Skip to main content

What Is RAII

RAII is a frequently used idiom in C++ that ensures the safe usage of resources by releasing them when an object's scope ends. In C++, resources allocated on the heap are not released unless explicitly done so, but those allocated on the stack are automatically released when their scope ends, triggering their destructor. Originally, RAII was used to guard against unexpected changes in control flow, such as exceptions.

In the above code example, the unsafeFunction() function is not safe. If the thisFunctionCanThrowException() throws an exception, the resource may not be released. The unmaintanableFunction releases the resource, but it is not easy to read and maintain.

The safeFunction example uses unique_ptr, a smart pointer introduced at C++11, for RAII. unique_ptr automatically releases the memory it holds when it is destroyed, ensuring that the resource is released when the function exits.

The resource does not only refer to heap memory but also includes files, databases, and other things that can be safely used with RAII. Furthermore, RAII can be used to handle code that must always be executed when a specific scope ends, similar to the finally statement in other languages. In fact, Bjarne Stroustrup, the creator of C++ and the term RAII, stated that there was no need for a finally statement in C++ due to the existence of RAII.

This article is a translation of a Korean post written in 2014. If you would like to view the original, please refer to this link.


Popular posts from this blog

[C++] Handling Exceptions in Constructors

When you use RAII idiom, there are often situations where constructors have to do complex tasks. These complex tasks can sometimes fail, resulting in throwing exceptions. This raises a concern: Is it okay to throw exceptions in constructors? The first concern is memory leaks. Fortunately, memory leaks do not occur. Variables created on the stack are released through stack unwinding, and if an exception occurs during heap allocation with the new operator, the new operator automatically deallocates the memory and returns nullptr . The next concern is whether the destructor of the member variables will be called correctly. However, this is also not a problem. When an exception occurs, member variables can be divided into three categories: fully initialized member variables, member variables being initialized, and uninitialized member variables. Fully initialized member variables have had their constructors called and memory allocations completed successfully. In the example code, t

Iterator Adapters in Rust

An Iterator that takes another iterator and returns a new one is called an iterator adapter . The name "adapter" comes from one of the GoF's design patterns, the adapter pattern . However, in reality, it corresponds more to the decorator pattern , so if you pay too much attention to the name, you might get confused about its purpose. So it's better not to worry too much about the name. Enough complaining about the name, what does an iterator adapter do? An iterator adapter adds a task to be performed when the iterator iterates. This will be easier to understand when you see an example. The map function is one of the famous adapters. The iterator returned by the map function for those who have used functional languages iterates over new values transformed from the original values. Besides, various adapters are already implemented in the standard library. Among them, the most frequently used are those that are convenient to use with loops. Examples include the

What is the size of an empty object?

Consider a class like the one above. Commonly called an "empty class," this class has no internal variables. So, how big is this empty class? At first glance, the size should be 0 since there are no member variables. However, the size is never 0 in any language, whether Java, C#, C (in this case, a struct), or C++. This is to ensure that two different objects never have the same address. Empty classes typically have a size of 1 byte in a 32-bit environment and 2 bytes in a 64-bit environment. However, the exact size cannot be determined. According to the specification, the size just needs to be non-zero. The precise size depends on the implementation. This is a translation of my old Korean post written in 2015. Because the size can vary depending on the implementation, it is now possible to have different sizes (although still not 0). And Languages like Rust have even introduced zero-sized types . We will look at this topic in more detail at a future opportunity.