convert bitset to int in c++

Question

In c++. I initialize a bitset to -3 like:

std::bitset<32> mybit(-3);

Is there a grace way that convert mybit to -3. Beacause bitset object only have methods like to_ulong and to_string.

As [the documentation says](http://en.cppreference.com/w/cpp/utility/bitset), `std::bitset` has function to convert the value to a ulong. So as @Barmar says, cast that long to a int. So whats your problem? Have you readed the documentation or tried anything before posting the question? — Manu343726, Oct 25 '13 at 07:38
@Johnsyweb He probably wants to handle negative values, as his example shows. — Kaidjin, Oct 25 '13 at 07:51
Convert that `ulong` to `long`, then `int` i.e. `int(long(mybit.to_ulong()))` — Erkin Alp Güney, Dec 09 '16 at 19:04

Barmar · Accepted Answer · 2016-08-05T13:05:29.190

63

Use to_ulong to convert it to unsigned long, then an ordinary cast to convert it to int.

int mybit_int;

mybit_int = (int)(mybit.to_ulong());

DEMO

edited Aug 05 '16 at 13:05

answered Oct 25 '13 at 07:43

Barmar

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As this is C++ I would suggest using [static_cast(mybit.to_ulong())](http://stackoverflow.com/questions/103512/in-c-why-use-static-castintx-instead-of-intx) rather than a C-style cast. – Steve Oct 25 '13 at 08:01
2

not working, still spitting out unsigned integer. -3 is becoming 251 – FluorescentGreen5 Aug 05 '16 at 09:11
@FluorescentGreen5 I've added a demo showing that it works. – Barmar Aug 05 '16 at 13:08
@Barmar oh, i used <8> instead of <32> cutting of the signature – FluorescentGreen5 Aug 05 '16 at 13:51
@Steve: Primitive types have no difference in static(C-style cast is also static) and dynamic casts. Only `reinterpret_cast` is different for them. – Erkin Alp Güney Dec 09 '16 at 19:07
1

@ErkinAlpGüney I think his suggestion is about style -- in C++ you should use the template syntax rather than the C syntax for casts, because it makes your intent clearer. But I'm not that much of a purist, and I find the verbose syntax off-putting. – Barmar Dec 09 '16 at 19:40
2

@ErkinAlpGüney - my suggestion is about safety of the operation. If you use a `static_cast` the code is more resilient to someone coming in and changing types. http://stackoverflow.com/a/103868/1517648 I've seen various occasions where C-style casts were ok, until someone changed something and then they weren't. A `static_cast` would pick this up with a compiler error. – Steve Jan 12 '17 at 21:19

score 0 · Answer 2 · answered Jul 17 '23 at 06:13

One comple example worked for both 32bit and not-32bit bitsets

#include <bitset>
#include <iostream>

template<std::size_t SIZE>
int bitSetToInt(std::bitset<SIZE> bitSet) {
    if (!bitSet[SIZE - 1]) return bitSet.to_ulong();
    bitSet.flip();
    return -(bitSet.to_ulong() + 1);
}

int main() {
    std::bitset<31> bitSet1(7);
    std::bitset<31> bitSet2(-7);
    std::bitset<32> bitSet3(7);
    std::bitset<32> bitSet4(-7);
    std::cout << bitSet1 << " = " << bitSetToInt(bitSet1) << std::endl;
    std::cout << bitSet2 << " = " << bitSetToInt(bitSet2) << std::endl;
    std::cout << bitSet3 << " = " << bitSetToInt(bitSet3) << std::endl;
    std::cout << bitSet4 << " = " << bitSetToInt(bitSet4) << std::endl;
}

Output

0000000000000000000000000000111 = 7
1111111111111111111111111111001 = -7
00000000000000000000000000000111 = 7
11111111111111111111111111111001 = -7

The dynamic version

#include <iostream>
#include <boost/dynamic_bitset.hpp>

int dynamicBitSetToInt(boost::dynamic_bitset<> bitSet) {
    if (!bitSet[bitSet.size() - 1]) return (int) bitSet.to_ulong();
    bitSet.flip();
    return (int) -(bitSet.to_ulong() + 1);
}

int main() {
    boost::dynamic_bitset bitSet1(31, 7);
    boost::dynamic_bitset bitSet2(31, -7);
    boost::dynamic_bitset bitSet3(32, 7);
    boost::dynamic_bitset bitSet4(32, -7);
    std::cout << bitSet1 << " = " << dynamicBitSetToInt(bitSet1) << std::endl;
    std::cout << bitSet2 << " = " << dynamicBitSetToInt(bitSet2) << std::endl;
    std::cout << bitSet3 << " = " << dynamicBitSetToInt(bitSet3) << std::endl;
    std::cout << bitSet4 << " = " << dynamicBitSetToInt(bitSet4) << std::endl;
}

convert bitset to int in c++

2 Answers2

One comple example worked for both 32bit and not-32bit bitsets

Output

The dynamic version

Linked