std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::max_bucket_count

From cppreference.com

 
C++
 
Containers library
 
std::unordered_map
Member types
Member functions
Iterators
Capacity
Modifiers
(C++23)
(C++17)
(C++17)
(C++17)
(C++17)
Lookup
(C++20)
Bucket interface
unordered_map::max_bucket_count
Hash policy
Observers
Non-member functions
(until C++20)
(C++20)
Deduction guides(C++17)
 
size_type max_bucket_count() const;
 (since C++11)

Returns the maximum number of buckets the container is able to hold due to system or library implementation limitations.

Parameters

(none)

Return value

Maximum number of buckets.

Complexity

Constant.

Example

Run this code
#include <iostream>
#include <unordered_map>
 
int main()
{
    struct Ha { std::size_t operator()(long x) const { return std::hash<long>{}(x); }; };
 
    auto c1 = std::unordered_map<char, long>{};
    auto c2 = std::unordered_map<long, long>{};
    auto c3 = std::unordered_map<long, long, std::hash<int>>{};
    auto c4 = std::unordered_map<long, long, Ha>{};
 
    std::cout
        << "Max bucket count of\n" << std::hex << std::showbase
        << "c1: " << c1.max_bucket_count() << '\n'
        << "c2: " << c2.max_bucket_count() << '\n'
        << "c3: " << c3.max_bucket_count() << '\n'
        << "c4: " << c4.max_bucket_count() << '\n'
        ;
}

Possible output: 

Max bucket count of
c1: 0xfffffffffffffff
c2: 0xfffffffffffffff
c3: 0xfffffffffffffff
c4: 0xaaaaaaaaaaaaaaa

See also

(C++11)
 returns the number of buckets
(public member function)
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