从 istream 到 ostream 的快速受控复制 [英] Fast controlled copy from istream to ostream

问题描述

我必须将几个字节从 istream 复制到 ostream，我知道有两种方法可以执行此复制.

I have to copy several bytes from a istream to a ostream, there are 2 ways that I know to perform this copy.

myostream << myistream.rdbuf();

和

copy( istreambuf_iterator<char>(myistream),
      istreambuf_iterator<char>(),
      ostreambuf_iterator<char>(myostream)
);

我发现 rdbuf 版本的速度至少是 copy 版本的两倍.
我还没有找到仅复制 100 字节的方法，但由于要复制的大小可能会很大，我希望能够使用 rdbuf 版本(如果可能).

I've found that rdbuf version is at least twice as fast as the copy.
I haven't found yet the way of copying just, say 100 bytes, but as the size to be copied will probably be quite big I would like to be able to use the rdbuf version if posible.

如何将这些副本限制为给定的字节数?

How to limit those copies to a given number of bytes?

推荐答案

你能用 0x 吗?如果是这样，那么您可以使用 copy_n:

Can you use 0x? If so, then you can use copy_n:

copy_n( istreambuf_iterator<char>(myistream),
        100,
        ostreambuf_iterator<char>(myostream)
);

编辑 1:

我知道您可能正在寻找一个库解决方案，并且您可能已经自己解决了这个问题.但如果你没有想到这样的事情，这就是我会做的(如果我没有 copy_n):

I know you're probably looking for a library solution, and you could probably have figured this out on your own. But in case you haven't thought of something like this, here's what I would do(if I didn't have copy_n):

void stream_copy_n(std::istream & in, std::size_t count, std::ostream & out)
{
    const std::size_t buffer_size = 4096;
    char buffer[buffer_size];
    while(count > buffer_size)
    {
        in.read(buffer, buffer_size);
        out.write(buffer, buffer_size);
        count -= buffer_size;
    }

    in.read(buffer, count);
    out.write(buffer, count);
}

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