Would anyone know how to extract the size of a bit-field member. The below code naturally gives me the size of an integer, but how do I find out how many bits or bytes are in mybits.one
? I've tried sizeof(test.one)
but which clearly won't work. I realize this is a measure of bits:
#include <iostream>
using namespace std;
int main()
{
struct mybits {
unsigned int one:15;
};
mybits test;
test.one = 455;
cout << test.one << endl;
cout << "The size of test.one is: " << sizeof(test) << endl;
}
Runtime solution: set the value of the whole structure to zero, then inverse the field using “~”, and then count the number of bits set to “1”.
Implementation. Bit fields can be used to reduce memory consumption when a program requires a number of integer variables which always will have low values. For example, in many systems storing an integer value requires two bytes (16-bits) of memory; sometimes the values to be stored actually need only one or two bits.
Pointers and non-const references to bit-fields are not possible.
Again, storage of bit fields in memory is done with a byte-by-byte, rather than bit-by-bit, transfer.
The draft C++ standard says sizeof shall not be applied to a bit-field in section 5.3.3
Sizeof paragraph 1. If you have control of the source then using an enum sounds much simpler and neater:
struct mybits
{
enum bitFieldSizes
{
field1 = 15,
field2 = 2,
field3 = 4,
field4 = 8,
field5 = 31
};
unsigned int one : field1 ;
unsigned int two : field2 ;
unsigned int three : field3 ;
unsigned int four : field4 ;
unsigned int five : field5 ;
};
If you don't have control of the source it is possible to use bit hacks to obtain the size of your bit-field and std::bitset makes it easier:
#include <iostream>
#include <bitset>
struct mybits
{
unsigned int one : 15 ;
unsigned int two : 2 ;
unsigned int three : 4 ;
unsigned int four : 8 ;
unsigned int five : 31 ;
};
int main()
{
mybits mb1 ;
mb1.one = ~0 ;
mb1.two = ~0 ;
mb1.three = ~0 ;
mb1.four = ~0 ;
mb1.five = ~0 ;
std::bitset<sizeof(unsigned int)*8> b1(mb1.one);
std::bitset<sizeof(unsigned int)*8> b2(mb1.two);
std::bitset<sizeof(unsigned int)*8> b3(mb1.three);
std::bitset<sizeof(unsigned int)*8> b4(mb1.four);
std::bitset<sizeof(unsigned int)*8> b5(mb1.five);
std::cout << b1 << ":" << b1.count() << std::endl ;
std::cout << b2 << ":" << b2.count() << std::endl ;
std::cout << b3 << ":" << b3.count() << std::endl ;
std::cout << b4 << ":" << b4.count() << std::endl ;
std::cout << b5 << ":" << b5.count() << std::endl ;
}
which produces the following output:
00000000000000000111111111111111:15
00000000000000000000000000000011:2
00000000000000000000000000001111:4
00000000000000000000000011111111:8
01111111111111111111111111111111:31
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