一个从二进制文件中读取一个字节而 [英] Reading bytes one by one from binary file

问题描述

这是我的问题，我想打开.jpg文件，并写入每个​​字节与逗号分隔，到另一个.txt文件一个十进制数（0-255）。现在它应该能够重新建立.jpf文件使用txt文件。这是我想做到这一点。

this is my question, i want to open a .jpg file and write each byte as a decimal number (0-255) separated with a comma, into another .txt file. now it should be able to build the .jpf file again using that txt file. this is how i tried to do it.

#include<iostream>
#include<fstream>
using namespace std;
int main()
{
long x;
char *s;

ifstream ifs("image.jpg",ios::binary);
ifs.seekg(0,ios::end);
x=ifs.tellg();
ifs.seekg(0,ios::beg);

s=new char[x];
ifs.read(s,x);
ifs.close();

ofstream is("image.txt");

for(int i=0;i<x;i++){
is<<(unsigned int)s[i]<<",";
}

现在这个节目及牡丹image.txt与十进制数如下，
 4294967295,4294967256,4294967295,4294967264,0,16,74,70,73,70,0,1，......
这里有些数字似乎是4字节长，S [I]是指只有一个字节，所以如何能（INT）S [I]返回大量超过255请一些可以一个可以帮助我这个....谢谢..上

now this program creats image.txt with decimal numbers as follows, 4294967295,4294967256,4294967295,4294967264,0,16,74,70,73,70,0,1,...... here some numbers seems to be 4bytes long, s[i] refers only one byte, so how can (int)s[i] return a large number than 255. please can some one help me on this.... thanks..

推荐答案

看来你的机器字符上的签署的。所以，当你施放一个负数 unsigned int类型，你会得到一个很大的价值。重新$ P $使用字符 psenting他们时，在输出的大值是负值。请注意，当字符是签署的，其值可 -128 到 127 不过的字节的可以是 255 0 $ C>。因此，大于任何值 127 将成为范围之间的负 -128〜-1 。

It seems on your machine char is signed. So when you cast a negative number to unsigned int, you get a big value. The big values in the output are negative values when representing them using char. Note that when char is signed, its value can be -128 to 127 but a byte can be between 0 to 255. So any value greater than 127 would become negative between the range -128 to -1.

使用 unsigned char型为：

unsigned char *s;

还是做到这一点：

Or do this:

is<< static_cast<unsigned int> (static_cast<unsigned char>(s[i]) )<<",";
                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                                casting to unsigned char first
     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
               then casting to unsigned int 

也就是说，第一个字符转换为 unsigned char型，然后无符号整型。

那么这是所有关于你所遇到的问题。现在，在风格和成语的一些注意事项。在C ++中，你应该避免使用新尽可能。你的情况，你可以使用的std ::矢量为：

Well that is all about the issue you're facing. Now some notes on style and idioms. In C++, you should avoid using new as much as possible. In your case, you can use std::vector as:

//define file stream object, and open the file
std::ifstream file("image.jpg",ios::binary);

//prepare iterator pairs to iterate the file content!
std::istream_iterator<unsigned char> begin(file), end;

//reading the file content using the iterator!
std::vector<unsigned char> buffer(begin,end);

最后一行从该文件中的所有数据读入缓存。现在，你可以把它们打印为：

The last line reads all the data from the file into buffer. Now you can print them as:

std::copy(buffer.begin(), 
          buffer.end(), 
          std::ostream_iterator<unsigned int>(std::cout, ","));

有关所有这些工作，你需要在除了你已经在你的code已经添加以下​​标题：

For all these to work, you need to include the following headers in addition to what you have already added in your code:

#include <vector>     //for vector
#include <iterator>   //for std::istream_iterator and std::ostream_iterator
#include <algorithm>  //for std::copy

正如你可以看到，这个地道的解决方案不使用的指针和新，它也不使用的投！

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