The C++17 standard came with a lot of great features and std::optional was one of the main features of today’s modern C++. std::optional<T> is a class template that is defined in the <optional> header and represents either a T value or no value. In this post, we explain, what is optional in modern C++ and how we can use it efficiently.
What is the optional class template in C++ 17 and beyond?
The std::optional feature is a class template that is defined in the <optional> header and represents either a T value or no value (which is signified by the tag type nullopt_t). In some respects, this can be thought of as equivalent to variant, but with a purpose-built interface.
Here is the definition of the std::optional class template.
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template< class T > class optional; |
Optional can be used to define any type of variables as below.
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std::optional<int> a(5); std::optional<int> b; |
An optional variable can be checked by has_value() method if it has a value or not.
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if (a.has_value()) { } |
Here is another example that has a function with an optional return.
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std::optional<std::string> testopt(std::string s) { if(s.length()==0) return {}; else return s; } |
as you see our function may return a string as an option or it may have no return value.
A common use case for optional is the return value of a function that may fail. Any instance of optional<T> at any given point in time either contains a value or does not contain a value. If an optional<T>contains a value, the value is guaranteed to be allocated as part of the optional object footprint, i.e. no dynamic memory allocation ever takes place. Thus, an optional object models an object, not a pointer, even though operator*() and operator->() are defined.
Is there a simple example about the optional class template in C++ 17?
Here is a simple example about the std::optional,
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#include <iostream> #include <optional> int main() { std::optional<int> a(5); // a = 5 std::optional<int> b; // b has no value if (a.has_value()) { int z = a.value() + b.value_or(0); std::cout << z; } system("pause"); return 0; } |
Is there a full example about the optional class template in C++ 17?
Here is a full example that we can use optional return for functions. Let’s create a function that may return a string as an option or no return. We can obtain optional value by .value or by .value_or() method if no value. The C++ Builder application example below explains all.
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#include <iostream> #include <optional> #include <string> std::optional<std::string> testopt(std::string s) { if(s.length()==0) return {}; else return s; } int main() { std::string str; str = testopt("").value_or("- no return -"); std::cout << str << std::endl; str = testopt("testing optional").value(); std::cout << str << std::endl; system("pause"); return 0; } |
and here is the output:
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- no return - testing optional |
The std::optional class template has swap(), reset(), emplace() modifiers as well as ->, *, bool, has_value(), value(), value_or() observers. It has non-member functions to compare optional objects such as =, ==, !=, <, >, <=, >=, operators in C++17 and <=> operator in C++20 . In C++23, It has and_then(), or_else(), transform() monadic operations. The std::optional has helpers and helper classes too.
For more details about this feature in C++17 standard, please see these papers; P0220R1, P0307R2, P0032R3, P0504R0
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