std::unordered_map max_bucket_count() method

since C++11

// Const version only
size_type max_bucket_count() const;

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 - O(1).

Exceptions

(none)

Example

#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