在C ++蟒蛇发电机等效的缓存读取 [英] Equivalent of a python generator in C++ for buffered reads

问题描述

吉多·范罗苏姆展示的Python在这个简单的文章，并利用这个功能进行缓冲读取未知长度的文件：

Guido Van Rossum demonstrates the simplicity of Python in this article and makes use of this function for buffered reads of a file of unknown length:

def intsfromfile(f):
    while True:
        a = array.array('i')
        a.fromstring(f.read(4000))
        if not a:
            break
        for x in a:
            yield x

我需要做同样的事情在C ++的速度的原因！我有一个包含了，我需要合并无符号的64位整数排序的列表多个文件。我发现这个漂亮的一块 code 用于合并向量。

我坚持就如何使一个 ifstream的作为未知长度present本身就是一个文件作为向量可以愉快地重复一遍，直到年底该文件被达到。有什么建议么？难道我吠叫了正确的树与 istreambuf_iterator

I am stuck on how to make an ifstream for a file of unknown length present itself as a vector which can be happily iterated over until the end of the file is reached. Any suggestions? Am I barking up the correct tree with an istreambuf_iterator?

推荐答案

为了掩饰一个 ifstream的（或者说真的，任何输入流）的行为像一个表格一个迭代器，要使用 istream_iterator 或 istreambuf_iterator 模板类。前者是为文件，其中的格式是一个令人关注的是有用的。例如，全空格分隔的整数的文件可以被读入向量的迭代器区间构造如下：

In order to disguise an ifstream (or really, any input stream) in a form that acts like an iterator, you want to use the istream_iterator or the istreambuf_iterator template class. The former is useful for files where the formatting is of concern. For example, a file full of whitespace-delimited integers can be read into the vector's iterator range constructor as follows:

#include <fstream>
#include <vector>
#include <iterator> // needed for istream_iterator

using namespace std;

int main(int argc, char** argv)
{
    ifstream infile("my-file.txt");

    // It isn't customary to declare these as standalone variables,
    // but see below for why it's necessary when working with
    // initializing containers.
    istream_iterator<int> infile_begin(infile);
    istream_iterator<int> infile_end;

    vector<int> my_ints(infile_begin, infile_end);

    // You can also do stuff with the istream_iterator objects directly:
    // Careful! If you run this program as is, this won't work because we
    // used up the input stream already with the vector.

    int total = 0;
    while (infile_begin != infile_end) {
        total += *infile_begin;
        ++infile_begin;
    }

    return 0;
}

istreambuf_iterator 用于阅读文件的单个字符的时间，不顾输入的格式。也就是说，它会回报你的所有字符，包括空格，换行字符，等等。根据您的应用程序，这可能是更合适的。

istreambuf_iterator is used to read through files a single character at a time, disregarding the formatting of the input. That is, it will return you all characters, including spaces, newline characters, and so on. Depending on your application, that may be more appropriate.

注：斯科特迈尔斯解释的有效STL 的，为什么上面需要独立的变量声明 istream_iterator 。通常情况下，你会做这样的事情：

Note: Scott Meyers explains in Effective STL why the separate variable declarations for istream_iterator are needed above. Normally, you would do something like this:

ifstream infile("my-file.txt");
vector<int> my_ints(istream_iterator<int>(infile), istream_iterator<int>());

不过，C ++实际上是在分析一个令人难以置信的怪异的方式，第二行。它把它看作这需要两个参数，并返回一个矢量＆lt名为 my_ints函数的声明; INT＆GT; 。第一个参数是类型 istream_iterator＆LT的; INT＆GT; 并命名为 INFILE （在括号混合被忽略）。第二个参数是一个函数指针没有名称发生（因​​为括号混合的）零参数和返回类型的对象 istream_iterator＆LT; INT＆GT;

However, C++ actually parses the second line in an incredibly bizarre way. It sees it as the declaration of a function named my_ints that takes in two parameters and returns a vector<int>. The first parameter is of type istream_iterator<int> and is named infile (the parantheses are ignored). The second parameter is a function pointer with no name that takes zero arguments (because of the parantheses) and returns an object of type istream_iterator<int>.

pretty的清凉，同时也pretty的加重，如果你没有看出来了。

Pretty cool, but also pretty aggravating if you're not watching out for it.

修改

下面是一个使用一个例子 istreambuf_iterator 读取的64位数字的文件奠定了终端到终端的：

Here's an example using the istreambuf_iterator to read in a file of 64-bit numbers laid out end-to-end:

#include <fstream>
#include <vector>
#include <algorithm>
#include <iterator>

using namespace std;

int main(int argc, char** argv)
{
    ifstream input("my-file.txt");
    istreambuf_iterator<char> input_begin(input);
    istreambuf_iterator<char> input_end;

    // Fill a char vector with input file's contents:
    vector<char> char_input(input_begin, input_end);
    input.close();

    // Convert it to an array of unsigned long with a cast:
    unsigned long* converted = reinterpret_cast<unsigned long*>(&char_input[0]);
    size_t num_long_elements = char_input.size() * sizeof(char) / sizeof(unsigned long);

    // Put that information into a vector:
    vector<unsigned long> long_input(converted, converted + num_long_elements);

    return 0;
}

现在，我个人很不喜欢这个解决方案（使用 reinter pret_cast ，露出 char_input 的数组），但我不与 istreambuf_iterator 不够熟悉而舒适地使用一个模板化的超过64位字符，这将使这个容易得多。

Now, I personally rather dislike this solution (using reinterpret_cast, exposing char_input's array), but I'm not familiar enough with istreambuf_iterator to comfortably use one templatized over 64-bit characters, which would make this much easier.

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