CODEWITHSUNDEEP
c++
NeomerArcana
NeomerArcana

Reputation: 2311

Why can't I pack these ints together?

I have the following code. The goal is to combine the two uint32_ts into a single uint64_t and then retrieve the values.

#include <iostream>
#include <cstdint>

int main()
{
    uint32_t first = 5;
    uint32_t second = 6;

    uint64_t combined = (first << 32) | second;

    uint32_t firstR = combined >> 32;
    uint32_t secondR = combined & 0xffffffff;

    std::cout << "F: " << firstR << "  S: " << secondR << std::endl;
}

It outputs

F: 0  S: 7

How do I successfully retrieve the values correctly?

Upvotes: 0

Views: 80

Answers (3)

NeomerArcana
NeomerArcana

Reputation: 2311

As per the comments:

#include <iostream>
#include <cstdint>

int main()
{
    uint32_t first = 5;
    uint32_t second = 6;

    uint64_t combined = (uint64_t(first) << 32) | second;

    uint32_t firstR = combined >> 32;
    uint32_t secondR = combined & 0xffffffff;

    std::cout << "F: " << firstR << "  S: " << secondR << std::endl;
}

The bit manipulation operators return a type of the first parameter. So you need to cast it to uint64_t in order for it to have room for the second value.

Upvotes: 0

Remy Lebeau
Remy Lebeau

Reputation: 597215

When you perform first << 32, you are shifting 32 bits within the space of 32 bits, so there are no bits remaining after the shift. The result of the shift is 0. You need to convert the first value to 64 bits before you shift it:

uint64_t combined = (uint64_t(first) << 32) | second;

Upvotes: 1

dreamlax
dreamlax

Reputation: 95355

first is a 32-bit type and you bit-shift it by 32 bits. This is technically undefined behaviour, but probably the most likely outcome is that the result of the expression is 0. You need to cast it to a larger type before bit-shifting it.

uint64_t combined = (static_cast<uint64_t>(first) << 32) | second;

Upvotes: 2

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