将文件的一部分读取到 std：：vector 中的有效方法<char>？

参考这个问题：将文件读取到std::vector<char>中的有效方法？我需要一个做以下事情的函数：

void readFromFile( std::vector< unsigned char >& buffer,
                   string filename,
                   size_t offset, size_t count );

所以从文件中读取的函数从偏移量到偏移量+计数到矢量；

void readFromFile( std::vector< unsigned char >& buffer,
                   string filename,
                   size_t offset, size_t count )
{
    // get file size and reallocate the buffer
    size_t fsize = filesize( filename );
    buffer.reserve( buffer.size() + size );
    // open the file
    ifstream file( filename );
    // first way
    file.seekg( offset );
    file.read( ???? )
    // second way
    istreambuf_iterator< unsigned char > from( file );
    istreambuf_iterator< unsigned char > eof;
    advance( from, offset );
    copy( from, eof, back_inserter( buffer );
}

第一种方法是，我不知道如何立即读取文件。在第二种方式中，读取操作相当慢，因为我是逐字节读取的。

有更好的替代方案吗？

编辑

感谢@Ben Voigt

我写了两个简单的函数：

inline std::streamsize filesize( const std::string& filename )
{
    std::ifstream in( filename, std::ifstream::in | std::ifstream::binary );
    if ( !in ) throw std::invalid_argument
    {
        "filesize error: invalid filename"
    };
    in.seekg( 0, std::ifstream::end );
    return in.tellg();
    // here the file is closed. so no need to restore the get pointer
}
inline std::streamsize filesize( std::ifstream& file )
{
    file.seekg( 0, std::ifstream::end );
    const auto size = file.tellg();
    file.seekg( 0 );      // restore the get pointer
    return size;
}
template< typename RAIter >
inline void read_file( std::istream& file,
                       RAIter first, RAIter last,
                       std::streamsize offset = 0
                        )
{
    const auto size = last - first;
    file.seekg( offset, std::ifstream::beg );
    file.read( reinterpret_cast< char* >( &*first ), size );
}
template<>
inline void read_file( std::istream& file,
                       unsigned char*  first, unsigned char*  last,
                       std::streamsize offset /*= 0 no default argument in template spacalization. */
        )
{
    const auto size = last - first;
    file.seekg( offset, std::ifstream::beg );
    file.read( reinterpret_cast< char* >( first ), size );
}

所以这个功能现在变得很简单了：

vector< unsigned char > buffer;
// do something with buffer
const string filename{ "blabla" };
const auto size = filesize( filename );
// resize the buffer
auto const OLD_LEN = buffer.size();
buffer.resize( OLD_LEN + size );
size_t startOffset = 0;       // from where to star reading from file
size_t cont = size;           // how manny bytes read from file
// read filename from startOffset to startOffset + count, appendeing in buffer
ifstream file( filename );
read_file( file,
           buffer.data() + OLD_LEN,
           buffer.data() + OLD_LEN + count,
           startOffset
           );