In c / c++, we can define a variable with 1 bit in memory: like unsigned char value : 1;
Is there a way to declare an array of 1 bit elements? like in sudo code below:
unsigned char : 1 data[10];
-
5en.cppreference.com/w/cpp/utility/bitsettkausl– tkausl2018-11-27 07:30:55 +00:00Commented Nov 27, 2018 at 7:30
-
Some related information here: stackoverflow.com/questions/8304723/…Damien– Damien2018-11-27 10:30:26 +00:00Commented Nov 27, 2018 at 10:30
-
people.eecs.berkeley.edu/~rcs/research/interactive_latency.htmlXiong Zou– Xiong Zou2018-11-29 03:13:57 +00:00Commented Nov 29, 2018 at 3:13
Add a comment
|
1 Answer
The problem is that in most implementations, the 1 bit variable will still occupy 1 byte of memory because that's the way the memory is addressed. If you have a big array of such values, however, then you can work around that. One such solution is std::bitset. You can make one like this:
#include <bitset>
std::bitset<64> data;
You can manipulate bits using the set, reset and flip operations (setting it to 1, 0 or toggling it). You can access a bit using [], for instance
if (data[5]) { ...
Check out a nice example here.
7 Comments
Xiong Zou
Thanks for your solution, however we cannot control the exact memory layout of std::bitset, as what we can with array. for example sizeof(std::bitset<8>) or std::sizeof(std::bitset<64>) always return 8 bytes. it limits its usage especially when we would like to use union to get bits of an integer.
Xiong Zou
And it is not efficient as array. std::bitset<> would use bit operations << or >> and & to get bit value at an index. If we have no other choices, we might write our own bitset to control the memory layout. I am wondering whether we can define array with elements with size of 1 bit each that we donot need extra bit operations.
Blaze
You could pack 8 bit fields (
unsigned int a: 1;) into one struct and if you're lucky, the compiler will optimize it properly. You can then use those structs to make your own bitset. I wouldn't recommend it, however.Xiong Zou
Yes, you are right, @Blaze. however if we do that, then we cannot access them by operator[] directly. We now need to do if else check to choose the correct bits. Branching is more costly than bit operations. branch mis-predict is about 3ns each referring to this url: people.eecs.berkeley.edu/~rcs/research/interactive_latency.html
Xiong Zou
It seems array of 1 bit size element cannot be created in c / c++ for now. I will accept your answer. Thanks
|