C++ 从文件流中读取无符号字符 [英] C++ reading unsigned char from file stream

问题描述

我想从二进制文件中读取无符号字节.所以我写了下面的代码.

I want to read unsigned bytes from a binary file. So I wrote the following code.

#include <iostream>
#include <fstream>
#include <vector>
#include <istream>

std::string filename("file");
size_t bytesAvailable = 128;
size_t toRead = 128;

std::basic_ifstream<unsigned char> inf(filename.c_str(), std::ios_base::in | std::ios_base::binary) ;
if (inF.good())
{
    std::vector<unsigned char> mDataBuffer;
    mDataBuffer.resize(bytesAvailable) ;
    inF.read(&mDataBuffer[0], toRead) ;
    size_t counted = inF.gcount() ;
}

这导致始终读取 0 个字节，如计数的变量所示.

This results in reading in always 0 bytes as shown by the variable counted.

网络上似乎有参考资料说我需要设置语言环境才能完成这项工作.我不清楚如何做到这一点.

There seem to be references on the web saying that I need to set the locale to make this work. How to do this exactly is not clear to me.

相同的代码使用数据类型char"而不是unsigned char"

The same code works using the data type 'char' instead of 'unsigned char'

上面使用 unsigned char 的代码似乎可以在 Windows 上运行，但在 colinux Fedora 2.6.22.18 中运行失败.

The above code using unsigned char seems to work on Windows but fails running in a colinux Fedora 2.6.22.18 .

我需要做什么才能让它在 linux 上工作?

What do I need to do to get it to work for linux?

推荐答案

C++ 确实要求实现只为两个版本的字符特征提供显式特化:

C++ does require the implementation only to provide explicit specializations for two versions of character traits:

std::char_traits<char>
std::char_traits<wchar_t>

流和字符串使用这些特征来计算各种事情，例如 EOF 值、一系列字符的比较、将字符扩展为 int 等等.

The streams and strings use those traits to figure out a variety of things, like the EOF value, comparison of a range of characters, widening of a character to an int, and such stuff.

如果你像这样实例化一个流

If you instantiate a stream like

std::basic_ifstream<unsigned char>

您必须确保流可以使用相应的字符特征特化，并且该特化确实可以做有用的事情.此外，流使用 facet 进行数字的实际格式化和读取.同样，您也必须手动提供这些专业化.该标准甚至不要求实现具有主模板的完整定义.所以你也可以得到一个编译错误:

You have to make sure that there is a corresponding character trait specialization that the stream can use and that this specialization does do useful things. In addition, streams use facets to do actual formatting and reading of numbers. Likewise you have to provide specializations of those too manually. The standard doesn't even require the implementation to have a complete definition of the primary template. So you could aswell get a compile error:

错误:无法实例化特化 std::char_traits.

error: specialization std::char_traits could not be instantiated.

我会使用 ifstream 代替(它是一个 basic_ifstream)，然后读入一个 vector.在解释向量中的数据时，您仍然可以稍后将它们转换为 unsigned char.

I would use ifstream instead (which is a basic_ifstream<char>) and then go and read into a vector<char>. When interpreting the data in the vector, you can still convert them to unsigned char later.

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