I have a string number ranging in uint256, such as "115792089237316195423570985008687907853269984665640564039457584007913129639935". I want to store the bytes of this number into a vector<unsigned char>. That is, I want to get 0xffffff...fff (256bit) stored in the vector, where the vector's size will not be larger than 32 bytes.
I have tried the following ways:
Using int to receive the string number and transfer, but the number is out of the int range;
Using boost::multiprecision::number<boost::multiprecision::cpp_int_backend<256, 256, boost::multiprecision::unsigned_magnitude, boost::multiprecision::unchecked, void>>. But I do not know how to transfer the string number to this type. I cannot find the details of using this type on the Internet.
This Boost.Multiprecision-based solution worked for me well:
std::string input { "115792089237316195423570985008687907853269984665640564039457584007913129639935" };
boost::multiprecision::uint256_t i { input };
std::stringstream ss;
ss << std::hex << i;
std::string s = ss.str();
std::cout << s << std::endl;
std::vector<unsigned char> v{s.begin(), s.end()};
It prints:
ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
Is it what you are looking for?
Live demo: https://godbolt.org/z/cW1hf61Wf.
EDIT
I might have originally misunderstood the question. If you want the vector to contain the binary representation of that number (that is to serialize that number into a vector), it is also possible, and even easier:
std::string input{ "115792089237316195423570985008687907853269984665640564039457584007913129639935" };
boost::multiprecision::uint256_t i { input };
std::vector<unsigned char> v;
export_bits(i, std::back_inserter(v), 8);
Live demo: https://godbolt.org/z/c1GvfndG9.
Corresponding documentation: https://www.boost.org/doc/libs/1_65_0/libs/multiprecision/doc/html/boost_multiprecision/tut/import_export.html.