C++中读取图片长度和宽度

看到一个用ASP写的读取图片文件的长度、宽度的程序，感觉有点意思，于是用C++也写了一个。

#include <iostream>
#include <fstream>
#include <string>

using namespace std;

class CImage
{
private:
long m_Width;
long m_Height;

int get_extension(string fname);
public:
CImage()
{
m_Width = 0;
m_Height = 0;

};
void LoadImage(char* fname);

long get_width()
{
return m_Width;
};

long get_height()
{
return m_Height;
};

};

int CImage::get_extension(string fname)
{
char c = fname.at(fname.length()-1);
char c2 = fname.at(fname.length()-3);

if ((c == 'f') && (c2 == 'g')){ // file extension name is gif
return 1;
}else if ((c == 'g') && (c2 == 'j')){ // file extension name is jpg
return 2;
}else if ((c == 'g') && (c2 == 'p')){ // file extension name is png
return 3;
}else if ((c == 'p') && (c2 == 'b')){ // file extension name is bmp
return 4;
}
return 0;
}

void CImage::LoadImage(char *fname)
{
m_Width = m_Height = 0;

ifstream ffin(fname, std::ios::binary);

if (!ffin){
cout<<"Can not open this file."<<endl;
return;
}
int result = get_extension(fname);
char s1[2] = {0}, s2[2] = {0};

switch(result)
{
case 1: // gif
ffin.seekg(6);
ffin.read(s1, 2);
ffin.read(s2, 2);
m_Width = (unsigned int)(s1[1])<<8|(unsigned int)(s1[0]);
m_Height = (unsigned int)(s2[1])<<8|(unsigned int)(s2[0]);
break;
case 2: // jpg
ffin.seekg(164);
ffin.read(s1, 2);
ffin.read(s2, 2);
m_Width = (unsigned int)(s1[1])<<8|(unsigned int)(s1[0]);
m_Height = (unsigned int)(s2[1])<<8|(unsigned int)(s2[0]);
break;
case 3: // png
ffin.seekg(17);
ffin.read(s1, 2);
ffin.seekg(2, std::ios::cur);
ffin.read(s2, 2);
m_Width = (unsigned int)(s1[1])<<8|(unsigned int)(s1[0]);
m_Height = (unsigned int)(s2[1])<<8|(unsigned int)(s2[0]);
break;
case 4: // bmp
ffin.seekg(18);
ffin.read(s1, 2);
ffin.seekg(2, std::ios::cur);
ffin.read(s2, 2);
m_Width = (unsigned int)(s1[1])<<8|(unsigned int)(s1[0]);
m_Height = (unsigned int)(s2[1])<<8|(unsigned int)(s2[0]);
break;
default:
cout<<"NO"<<endl;
break;
}
ffin.close();
};

int main(int argc, char *argv[])
{
if (argc < 2){
printf("usage: program imagefilename/n");
return 0;
}
CImage test;
test.LoadImage(argv[1]);
cout<<"width:"<<test.get_width()<<endl;
cout<<"height:"<<test.get_height()<<endl;

return 0;
}