Introduction

While new language features allow you to write more compact code, you also need the tools - in the form of the Standard Library types. The classes and systems that you can find in the Library can significantly enhance your productivity. C++17 offers even more handy instruments: for example the filesystem, new vocabulary types, and even parallel algorithms! We’ll now look at a prominent new feature called std::optional.

Nullable Types

One approach is to achieve “null-ability” by using unique values (-1, infinity, nullptr). Before use, you need to compare the object against the predefined value to see if it’s not empty. Such a pattern is widespread in programming. For instance string::find returns a value that represents the position or npos when it’s “null” or the pattern is not found.

Alternatively, you could try with std::unique_ptr<Type> and treat the empty pointer as not initialised. That works but comes with the cost of allocating memory for the object and is not a recommended technique.

Another technique is to build a wrapper that adds a boolean flag to other types. Such wrapper can quickly determine the state of the object. And this is how in a nutshell works std::optional.

Optional types that come from the functional programming world bring type safety and expressive- ness. Most other languages have something similar: for example std::option in Rust, Optional<T> in Java, Data.Maybe in Haskell.

std::optional

std::optional was added in C++17 and brings a lot of experience from boost::optional that has been available for many years. With C++17 you can just #include and use the type.

std::optional was available also in Library Fundamentals TS, so there’s a chance that your C++14 compiler could also support it in the <experimental/optional> header.

std::optional is still a value type (so it can be copied, via deep copy). Additionally, std::optional doesn’t need to allocate any memory on the free store.

std::optional is a part of C++ vocabulary types along with std::any, std::variant and std::string_view.

When To Use

You can usually use an optional wrapper in the following scenarios:

If You Want to Represent a Nullable Type

  • Rather than using unique values (like -1, nullptr, NO_VALUE or something)
  • For example, a user’s middle name is optional. You could assume that an empty string would work here, but knowing if a user entered something or not might be important. std::optional<std::string> gives you more information.

Return a Result of Some Computation (Processing) That Fails to Produce a Value and Is Not an Error

For example, finding an element in a dictionary: if there’s no element under a key, it’s not an error, but we need to handle the situation.

To Perform Lazy-Loading of Resources

For example, if the construction of a resource type is substantial, or if there’s no default constructor, you can define it as std::optional<Resource>. In that form, you can pass it around the system, and then initialise it (load a resource), when the application wants to access it for the first time.

To Pass Optional Parameters into Functions

The documentation for boost.optional has a useful summary on when we should use the type, see in When to use Optional:

It is recommended to use optional<T> in situations where there is exactly one, clear (to all parties) reason for having no value of type T, and where the lack of value is as natural as having any regular value of T.

While sometimes the decision to use optional might be blurry, it best suits the cases when the value is empty, and it’s a normal state of the program.

If this is a lot to take in, don’t worry. We’ll look at an example of how std::option​al actually works.

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