C++ Unordered_map::rehash() Function

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The C++ std::unordered_map::rehash() function is used to sets the number of buckets in the container to n or more. A rehash is the reconstruction of the hash table; all its elements are rearranged into the new set of buckets according to their new hash value.

If n is greater than the current number of buckets in the container, a rehash is occurred, similarly if n is less than the current number of buckets in the container, the function may have no effect on the bucket count and may not force a rehash.

Syntax

Following is the syntax of std::unordered_map::rehash() function.

 void rehash(size_type n); 

Parameters

• n − It inidicates the new number of buckets.

Return value

This function does not returns anything.

Example 1

In the following example, let's see the usage of rehash() function.

 #include <iostream> #include <unordered_map> using namespace std; int main (void) { unordered_map<char, int> uMap; cout << "Initial bucket_count: " << uMap.bucket_count() << endl; uMap.rehash(20); cout << "Current bucket_count: " << uMap.bucket_count() << endl; return 0; } 

Output

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

 Initial bucket_count: 1 Current bucket_count: 23 

Example 2

Consider the following example, where we are going to count the size and bucket count before and after the use of rehash() function.

 #include <iostream> #include <unordered_map> using namespace std; int main (void) { unordered_map<int, int> uMap={{3, 30}, {2, 20}, {1, 10}}; cout << "Size of container before use of rehash: "<<uMap.size() << endl; cout << "Initial bucket_count: " << uMap.bucket_count() << endl; uMap.rehash(22); cout << "Size of container after use of rehash: "<<uMap.size() << endl; cout << "Current bucket_count: " << uMap.bucket_count() << endl; return 0; } 

Output

Following is the output of the above code −

 Size of container before use of rehash: 3 Initial bucket_count: 13 Size of container after use of rehash: 3 Current bucket_count: 23 

Example 3

Let's look at the following example, where we are going to store the keys/values pair of integral types and using the rehash() function to set the number of buckets in the container.

 #include <iostream> #include <unordered_map> using namespace std; int main() { unordered_map<int, int> uMap; uMap = { { 2, 2 }, { 3, 4 }, { 4, 6 }, { 5, 8 } }; cout << "Size of container : " << uMap.size() << endl; cout << "Initial bucket count : " << uMap.bucket_count() << endl; uMap.rehash(15); cout << "Size of container : " << uMap.size() << endl; cout << "current bucket count is : " << uMap.bucket_count() << endl; for(unsigned int i = 0; i < uMap.bucket_count(); i++){ cout<<"The bucket #"<<i <<endl; } return 0; } 

Output

Output of the above code is as follows −

 Size of container : 4 Initial bucket count : 5 Size of container : 4 current bucket count is : 17 The bucket #0 The bucket #1 The bucket #2 The bucket #3 The bucket #4 The bucket #5 The bucket #6 The bucket #7 The bucket #8 The bucket #9 The bucket #10 The bucket #11 The bucket #12 The bucket #13 The bucket #14 The bucket #15 The bucket #16