title: How to convert a large string number (uint256) into vector<unsigned char> in C++
body: <p>I have a string number ranging in <code>uint256</code>, such as <code>&quot;115792089237316195423570985008687907853269984665640564039457584007913129639935&quot;</code>.  I want to store the bytes of this number into a <code>vector&lt;unsigned char&gt;</code>. That is, I want to get <code>0xffffff...fff</code> (256bit) stored in the <code>vector</code>, where the <code>vector</code>'s size will not be larger than 32 bytes.</p>
  <p>I have tried the following ways:</p>
  <ol>
  <li><p>Using <code>int</code> to receive the string number and transfer, but the number is out of the <code>int</code> range;</p>
  </li>
  <li><p>Using <code>boost::multiprecision::number&lt;boost::multiprecision::cpp_int_backend&lt;256, 256, boost::multiprecision::unsigned_magnitude, boost::multiprecision::unchecked, void&gt;&gt;</code>. 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.</p>
  </li>
  </ol>