patterncppModerate

Combining two 32-bit integers into one 64-bit integer

Submitted by: @import:stackexchange-codereview·Mar 10, 2026·

Viewed 0 times

codereview cpp stackoverflow c++bitwise integer

bitcombiningintotwooneintegersinteger

Problem

long long combine(unsigned int high, unsigned int low) {
    return ((unsigned long long) high) << 32 || low;
}

Is there a better name for the operation?

Is the implicit cast from unsigned long long to long long a reinterpret_cast, or what happens if the unsigned number is too large for the signed data type?

Solution

You should return unsigned long long and let the user decide what they want to do with the cast, especially if you want this to be generic.

I'd prefer a name such as u32tou64, uinttoull, or something more descriptive than combine. A lot of this will depend on your own naming standards, though.

Also, I'd consider being more pedantic:

return (((uint64_t) high) << 32) | ((uint64_t) low);

It's unlikely to make a difference because the code is essentially the same as yours, but it's easier to read and avoids extremely rare (but very troublesome to debug) casting issues. It may require a custom types header if your compiler doesn't support this type notation.

Also, consider making it a macro. There's little benefit to having it as a function - the operation itself will take far less time than the function call setup, call, and return, so the performance of a macro will be much higher. Further, it isn't going to take up much more program space than a function call, so there's little to gain by leaving it a real function.

Code Snippets

return (((uint64_t) high) << 32) | ((uint64_t) low);

Context

StackExchange Code Review Q#2607, answer score: 10

Revisions (0)

No revisions yet.