C++ optional::value() Function

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The std::optional::value() function in C++, is used to retrieve the value stored in the optional object. It is used when working with optional types, allowing the safe handling of the absent values while providing a mechanism to detect and handle such cases.

When we try to invoke this function on the empty optional object, it will throws a std::bad_optional_access exception.

Syntax

Following is the syntax for std::optional::value() function.

T& value() &;
or
const T& value() const &;

Parameters

It does not accepts any parameter.

Return value

This function returns a reference to the contained value.

Example 1

Let's look at the following example, where we are going to consider the basic usage of the value() function.

#include <iostream>
#include <optional>
int main() {
   std::optional < int > a = 1121;
   if (a) {
      std::cout << "Result : " << a.value() << std::endl;
   }
   return 0;
}

Output

Output of the above code is as follows −

Result : 1121

Example 2

Consider the following example, where we are going to access the empty optional, which throws a bad_optional_access exception.

#include <iostream>
#include <optional>
#include <stdexcept>
int main() {
   std::optional < int > a;
   try {
      int x = a.value();
      std::cout << "Result : " << x << std::endl;
   } catch (const std::bad_optional_access & e) {
      std::cout << "Exception : " << e.what() << std::endl;
   }
   return 0;
}

Output

Output of the above code is as follows −

Exception : bad optional access

Example 3

In the following example, we are going to initializes an optional with a string, modifies it and retrieve the updated value using the value() function.

#include <iostream>
#include <optional>
int main() {
   std::optional < std::string > x = "TP";
   if (x.has_value()) {
      std::cout << "Original Value: " << x.value() << std::endl;
      x = "TutorialsPoint";
      std::cout << "Modified Value: " << x.value() << std::endl;
   }
   return 0;
}

Output

If we run the above code it will generate the following output −

Original Value: TP
Modified Value: TutorialsPoint