GMP - Store 64 bit interger in mpz_t/mpz_class and get 64 bit integers back

Question

I want to assign the values from 64 bit integers into mpz_class/mpz_t variables and later get 64 bit integers back. However GMP only provides this functionality for 32 bit and lower integers.

So how would I turn an 64 bit integer into a mpz_class/mpz_t variable and vice versa. (I need it for both signed and unsigned integers)

Answer 1

This could be achieved with mpz_import() and mpz_export() functions.
Code sample (tested on LP64 data model):

using Type = int64_t;

// We start with some 64bit integer
const Type builtIn64 = std::numeric_limits<Type>::min();
std::cout << builtIn64 << '\n';

// Import the integer into the mpz_class
mpz_t mpzNum;
mpz_init(mpzNum);
mpz_import(mpzNum, 1, 1, sizeof(Type), 0, 0, &builtIn64);
if (builtIn64 < 0) {
    mpz_neg(mpzNum, mpzNum);
}
std::cout << mpz_class(mpzNum) << '\n';

// Export the mpz_t value to a buffer allocated by the function and given
// the word size, get also the number of words required to hold the value
const size_t wordSize = sizeof(Type);
size_t wordCount = 0;
void* outRaw = mpz_export(nullptr, &wordCount, 1, wordSize, 0, 0, mpzNum);

// Make sure that our integer type can still hold the value
if (wordCount == 1) {
    const Type out = *static_cast<Type*>(outRaw);
    std::cout << out << '\n';
}

// Free the allocated memory by mpz_export
void (*freeFunction)(void*, size_t);
mp_get_memory_functions(nullptr, nullptr, &freeFunction);
freeFunction(outRaw, wordCount * wordSize);

// Don't forget to free the allocated memory
mpz_clear(mpzNum);

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