问题描述
我一直听说 C++ 文件 I/O 操作比 C 风格 I/O 慢得多.但是我没有找到任何关于它们实际上有多慢的实用参考，所以我决定在我的机器上测试它(Ubuntu 12.04、GCC 4.6.3、ext4 分区格式).
I've always heard that C++ file I/O operations are much much slower than C style I/O. But I didn't find any practical references on comparatively how slow they actually are, so I decided to test it in my machine (Ubuntu 12.04, GCC 4.6.3, ext4 partition format).
首先我在磁盘中写入了一个 ~900MB 的文件.
First I wrote a ~900MB file in the disk.
C++ (ofstream):163s
C++ (ofstream): 163s
ofstream file("test.txt");
for(register int i = 0; i < 100000000; i++)
file << i << endl;
C (fprintf):12s
FILE *fp = fopen("test.txt", "w");
for(register int i = 0; i < 100000000; i++)
fprintf(fp, "%d
", i);
我期待这样的输出，它表明在 C++ 中写入文件比在 C 中慢得多.然后我使用 C 和 C++ I/O 读取同一个文件.是什么让我惊叹从文件读取时性能几乎没有差异.
I was expecting such output, it shows that writing to a file is much slower in C++ than in C. Then I read the same file using C and C++ I/O. What made me exclaimed that there is almost no difference in performance while reading from file.
C++ (ifstream):12s
int n;
ifstream file("test.txt");
for(register int i = 0; i < 100000000; i++)
file >> n;
C (fscanf):12s
FILE *fp = fopen("test.txt", "r");
for(register int i = 0; i < 100000000; i++)
fscanf(fp, "%d", &n);
那么，为什么使用流执行写入要花这么长时间?或者，为什么使用流读取比写入快?
So, why is taking so long to execute writing using stream? Or, why reading using stream is so fast compared to writing?
结论:罪魁祸首是 std::endl，正如答案和评论所指出的那样.换线<代码>文件<<我＜＜endl;到<代码>文件<<我＜＜'
'; 已将运行时间从 163 秒减少到 16 秒.
Conclusion: The culprit is the std::endl, as the answers and the comments have pointed out. Changing the line
file << i << endl;
to
file << i << '
'; has reduced running time to 16s from 163s.
推荐答案
您正在使用 endl 打印换行符.这就是问题所在，因为它更多不仅仅是打印换行符 —endl 还刷新缓冲区，这是一个昂贵的操作(如果你在每次迭代中都这样做).
You're using endl to print a newline. That is the problem here, as it does more than just printing a newline — endl also flushes the buffer which is an expensive operation (if you do that in each iteration).
如果您的意思是这样，请使用
:
Use
if you mean so:
file << i << '
';
此外，必须在发布模式下编译您的代码(即打开优化).
And also, must compile your code in release mode (i.e turn on the optimizations).
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