Example of C++ Bit Fields
The following examples demonstrate the use of bit fields in both structures and classes.
Example 1: Bit Fields in Structures
In the below example, we will compare the size difference between the structure that does not specify bit fields and the structure that has specified bit fields.
C++
// C++ program to demonstrate the size occupied by a struct// with and without bitfields#include <iostream>using namespace std;// Define a struct without bit fieldsstruct Loan1 { unsigned int principal; unsigned int interestRate; unsigned int period;};// Define a struct with bit-fieldsstruct Loan2 { // principal variable can store maximum value of // 1,048,575 unsigned int principal : 20; // Maximum interest rate of 63 unsigned int interestRate : 6; // Maximum period of 63 months unsigned int period : 6;};int main(){ // printing the size of both structures cout << "Size of Structure without Bit Fields: "; cout << sizeof(Loan1) << " Bytes" << endl; cout << "Size of Structure with Bit Fields: "; cout << sizeof(Loan2) << " Bytes" << endl; return 0;} |
Size of Structure without Bit Fields: 12 Bytes Size of Structure with Bit Fields: 4 Bytes
Explanation
- For the structure ‘Loan’, the size is 12 bytes which accounts for 4 bytes for each of the three integer members.
- In structure ‘Loan2″, we have used bit fields to specify the memory allocated to the member variables of a struct in bits.
- We have specified 20 bits for ‘principal’, 6 bits for ‘interest rate’, and 6 bits for ‘period’.
- They all contributed to 32 bits which is equal to 4 Bytes which is why the size of ‘Loan2″ is 4 bytes.
Example 2: Class with Bit Fields in C++
In the below example, we have used bit fields with classes instead of struct.
C++
// C++ program to demonstrate the size occupied by a class// by specifying the bits to member variables of the class// using bit fields#include <iostream>using namespace std;// Define a class with bit-fields for loan informationclass Loan {public: // Maximum principal of 1,048,575 unsigned int principal : 20; // Maximum interest rate of 63 unsigned int interestRate : 6; // Maximum period of 63 months unsigned int period : 6;};int main(){ Loan loan1; loan1.principal = 500000; loan1.interestRate = 15; loan1.period = 36; // Print the size of loan1 // (20+6+6)/8 = 4 Bytes // 1 Byte = 8 Bits cout << sizeof(Loan) << " Bytes" << endl; return 0;} |
4 Bytes
Explanation
In the above example, we have used bit fields to specify the memory allocated to the member variables of a class in bits. We have specified 20 bits for ‘principal’, 6 bits for ‘interest rate’, and 6 bits for ‘period’. They all contributed to 32 bits which is equal to 4 Bytes.
C++ Bit Fields
When we declare the members in a struct and classes, a pre-determined size is allocated for the member variables. In C++, we can compress that allocated size using bit fields.
Bit Field is a feature in C++ for specifying the size of struct and class members so that they occupy a specific number of bits, not the default memory size.
For example, an integer takes 4 bytes of size but this size is large according to our requirements so we can change it to 2 bytes using bit fields.