And use Boost.Filesystem's recursive_directory_iterator to do it. For example, if you're not worried about symlinks, you can do this:
#include <boost/cstdint.hpp>
#include <algorithm>
#include <boost/filesystem.hpp>
#include <boost/bind.hpp>
#include <boost/iterators/transform_iterator.hpp>
namespace fs = boost::filesystem;
boost::uintmax_t dir_size(const fs::path &p)
{
return std::accumulate(
boost::make_transform_iterator(&fs::file_size, fs::recursive_directory_iterator(p)),
boost::make_transform_iterator(&fs::file_size, fs::recursive_directory_iterator()),
static_cast<boost::uintmax_t>(0));
}