How to set bits?

Question

I have a remote service that receives a number of parameters and an integer to flag which ones are null:

byte<(param_count + 7)/8> null bitmap

I've tried a naive implementation but since I've no experience in bit shifting I'd rather not show it.

So, given a vector of booleans, how can I create my bit map?

`std::vector` is a bitmap. Just like `std::bitset`: http://en.cppreference.com/w/cpp/utility/bitset — Jonas, Jul 04 '17 at 13:09
If you use an `bitset` you can use the method `to_ulong`. The vector implementation may use a bitset under the hood but it doesn't have to. — Andre Kampling, Jul 04 '17 at 13:24

Jonas · Accepted Answer · 2017-07-05T07:30:59.713

4

If param_count is known at compile time you can use std::bitset. Here is an example:

// Define a bitmap with 'param_count + 7' elements
std::bitset<param_count + 7> b;

// Set the fifth bit, zero is the first bit
b[4] = 1;

// Convert to 'unsigned long', and the casting it to an int.
int a = int(b.to_ulong());

If param_count is not known at compile time, you can use std::vector<bool>. Here is another example:

// Define a bitmap with 'param_count + 7' elements
std::vector<bool> b(param_count + 7);

// Set the fifth bit, zero is the first bit
b[4] = 1;

// Convert to 'int'
int a = std::accumulate(b.rbegin(), b.rend(), 0, [](int x, int y) { return (x << 1) + y; });

The conversion from std::vector<bool> to int is taken from this answer.

edited Jul 05 '17 at 07:30

answered Jul 04 '17 at 13:14

Jonas

6,915
8
35
53

@Rakete1111 You are right, I'm adding a `vector` version. – Jonas Jul 04 '17 at 13:17
param_count is not known at compile time. – ruipacheco Jul 04 '17 at 13:22

How to set bits?

1 Answers1