Does boost::multiprecision::mpz_int constructor copy the mpz_t?

Question

I am using boost multiprecision with the gmp backend. Consider the following code to construct a multiprecision number from a binary representation:

typedef boost::multiprecision::mpz_int BigInt;

BigInt createNumber(const unsigned char* in, size_t length)
{
    mpz_t number;
    mpz_init2(number, length);
    mpz_import(number, 1, 1, length, 1, 0, in);
    BigInt id(number);
    mpz_clear(number);
    return number;
}

I was under the impression that the call to mpz_clear(number)´ is required to not leak thempz_t, because theboost::multiprecision::mpz_intconstructor copies the value of thempz_t` and therefore takes no ownership of it. By using the debugger I figured out that this constructor is called:

gmp_int(const mpz_t val)
{
    mpz_init_set(this->m_data, val);
}

This seems to confirm my suspicion since mpz_init_set initializes the internal mpz_t variable with the value of the operand instead of just copying the mpz_t.

But when I do the mpz_clear(number)to free the memory I get wrong values for the constructed BigInt. Removing the mpz_clear(number) yields the correct result. What am I missing here?

Answer 1

Okay after crafting this answer and looking at the code I saw the mistake. I was returning the number when I should be returning the id. This is why conversion constructors should be explicit :)

So yes the constructor does copy the mpz_int. But I was freeing it before it could be copied (on function return). In addition I leaked one mpz_t every time.