why bits sets are decreased after setting flag manipulator?

1

Code

#include<iostream>
#include<bitset>
int main()
{
    
    std::ios_base::fmtflags flag=std::ios::showpos;
    
    std::cout<<std::cout.flags()<<"\n";
    std::cout<<flag<<"\n";

    std::cout.flags(flag);
    std::cout<<std::cout.flags()<<"\n";
    
    std::cout<<59.20<<"\n";

    std::cout<<std::bitset<16>(4098)<<"\n";
    std::cout<<std::bitset<16>(2048)<<"\n";
}

Output

4098
2048
+2048
+59.2
0001000000000010
0000100000000000

see here I set showpos flag manipulator and then I value returned by cout.flags() (current format setting) decreased bits set.

2

Code

#include<iostream>
#include<bitset>
int main()
{
    
    std::cout<<std::cout.flags()<<"\n";
    
    std::cout<<std::showpos;
    std::cout<<std::cout.flags()<<"\n";
    
    std::cout<<59.20<<"\n";

    std::cout<<std::bitset<16>(4098)<<"\n";
    std::cout<<std::bitset<16>(6146)<<"\n";
}

Output

4098
+6146
+59.2
0001000000000010
0001100000000010

here as expected bits set value increased by 1 as I added one flag manipulator.

Why when I set flag with help of cout.flags() function bit sets decrease rather than increase ?

The flags function takes the new value, not a flag to add. What you're doing is replacing the flags entirely with showpos.

If you want to do it with flags, you need to add the flag yourself:

std::cout.flags(std::cout.flags() | std::ios::showpos);