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<h1 id="use-bit-fields"><a class="header" href="bit_fields.html#use-bit-fields">use bit fields</a></h1>
<p>If we need a compound data which has bit sections which correspond to different values...</p>
<p>we can use bit-fields or we can use more complex solutions...</p>
<p>c++ has bit-fields which can easily solve this problem... compiler can detect wrong assignments too. it should have at most it's type maximum bits so x can be maximum 32 bits.</p>
<p>according to total bits wrapper type will have a 2n size.... for Version1 => 32 bits...</p>
<p>other solution is very classic...instead of bit-fields combining all into one variable but this time we need masks to set / get the data.</p>
<pre><code class="language-c++">
struct Version1 {
uint32_t x : 4;
uint32_t y : 4;
uint32_t z : 12;
uint32_t get_x() { return x; }
uint32_t get_y() { return y; }
uint32_t get_z() { return z; }
};
struct Version2 {
uint32_t data;
const uint32_t mask_x = 0x0F;
const uint32_t mask_y = 0x0F;
const uint32_t mask_z = 0x0FFF;
uint32_t get_x() { return (data >> 16) & mask_x; }
uint32_t get_y() { return (data >> 12) & mask_y; }
uint32_t get_z() { return (data >> 0) & mask_z; }
void set_data(uint32_t x, uint32_t y, uint32_t z) {
data = (x << 16) | (y << 12) | z;
}
};
int main() {
const uint32_t x = 13;
const uint32_t y = 11;
const uint32_t z = 777;
Version1 v{.x = x, .y = y, .z = z};
Version2 v2;
v2.set_data(x, y, z);
std::cout << v.x << ":" << v.y << ":" << v.z << std::endl;
std::cout << v2.get_x() << ":" << v2.get_y() << ":" << v2.get_z()
<< std::endl;
return 0;
}
</code></pre>
<pre><code class="language-c++">#include <bitset>
#include <iostream>
using namespace std;
union U {
struct {
int i : 4;
int j : 4;
} s_data;
int arr;
};
int main() {
U u{};
u.s_data.i = 0b0011;
u.s_data.j = 0b1100;
bitset<12> bits = u.arr;
cout << bits << endl;
return 0;
}
</code></pre>
<p>another example...</p>
<p>Note: size of(FirstIP4word) can't be lower than field type ( maximum one) so in this example: minimum size of FirstIP4word == size of(uint32_t) .. ( 4 bytes) but if you use 32 bits for each...4 x 4 = 16 bytes will be used. in example 32 bits used so 4 bytes enough to store this data</p>
<p>Also when assigning you should be careful.</p>
<p>Always assign directly to bit fields.</p>
<pre><code class="language-c++">
#include <iostream>
struct FirstIP4word {
uint32_t version : 4;
uint32_t header : 4;
uint32_t tos : 8;
uint32_t length : 16;
};
int main() { std::cout << sizeof(FirstIP4word) << std::endl; }
</code></pre>
<pre><code class="language-c++">4
</code></pre>
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