destroy std::mutex vs pthread_mutex_t

Question

What is the difference between std::mutex and pthread_mutex_t in CPP?

What is the correct way to destroy each one of the mutex types when they are dynamically allocated as follows:

pthread_mutex_t * mutex1 = new pthread_mutex_t();

std::mutex * mutex2 = new std::mutex ();

Answer 1

The difference is that they are from different libraries. They are both mutexes (mutices?).

pthread_mutex is from a library designed for C, so it does not use constructors or destructors. Simply creating a pthread_mutex_t object does not create a mutex. You have to call pthread_mutex_init to create the mutex. Likewise you have to call pthread_mutex_destroy to destroy it, because it has no destructor which does this.

std::mutex is designed for C++ so of course the object itself always acts as a mutex, with no need for separate function calls.

pthread_mutex_t * mutex1 = new pthread_mutex_t; // () not needed
// you created a pthread_mutex_t object, but didn't initialize it.
// You created it with new, so destroy it with delete:
delete mutex1;

pthread_mutex_t * mutex1 = new pthread_mutex_t; // () not needed
// you created a pthread_mutex_t object
pthread_mutex_init(&mutex1, nullptr); // can check return value for errors
// and used that space to store whatever data a mutex has in it (perhaps pthread_mutex_init allocates other memory)
// Reverse pthread_mutex_init with pthread_mutex_destroy:
pthread_mutex_destroy(&mutex1); // can check return value for errors
// You created it with new, so destroy it with delete:
delete mutex1;

std::mutex * mutex3 = new std::mutex; // () not needed
// You created it with new, so destroy it with delete
delete mutex3;

However, you don't need to allocate everything with new. C++ isn't Java. If you want an object, then in the vast majority of cases you can just have one:

std::mutex mutex4;
// automatically destroyed when it goes out of scope

and the C version:

pthread_mutex_t mutex5;
pthread_mutex_init(&mutex5, nullptr); // can check return value for errors
// Reverse pthread_mutex_init with pthread_mutex_destroy:
pthread_mutex_destroy(&mutex5); // can check return value for errors
// Variable is destroyed when it goes out of scope.
// Of course you can reuse the same variable for a new mutex by calling init again
// and destroy when you are done