C ++读取二进制文件 [英] C++ reading binary files

问题描述

我想了解如何在C ++中读取二进制文件.我的代码:

I want to understand how does reading binary files work in C++. My code:

int main() {
    ifstream ifd("input.png",ios::binary |ios::ate);
    int size = ifd.tellg();
    ifd.seekg(0,  ios::beg);
    vector<char> buffer;
    buffer.reserve(size);
    ifd.read(buffer.data(), size);

    cout << buffer.data();
    return 0;
}

我认为如果我退出缓冲区，我将以二进制形式获取结果，但事实并非如此.

I thought that if I cout my buffer I would get the result in binary but that is not the case.

我的输出是:˙Ř˙á6Exif

My output is: ˙Ř˙á6Exif

如果我阅读了文本文件，它将以正常形式而不是二进制形式显示文本.显然我的逻辑不对.如何将文件读取到缓冲区中，使其包含二进制值?P.s.我这样做是为了实现Shannon-Fano算法，因此，如果有人对读取二进制文件有任何建议，我将不胜感激.

And if I read the text file it displays the text in normal form not in binary. Obviously my logic is not right here. How can I read files to a buffer so it will contain binary values? P.s. I`m doing this for implementing a Shannon-Fano algorithm so if anyone has any advice on reading a binary file I would be grateful.

推荐答案

您需要调整向量的大小，而不是保留向量:

You need to resize your vector, not reserve it:

int main()
{
    ifstream ifd("input.png", ios::binary | ios::ate);
    int size = ifd.tellg();
    ifd.seekg(0, ios::beg);
    vector<char> buffer;
    buffer.resize(size); // << resize not reserve
    ifd.read(buffer.data(), size);

    cout.write(buffer.data(), buffer.size()); // you cannot just output buffer to cout as the buffer won't have '\0' ond-of-string terminator
}

否则，您的代码会尝试将 size 个字符读取到一个空缓冲区中.您也可以使用设置向量大小的向量构造函数: vector< char>缓冲区(大小);

Otherwise your code tries to read size characters into an empty buffer. You may as well use vector constructor that sets vector size: vector<char> buffer(size);

您可以通过以下方式输出缓冲区的字节值:

You can output byte values of your buffer this way:

void dumpbytes(const vector<char>& v)
{
    for (int i=0; i<v.size(); ++i)
    {
        printf("%u ", (unsigned char)v[i]);
        if ((i+1) % 16 == 0)
            printf("\n");
    }
    printf("\n");
}

或者像普通十六进制编辑器一样为十六进制输出做的事情:

Or something like common hex editors do for hex output:

void dumphex(const vector<char>& v)
{
    const int N = 16;
    const char hex[] = "0123456789ABCDEF";
    char buf[N*4+5+2];
    for (int i = 0; i < v.size(); ++i)
    {
        int n = i % N;
        if (n == 0)
        {
            if (i)
                puts(buf);
            memset(buf, 0x20, sizeof(buf));
            buf[sizeof(buf) - 2] = '\n';
            buf[sizeof(buf) - 1] = '\0';
        }
        unsigned char c = (unsigned char)v[i];
        buf[n*3+0] = hex[c / 16];
        buf[n*3+1] = hex[c % 16];
        buf[3*N+5+n] = (c>=' ' && c<='~') ? c : '.';
    }
    puts(buf);
}

使用"Hello World！"缓冲数据将打印如下:

Buffer with "Hello World!" data would be printed as follows:

48 65 6C 6C 6F 20 57 6F 72 6C 64 21                  Hello World!

这篇关于C ++读取二进制文件的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！