Resource Management

Introduction#

One of the hardest things to do in C and C++ is resource management. Thankfully, in C++, we have many ways to go about designing resource management in our programs. This article hopes to explain some of the idioms and methods used to manage allocated resources.

Resource Acquisition Is Initialization

Resource Acquisition Is Initialization (RAII) is a common idiom in resource management. In the case of dynamic memory, it uses smart pointers to accomplish resource management. When using RAII, an acquired resource is immediately given ownership to a smart pointer or equivalent resource manager. The resource is only accessed through this manager, so the manager can keep track of various operations. For example, std::auto_ptr automatically frees its corresponding resource when it falls out of scope or is otherwise deleted.

#include <memory>
#include <iostream>
using namespace std;

int main() {
    {
        auto_ptr ap(new int(5)); // dynamic memory is the resource
        cout << *ap << endl; // prints 5
    } // auto_ptr is destroyed, its resource is automatically freed
}

std::auto_ptr’s main problem is that it can’t copied without transferring ownership:

#include <memory>
#include <iostream>
using namespace std;

int main() {
    auto_ptr ap1(new int(5));
    cout << *ap1 << endl; // prints 5
    auto_ptr ap2(ap1); // copy ap2 from ap1; ownership now transfers to ap2
    cout << *ap2 << endl; // prints 5
    cout << ap1 == nullptr << endl; // prints 1; ap1 has lost ownership of resource
}

Because of these weird copy semantics, std::auto_ptr can’t be used in containers, among other things. The reason it does this is to prevent deleting memory twice: if there are two auto_ptrs with ownership of the same resource, they both try to free it when they’re destroyed. Freeing an already freed resource can generally cause problems, so it is important to prevent it. However, std::shared_ptr has a method to avoid this while not transferring ownership when copying:

#include <memory>
#include <iostream>
using namespace std;

int main() {
    shared_ptr sp2;
    {
        shared_ptr sp1(new int(5)); // give ownership to sp1
        cout << *sp1 << endl; // prints 5
        sp2 = sp1; // copy sp2 from sp1; both have ownership of resource
        cout << *sp1 << endl; // prints 5
        cout << *sp2 << endl; // prints 5
    } // sp1 goes out of scope and is destroyed; sp2 has sole ownership of resource
    cout << *sp2 << endl;        
} // sp2 goes out of scope; nothing has ownership, so resource is freed

Mutexes & Thread Safety

Problems may happen when multiple threads try to access a resource. For a simple example, suppose we have a thread that adds one to a variable. It does this by first reading the variable, adding one to it, then storing it back. Suppose we initialize this variable to 1, then create two instances of this thread. After both threads finish, intuition suggests that this variable should have a value of 3. However, the below table illustrates what might go wrong:

As you can see, at the end of the operation, 2 is in the variable, instead of 3. The reason is that Thread 2 read the variable before Thread 1 was done updating it. The solution? Mutexes.

A mutex (portmanteau of mutual exclusion) is a resource management object designed to solve this type of problem. When a thread wants to access a resource, it “acquires” the resource’s mutex. Once it is done accessing the resource, the thread “releases” the mutex. While the mutex is acquired, all calls to acquire the mutex will not return until the mutex is released. To better understand this, think of a mutex as a waiting line at the supermarket: the threads go into line by trying to acquire the mutex and then waiting for the threads ahead of them to finish up, then using the resource, then stepping out of line by releasing the mutex. There would be complete pandemonium if everybody tried to access the resource at once.

std::mutex is C++11’s implementation of a mutex.

#include <thread>
#include <mutex>
#include <iostream>
using namespace std;

void add_1(int& i, const mutex& m) { // function to be run in thread
    m.lock();
    i += 1;
    m.unlock();
}

int main() {
    int var = 1;
    mutex m;

    cout << var << endl; // prints 1
    
    thread t1(add_1, var, m); // create thread with arguments
    thread t2(add_1, var, m); // create another thread
    t1.join(); t2.join(); // wait for both threads to finish
    
    cout << var << endl; // prints 3
}