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TheC++ std::unordered_set::max_size() function is used to return the maximum number of elements that the unordered_set container can hold due to system and library implementation limitations.

The maximum size will always be the same in the particular program; it does not depend upon the unordered_set size or type, whether the unordered_set is empty or not.

Syntax

Following is the syntax of std::unordered_set::max_size() function.

size_type max_size() const noexcept;

Parameters

This function does not accepts any parameter.

Return Value

This function returns the maximum number of elements that the unordered_set container can hold.

Example 1

Consider the following example, where we are going to demonstrate the usage of unordered_set::max_size() function.

#include <iostream>
#include <string>
#include <unordered_set>

int main () {
   std::unordered_set<std::string> myset;
   std::cout << "0. size: " << myset.max_size() << std::endl;

   myset = {"milk","potatoes","eggs"};
   std::cout << "1. size: " << myset.max_size() << std::endl;

   myset.insert ("pineapple");
   std::cout << "2. size: " << myset.max_size() << std::endl;

   myset.erase ("milk");
   std::cout << "3. size: " << myset.max_size() << std::endl;

   return 0;
}

Output

Let us compile and run the above program, this will produce the following result −

0. size: 768614336404564650
1. size: 768614336404564650
2. size: 768614336404564650
3. size: 768614336404564650

Example 2

Let's look at the following example, where we are going to use the unordered_set::max_size() function to get the maximum number of sizes that a container can hold.

#include <iostream>
#include <iostream>
#include <locale>
#include <unordered_set>
using namespace std;

int main() {
   unordered_set<int> uSet = {1, 2, 3};
   // Check if the locale is available on the system
   try {
      cout.imbue(locale("en_US.UTF-8"));
   } catch (const std::runtime_error& e) {
      cout << "Locale not found: " << e.what() << '\n';
      // Fallback to the default locale
      cout.imbue(locale(""));
   }
   cout << "Maximum size of an unordered_set is " << uSet.max_size() << '\n';
   return 0;
}

Output

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

Locale not found: locale::facet::_S_create_c_locale name not valid
Maximum size of an unordered_set is 576460752303423487

Example 3

In the following example, we are going to consider the unordered_sets that contain integer type and applying the unordered_set::max_size() function to display the maximum size of both unordered_sets.

#include <iostream>
#include <locale>
#include <unordered_set>
using namespace std;

int main() {
   unordered_set<int> uSet, myUset;
   uSet={1, 2, 3};
   myUset={2, 3, 4, 5};
   cout << "Maximum size of a unordered_set first is " << uSet.max_size() << '\n';
   cout << "Maximum size of a unordered_set second is " << myUset.max_size() << '\n';
}

Output

Following is the output of the above code −

Maximum size of a unordered_set first is 576460752303423487
Maximum size of a unordered_set second is 576460752303423487

Example 4

Following is another example of the usage of the unordered_set::max_size() function, where we are going to conider two unordered_sets, one is of int type and the other is char type, and display the maximum size of both unordered_sets.

#include <iostream>
#include <locale>
#include <unordered_set>
using namespace std;

int main() {
   unordered_set<int> uSet = {1, 2, 3};
   unordered_set<char> myUset = {'a', 'b', 'c'};
   cout << "Maximum size of a unordered_set first is " << uSet.max_size() << '\n';
   cout << "Maximum size of a unordered_set second is " << myUset.max_size() << '\n';
}

Output

Output of the above code is as follows −

Maximum size of a unordered_set first is 576460752303423487
Maximum size of a unordered_set second is 576460752303423487