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;
}

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