为什么 ostringstream 比 ofstream 快

发布于 2024-10-20 01:04:06 字数 572 浏览 7 评论 0原文

要将多条数据写入文件，我有2种方法：

直接一条一条写入ofstream

ofstream 文件("c:\\test.txt");
for (int i = 0; i < 10000; ++i)
{
    文件<<数据[i]；
}

先写入istingstream，然后一次性写入ofstream

ostringstream strstream;
for (int i = 0; i < 10000; ++i)
{
    字符串流<<数据[i]；
}
ofstream 文件("c:\\test.txt");
文件<< strstream.str();

毫不奇怪，第二种方法更快，事实上，在我的HP7800机器上它比第一种方法快4倍。

但为什么？我知道 ofstream 在内部使用 filebuf，而 ostringstream 使用 stringbuf - 作为缓冲区，它们都应该驻留在内存中，因此应该没有区别。

引擎盖下有什么区别？

原文

To write many piece of data to file, I have 2 approaches:

Write to ofstream one by one directly

ofstream file("c:\\test.txt");
for (int i = 0; i < 10000; ++i)
{
    file << data[i];
}

Write to istringstream first, and then write to ofstream at once

ostringstream strstream;
for (int i = 0; i < 10000; ++i)
{
    strstream << data[i];
}
ofstream file("c:\\test.txt");
file << strstream.str();

Not surprisingly, the second approach is faster, in fact, it is 4 times faster than the first approach on my HP7800 machine.

But why? I know ofstream is using filebuf inside, and ostringstream is using stringbuf - as a buffer they should all reside in memory thus should have no difference.

What is the difference under the hood?

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空‖城人不在 2024-10-27 01:04:06

您是否经常使用 std::endl 而不是 '\n' ？ std::endl 做了两件事：它将 '\n' 插入流中，然后将缓冲区刷新到磁盘< /强>。我见过代码说这样做会严重影响性能。（修复后代码运行速度提高了 5-10 倍。）
刷新到字符串缓冲区将比刷新到磁盘快得多，因此这可以解释您的发现。

如果不是这种情况，您可能会考虑增加缓冲区大小：

const std::size_t buf_size = 32768;
char my_buffer[buf_size];
ofstream file("c:\\test.txt");
file.rdbuf()->pubsetbuf(my_buffer, buf_size);

for (int i = 0; i < 10000; ++i)
{
    file << data[i];
}

Are you using std::endl a lot instead of '\n'? std::endl does two things: it inserts a '\n' into the stream and then flushes the buffer to disk. I've seen code talking a severe performance hit by doing so. (The code ran 5-10 times faster after that was fixed.)
Flushing to a string buffer will be much faster than flushing to the disk, so that would explain your findings.

If that's not the case you might consider is increasing the buffer size:

const std::size_t buf_size = 32768;
char my_buffer[buf_size];
ofstream file("c:\\test.txt");
file.rdbuf()->pubsetbuf(my_buffer, buf_size);

for (int i = 0; i < 10000; ++i)
{
    file << data[i];
}

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