Load an OpenCV::Mat as a .pgm - opencv

Question

I have a program which loads a .PGM image file into std::vector m_data :
void LoadPgm( std::string const& filename ){
std::ifstream in( filename.c_str(), std::ios::binary );
if ( !in ){
std::cerr << "Error in Image::LoadPgm: Could not open file " << filename << std::endl;
std::exit(1);
}
std::string data = std::string( std::istreambuf_iterator< char >( in ),
std::istreambuf_iterator< char >() );
if ( ( data[0] != 'p' && data[0] != 'P' ) || data[1] != '5' ){
std::cerr << "Error in Image::LoadPgm: Unsupported image format " << filename << std::endl;
std::exit(1);
}
unsigned char const* p = reinterpret_cast< unsigned char* >(&data[2]);
m_width = ReadNumber( p );
m_height = ReadNumber( p );
if ( ReadNumber( p ) > 255 )
{
std::cerr << "Error in Image::LoadPgm: Unsupported image deep "
<< filename << std::endl;
std::exit(1);
}
p++;
m_data.assign( p, p + m_width * m_height );
}
Where the ReadNumber() is :
unsigned long ReadNumber(unsigned char const*&buffer){
do{
if ( *(buffer++) == '#' ){
while ( *(buffer++) != 10 );
}
switch (*buffer)
{
case 9 :
case 10 :
case 13 :
case ' ' :
case '#' :
case '0' :
case '1' :
case '2' :
case '3' :
case '4' :
case '5' :
case '6' :
case '7' :
case '8' :
case '9' : break;
default :
{
std::cerr << "Error in Image::LoadPgm: invalid data" << std::endl;
exit( 1 );
}
}
}
while ( (*buffer < '0') || (*buffer > '9') );
unsigned long value = 0;
do
{
value = 10 * value;
value += *(buffer++) - '0';
}
while ( (*buffer>='0') && (*buffer<='9') );
return value;
}
My question is how can I load, instead of .pgm file, an OpenCV Mat into m_data (without creating a file).
EDIT
Here is my solution (thanks to ypnos) :
void LoadPgm(cv::Mat1b &cvimg ){
Mat1b matimage;
cvtColor(cvimg, matimage, CV_RGB2GRAY);
// Create a pgm object
std::vector<unsigned char> dest(matimage.rows*matimage.cols);
std::copy(matimage.begin(), matimage.end(), dest.begin());
std::stringstream in;
in << "P5" << std::endl;
in << matimage.cols << std::endl;
in << matimage.rows << std::endl;
in << "255" << std::endl;
in.write( reinterpret_cast<char*>( &dest[0] ), matimage.cols * matimage.rows );
std::string data = std::string( std::istreambuf_iterator< char >( in ), std::istreambuf_iterator< char >() );
if ( ( data[0] != 'p' && data[0] != 'P' ) || data[1] != '5' ){
std::cerr << "Error in Image::LoadPgm: Unsupported image format " << std::endl;
std::exit(1);
}
unsigned char const* p = reinterpret_cast< unsigned char* >(&data[2]);
m_width = ReadNumber( p );
m_height = ReadNumber( p );
if ( ReadNumber( p ) > 255 ){
std::cerr << "Error in Image::LoadPgm: Unsupported image deep" << std::endl;
std::exit(1);
}
p++;
m_data.assign( p, p + m_width * m_height );
}

Answer 1

If you want to reliably copy all pixels of a 1 channel unsigned char matrix into an array, do this:
cv::Mat1u source = …
std::vector<unsigned char> dest(source.rows*source.cols);
std::copy(source.begin(), source.end(), dest.begin());