小白虚心请教大佬

#include <iostream>
#include <random>
#include <array>
#include <tuple>

class foo
{
public:
    foo() noexcept
    {
        _rand_gen.seed( std::random_device()() );
    }
    std::array<unsigned,4> generate_4digits_unique() const noexcept
    {
        unsigned buf[] = { 0,1,2,3,4,5,6,7,8,9 };
        std::uniform_int_distribution<> gen;
        using param_type = std::uniform_int_distribution<>::param_type;
        std::swap( buf[0], buf[gen(_rand_gen,param_type{1,9})] );
        std::swap( buf[1], buf[gen(_rand_gen,param_type{1,9})] );
        std::swap( buf[2], buf[gen(_rand_gen,param_type{2,9})] );
        std::swap( buf[3], buf[gen(_rand_gen,param_type{3,9})] );
        return { buf[0], buf[1], buf[2], buf[3] };
    }
    std::array<unsigned,4> input_4digits_unique() const noexcept
    {
        auto is_unique = [](unsigned n) {
            bool buf[10] = {};
            for( ; n!=0; n/=10 ) {
                if( buf[n%10] )
                    return false;
                buf[n%10] = true;
            }
            return true;
        };

        unsigned n;
        std::cout << "please input a 4 digits number:" << std::endl;
        for( ; !(std::cin>>n) || n<1000 || n>9999 || !is_unique(n); )
        {
            std::cin.clear();
            std::cin.ignore( std::numeric_limits<std::streamsize>::max(), '\n' );
            std::cout << "your input is wrong!! please input again :" << std::endl;
        }
        return { n/1000, n/100%10, n/10%10, n%10 };
    }
    std::tuple<unsigned,unsigned> compare( const std::array<unsigned,4>& a, const std::array<unsigned,4>& b ) const noexcept
    {
        bool buf[10] = {};
        unsigned x=0, y=0;
        for( size_t i=0; i!=size(a); ++i )
        {
            buf[a[i]] = true;
            x += a[i]==b[i];
        }
        for( size_t i=0; i!=size(b); ++i )
            y += buf[b[i]];
        return { x, y-x };
    }
    bool game( size_t count=8 ) const noexcept
    {
        std::array<unsigned,4> key = generate_4digits_unique();
        std::cout << key[0] << key[1] << key[2] << key[3] << std::endl;

        for( size_t i=count; i!=0; --i )
        {
            std::array<unsigned,4> guess = input_4digits_unique();
            std::tuple<unsigned,unsigned> result = compare( key, guess );
            if(  std::get<0>(result) == 4 )
            {
                std::cout << "Bingo" << std::endl;
                return true;
            }
            std::cout << std::get<0>(result) << 'A' << std::get<1>(result) << 'B' << std::endl;
            std::cout << "you have " << (i-1) << " chance" << std::endl;
        }
        std::cout << "超过规定次数啦，重新来过吧" << std::endl;
        return false;
    }

private:
    mutable std::mt19937 _rand_gen;
};

int main( void )
{
    using namespace std;
    for( foo a; ; )
    {
        a.game(8);
        cout << "是否继续(Y/N)? ";
        char ch;
        if( !(cin>>ch) || (ch!='Y' && ch!='y') )
            break;
    }
}