthread t2(fun,1'000'000);}std::cout<<"after, g_i: "<< g_i<<"\n\n";}; test("safe_increment", safe_increment); test("unsafe_increment", unsafe_increment);}
Possible output:
safe_increment:before, g_i: 0thread #140121493231360, g_i: 1000000thread #140121484838656, g_i: 2000000after, g_i: 2000000 unsafe_increment:before, g_i: 0thread #140121484838656, g_i: 1028945thread #140121493231360, g_i: 1034337after, g_i: 1034337
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 2981 | C++17 | redundant deduction guide fromlock_guard<Mutex> was provided | removed |
(C++11) | implements movable mutex ownership wrapper (class template)[edit] |
(C++17) | deadlock-avoiding RAII wrapper for multiple mutexes (class template)[edit] |
