__builtin_popcount (GNU内建函数)

内部使用查表法实现
https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html

int __builtin_popcount(unsigned int)
int __builtin_popcountl(unsigned long int)
int __builtin_popcountll(unsigned long long int)

#include <cstdio>
int main(){
    int n, x;
    scanf("%d", &n);
    while(n--){
        scanf("%d", &x);
        printf("%d ", __builtin_popcount(x));
    }
    return 0;
}

std::popcount (C++20)

https://en.cppreference.com/w/cpp/numeric/popcount

template<class T>
constexpr int popcount(T x) noexcept;

#include <bit>
#include <cstdio>
int main(){
    int n, x;
    scanf("%d", &n);
    while(n--){
        scanf("%d", &x);
        printf("%d ", std::popcount((unsigned)x));
    }
    return 0;
}