c++11bit-manipulationbitwise-operatorsuint16

How to store into and read back multiple number values on an `uint16_t` using bitwise manipulation?


Using bitwise operations, is it possible to package and read back the following set of values into an uint16_t variable? I think yes but I am trying to figure out how using a sample program.

Lets say following is the set of values I want to package into an uint16_t.

unsigned int iVal1 = 165 // which is 8 bits
unsigned int iVal2 = 28  // which is 5 bits
unsigned int iVal3 = 3   // which is 2 bits
bool bVal = true;        // which can stored in 1 bit if we use 0 for true and 1 for false

Following is my program which aims to write in the values and needs to read back. How can write and read the values back using C++ 11?

#include <iostream>

uint16_t Write(unsigned int iVal1, unsigned int iVal2, unsigned int iVal3, bool bVal) {
    // Is this technique correct to package the 3 values into an uint16_t?
    return static_cast<uint16_t>(iVal1) + static_cast<uint16_t>(iVal2) + static_cast<uint16_t>(iVal3) + static_cast<uint16_t>(bVal);
}

unsigned int ReadVal1(const uint16_t theNumber) {
    // How to read back iVal1
}

unsigned int ReadVal2(const uint16_t theNumber) {
    // How to read back iVal2
}

unsigned int ReadVal3(const uint16_t theNumber) {
    // How to read back iVal3
}

bool ReadVal4(const uint16_t theNumber) {
    // How to read back bVal
}

int main() {
    unsigned int iVal1 = 165; // which is 8 bits
    unsigned int iVal2 = 28;  // which is 5 bits
    unsigned int iVal3 = 3;   // which is 2 bits
    bool bVal = true;        // which can stored in 1 bit if we use 0 for true and 1 for false

    const uint16_t theNumber = Write(iVal1, iVal2, iVal3, bVal);

    std::cout << "The first 8 bits contain the number: " << ReadVal1(theNumber) << std::endl;
    std::cout << "Then after 8 bits contain the number: " << ReadVal2(theNumber) << std::endl;
    std::cout << "Then after 2 bits contain the number: " << ReadVal3(theNumber) << std::endl;
    std::cout << "Then after 1 bit contains the number: " << ReadVal4(theNumber) << std::endl;
}

Solution

  • For this, you need to play on bit shifts.

    uint16_t Write(unsigned int iVal1, unsigned int iVal2, unsigned int iVal3, bool bVal) {
        // this will encode ival1 on the 8 first bits, ival2 on bits 4 to 8, 
        // ival3 on bits 2 and 3, and bval on last bit
        return (static_cast<uint16_t>(iVal1)<<8) + (static_cast<uint16_t>(iVal2)<<3) + (static_cast<uint16_t>(iVal3)<<1) + static_cast<uint16_t>(bVal);
    }
    

    Then your uint16_t will hold all the value you need. To read back, let us say ival2, you need to shift back and to use the and operator:

    unsigned int ReadVal1(const uint16_t theNumber) {
        // ival1 is the first 8 bits from place 9 to 16
        uint16_t check1 = 255; // in bits 0000000011111111
        return (theNumber>>8)&check1;
    }
    unsigned int ReadVal2(const uint16_t theNumber) {
        // ival2 is the 5 bits from place 3 to place 8
        uint16_t check2 = 31; // in bits 0000000000011111
        return (theNumber>>3)&check2;
    }
    unsigned int ReadVal3(const uint16_t theNumber) {
        // ival3 is the 2 bits on places 2 and 3
        uint16_t check3 = 3; // in bits 0000000000000011
        return (theNumber>>1)&check3;
    }
    bool ReadVal4(const uint16_t theNumber) {
        // ival4 is the last bit
        uint16_t check4 = 1; // in bits 0000000000000001
        return theNumber&check4;
    }
    

    NB : here true is equal to 1 and false to 0.