WKB hex decoder in C++

In PostGIS world, I often need to construct geometry from Well-Known-Binary (WKB) or PostGIS EWKB stream encoded as hex stream. It’s easy to do if I have access to PostgreSQL/PostGIS client which accepts SQL queries:

<code>SELECT
   ST_AsText(
      ST_GeomFromWKB(
         decode('0101000000e5d022dbf93e2e40dbf97e6abc743540', 'hex'),
         4326))</code>

I often need to do the same directly in C++ code - parse hex encoded binary stream to raw stream of bytes. Here is simple hex decoder I use:

<code>#include <sstream>
#include <string>
#include <vector>
typedef std::vector<unsigned char> ewkb_t;

// bytes [out] - buffer for binary output
void hex_to_bytes(std::string const& hexstr, ewkb_t& bytes)
{
    bytes.clear();
    for(std::string::size_type i = 0; i < hexstr.size() / 2; ++i)
    {                   
        std::istringstream iss(hexstr.substr(i * 2, 2));
        unsigned int n;
        iss >> std::hex >> n;
        bytes.push_back(static_cast<unsigned char>(n));
    }
}
</code>

For example, I use it to build FDO geometry objects using another utility CreateGeometryFromExtendedWkb defined in FDO provider for PostGIS:

<code>// POINT (1.234 5.678)
std::string ewkbhex("01010000005839B4C876BEF33F83C0CAA145B61640");
ewkb_t ewkb;
hex_to_bytes(ewkbhex, ewkb);
if (!ewkb.empty())
{
   FdoPtr<fdoigeometry> g = CreateGeometryFromExtendedWkb(ewkb);
   // ... use geometry
}</code>