CODEWITHSUNDEEP
c++binarybitset
Shravan
Shravan

Reputation: 2919

Why is the std::bitset<8> variable unable to handle 11111111?

Why is this program showing the following output ?

#include <bitset>
...

{
    std::bitset<8> b1(01100100); std::cout<<b1<<std::endl;
    std::bitset<8> b2(11111111); std::cout<<b2<<std::endl; //see, this variable
                                                           //has been assigned
                                                           //the value 11111111
                                                           //whereas, during
                                                           //execution, it takes
                                                           //the value 11000111
    std::cout << "b1 & b2: " << (b1 & b2) << '\n';
    std::cout << "b1 | b2: " << (b1 | b2) << '\n';
    std::cout << "b1 ^ b2: " << (b1 ^ b2) << '\n';
}

This is the OUTPUT:

01000000
11000111
b1 & b2: 01000000
b1 | b2: 11000111
b1 ^ b2: 10000111

First, I thought there is something wrong with the header file (I was using MinGW) so I checked using MSVCC. But it too showed the same thing. Please help.

Upvotes: 14

Views: 1257

Answers (2)

NPE
NPE

Reputation: 500307

Despite the appearance, the 11111111 is decimal. The binary representation of 1111111110 is 1010100110001010110001112. Upon construction, std::bitset<8> takes the eight least significant bits of that: 110001112.

The first case is similar except the 01100100 is octal (due to the leading zero). The same number expressed in binary is 10010000000010000002.

One way to represent a bitset with a value of 111111112 is std::bitset<8> b1(0xff).

Alternatively, you can construct a bitset from a binary string:

std::bitset<8> b1(std::string("01100100"));
std::bitset<8> b2(std::string("11111111"));

Upvotes: 48

JBentley
JBentley

Reputation: 6260

As per NPE's answer, you are constructing the bitset with an unsigned long, and not with bits as you were expecting. An alternative way to construct it, which enables you to specify the bits, is by using the string constructor as follows:

#include <bitset>
#include <cstdio>
#include <iostream>

int main()
{
    std::bitset<8> b1(std::string("01100100")); std::cout<<b1<<std::endl;
    std::bitset<8> b2(std::string("11111111")); std::cout<<b2<<std::endl;
    std::cout << "b1 & b2: " << (b1 & b2) << '\n';
    std::cout << "b1 | b2: " << (b1 | b2) << '\n';
    std::cout << "b1 ^ b2: " << (b1 ^ b2) << '\n';
getchar();
return 0;
}

Click here to view the output.

Upvotes: 7

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