C++ flat_map::at() Function

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The std::flat_map::at() function in C++, is used to access the value associated with a specific key in the flat_map. Unlike the operator[], which inserts a default value if the key is missing, at() ensures safe access without modifications.

This function is used when strict key existence is required. It throws an out_of_range exception if the key is not found.

Syntax

Following is the syntax for std::flat_map::at() function.

T& at( const Key& key );
or
const T& at( const Key& key ) const;

Parameters

• key − It indicates the key of the element to find.
• x − It indicates a value of any type that can be transparently compared with the key.

Return Value

This function returns a reference to the mapped value of the requested element.

Example 1

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

#include <boost/container/flat_map.hpp>
#include <iostream>
int main() {
   boost::container::flat_map < int, std::string > x = {
      {1,"HI"},
      {2,"Hello"},
      {3,"Welcome"}
   };
   std::cout << "Result : " << x.at(2) << std::endl;
   return 0;
}

Output

Output of the above code is as follows −

Result : Hello

Example 2

Consider the following example, where we are going to modify the value using the at() function.

#include <iostream>
#include <boost/container/flat_map.hpp>
int main() {
   boost::container::flat_map < int, std::string > x = {
      {1,"RX100"},
      {2,"BMW"}
   };
   x.at(1) = "AUDI";
   std::cout << "Result : " << x.at(1) << std::endl;
   return 0;
}

Output

Following is the output of the above code −

Result : AUDI

Example 3

In the following example, we are going to use the at() function to access the key that does not exists and observing the output.

#include <iostream>
#include <boost/container/flat_map.hpp>
#include <stdexcept>
int main() {
   boost::container::flat_map < int, std::string > a = {
      {1,"CRUZE"},
      {2,"CIAZ"}
   };
   try {
      std::cout << "Result : " << a.at(4) << std::endl;
   } catch (const std::out_of_range & e) {
      std::cout << "Exception: " << e.what() << std::endl;
   }
   return 0;
}

Output

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

terminate called after throwing an instance of 'boost::container::out_of_range'
  what():  flat_map::at key not found