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I'm working on some simple bit manipulation problems in C++, and came across this while trying to visualize my steps. I understand that the number of bits assigned to different primitive types may vary from system to system. For my machine, sizeof(int) outputs 4, so I've got 4 char worth of bits for my value. I also know now that the definition of a byte is usually 8 bits, but is not necessarily the case. When I output CHAR_BIT I get 8. I therefore expect there to be a total of 32 bits for my int values.
I can then go ahead and print the binary value of my int to the screen:
int max=~0; //All my bits are turned on now
std::cout<<std::bitset<sizeof(int)*CHAR_BIT>(max)<<std::endl;
$:11111111111111111111111111111111
I can increase the bitset size if I want though:
int max=~0;
std::cout<<std::bitset<sizeof(int)*CHAR_BIT*3>(max)<<std::endl;
$:000000000000000000000000000000001111111111111111111111111111111111111111111111111111111111111111
Why are there so many ones? I would have expected to have only 32 ones, padded with zeros. Instead there's twice as many, what's going on?
When I repeat the experiment with unsigned int, which has the same size as int, the extra ones don't appear:
unsigned int unmax=~0;
std::cout<<std::bitset<sizeof(unsigned int)*CHAR_BIT*3>(unmax)<<std::endl;
$:000000000000000000000000000000000000000000000000000000000000000011111111111111111111111111111111
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C++ bitset and its application Difficulty Level : Easy Last Updated : 04 Sep, 2018 A bitset is an array of bool but each Boolean value is not stored separately instead bitset optimizes the space such that each bool takes 1 bit space only, so space taken by bitset bs is less than that of bool bs [N] and vector bs (N).
However, a limitation of bitset is, N must be known at compile time, i.e., a constant (this limitation is not there with vector and dynamic array) As bitset stores the same information in compressed manner the operation on bitset are faster than that of array and vector.
A bitset is an array of bool but each Boolean value is not stored separately instead bitset optimizes the space such that each bool takes 1 bit space only, so space taken by bitset bs is less than that of bool bs[N] and vector bs(N).
We can access each bit of bitset individually with help of array indexing operator [] that is bs [3] shows bit at index 3 of bitset bs just like a simple array. Remember bitset starts its indexing backward that is for 10110, 0 are at 0th and 3rd indices whereas 1 are at 1st 2nd and 4th indices.
The constructor of std::bitset takes an unsigned long long, and when you try to assign a -1 (which is what ~0 is in an int) to an unsigned long long, you get 8 bytes (64 bits) worth of 1s.
It doesn't happen with unsigned int because you are assigning the value of 4294967295 instead of -1, which is 32 1s in a unsigned long long
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