3.1.5Destructors and releasing resources

What a destructor is

A destructor is a method that runs when an object is destroyed, to release resources (files, network connections, memory).

• C++ has destructors written as ~ClassName(), and they run when the object goes out of scope or is deleted.
• Java and Python have no destructors in the C++ sense. Memory is reclaimed automatically by the garbage collector (see 3.2.2). To release resources such as files, Java uses try-with-resources and the AutoCloseable interface, or an explicit close() method. The old finalize() method is deprecated and must not be used.

class Report implements AutoCloseable {
    Report() { System.out.println("Report opened"); }

    @Override
    public void close() { System.out.println("Report closed"); }
}

try (Report r = new Report()) {
    System.out.println("Working...");
}   // close() is called automatically here

Output: Report opened, Working..., Report closed.

#include <iostream>
using namespace std;

class Report {
public:
    Report()  { cout << "Report opened" << endl; }
    ~Report() { cout << "Report closed" << endl; }   // destructor
};

int main() {
    Report r;                          // constructor runs
    cout << "Working..." << endl;
    return 0;
}                                      // r goes out of scope: destructor runs

Why releasing resources matters

Memory is not the only thing an object can hold. Objects often hold resources that belong to the operating system or to another program: an open file, a database connection, a network socket or a printer. There is only a limited number of these. If a program opens files and never closes them, it eventually cannot open any more. A database server may refuse new connections, and data written to a file may never actually be saved to disk.

The garbage collector frees memory, but you cannot know when it will run (see 3.2.2). Waiting for it to close a file is not good enough. So in Java you close resources yourself, at a moment you choose.

How try-with-resources works

try (Report r = new Report()) {   // the resource is opened here
    r.write();
}                                  // r.close() is called here, automatically

• The class must implement the AutoCloseable interface, which means it has a close() method.
• When the try block ends, Java calls close() automatically: whether the block finishes normally, returns early or throws an exception.
• You can open several resources in one try (...). They are closed in the reverse order to the one they were opened in.

Before Java 7, programmers had to call close() in a finally block. That approach still works, but it is longer and easy to get wrong.

C++ destructors and RAII

In C++, a destructor runs at an exact, predictable moment: when a local object goes out of scope at the closing }, or when delete is used on an object created with new. C++ programmers use this to tie a resource to an object's lifetime: the constructor opens the resource and the destructor closes it. This idea is called RAII (Resource Acquisition Is Initialisation). Java's try-with-resources gives the same safety in a different way.