問題描述
在執行一些測試后，我注意到 printf 比 cout 快得多.我知道它依賴于實現，但在我的 Linux 機器上 printf 快 8 倍.所以我的想法是混合兩種打印方法:我想使用 cout 進行簡單的打印，我計劃使用 printf 來產生巨大的輸出(通常在循環中).我認為只要在切換到其他方法之前不要忘記刷新就可以安全:
After performing some tests I noticed that printf is much faster than cout. I know that it's implementation dependent, but on my Linux box printf is 8x faster. So my idea is to mix the two printing methods: I want to use cout for simple prints, and I plan to use printf for producing huge outputs (typically in a loop). I think it's safe to do as long as I don't forget to flush before switching to the other method:
cout << "Hello" << endl;
cout.flush();
for (int i=0; i<1000000; ++i) {
printf("World!
");
}
fflush(stdout);
cout << "last line" << endl;
cout << flush;
這樣可以嗎?
更新:感謝所有寶貴的反饋.答案摘要:如果您想避免棘手的解決方案，只需堅持使用 cout 但不要使用 endl 因為它會隱式刷新緩沖區(減慢進程).使用 "
" 代替.如果你產生大輸出，這會很有趣.
Update: Thanks for all the precious feedbacks. Summary of the answers: if you want to avoid tricky solutions, simply stick with cout but don't use endl since it flushes the buffer implicitly (slowing the process down). Use "
" instead. It can be interesting if you produce large outputs.
推薦答案
直接的回答是，是的，沒關系.
The direct answer is that yes, that's okay.
很多人就如何提高速度提出了各種想法，但似乎對哪種方法最有效存在相當多的分歧.我決定編寫一個快速測試程序，以至少了解哪些技術做了什么.
A lot of people have thrown around various ideas of how to improve speed, but there seems to be quite a bit of disagreement over which is most effective. I decided to write a quick test program to get at least some idea of which techniques did what.
#include <iostream>
#include <string>
#include <sstream>
#include <time.h>
#include <iomanip>
#include <algorithm>
#include <iterator>
#include <stdio.h>
char fmt[] = "%s
";
static const int count = 3000000;
static char const *const string = "This is a string.";
static std::string s = std::string(string) + "
";
void show_time(void (*f)(), char const *caption) {
clock_t start = clock();
f();
clock_t ticks = clock()-start;
std::cerr << std::setw(30) << caption
<< ": "
<< (double)ticks/CLOCKS_PER_SEC << "
";
}
void use_printf() {
for (int i=0; i<count; i++)
printf(fmt, string);
}
void use_puts() {
for (int i=0; i<count; i++)
puts(string);
}
void use_cout() {
for (int i=0; i<count; i++)
std::cout << string << "
";
}
void use_cout_unsync() {
std::cout.sync_with_stdio(false);
for (int i=0; i<count; i++)
std::cout << string << "
";
std::cout.sync_with_stdio(true);
}
void use_stringstream() {
std::stringstream temp;
for (int i=0; i<count; i++)
temp << string << "
";
std::cout << temp.str();
}
void use_endl() {
for (int i=0; i<count; i++)
std::cout << string << std::endl;
}
void use_fill_n() {
std::fill_n(std::ostream_iterator<char const *>(std::cout, "
"), count, string);
}
void use_write() {
for (int i = 0; i < count; i++)
std::cout.write(s.data(), s.size());
}
int main() {
show_time(use_printf, "Time using printf");
show_time(use_puts, "Time using puts");
show_time(use_cout, "Time using cout (synced)");
show_time(use_cout_unsync, "Time using cout (un-synced)");
show_time(use_stringstream, "Time using stringstream");
show_time(use_endl, "Time using endl");
show_time(use_fill_n, "Time using fill_n");
show_time(use_write, "Time using write");
return 0;
}
在使用 VC++ 2013(x86 和 x64 版本)編譯后，我在 Windows 上運行了它.一次運行的輸出(輸出重定向到磁盤文件)如下所示:
I ran this on Windows after compiling with VC++ 2013 (both x86 and x64 versions). Output from one run (with output redirected to a disk file) looked like this:
Time using printf: 0.953
Time using puts: 0.567
Time using cout (synced): 0.736
Time using cout (un-synced): 0.714
Time using stringstream: 0.725
Time using endl: 20.097
Time using fill_n: 0.749
Time using write: 0.499
正如預期的那樣，結果各不相同，但我覺得有幾點很有趣:
As expected, results vary, but there are a few points I found interesting:
- 在寫入 NUL 設備時，printf/puts 比 cout 快得多
- 但是 cout 在寫入真實文件時保持得很好
- 在我的測試中，fill_n 與其他任何東西一樣快
我最近編輯了代碼以強制調用printf.Anders Kaseorg 很友好地指出——g++ 識別特定序列 printf("%s
", foo); 等價于 puts(foo);，并相應地生成代碼(即生成代碼來調用 puts 而不是 printf).將格式字符串移動到全局數組，并將其作為格式字符串傳遞會產生相同的輸出，但強制它通過 printf 而不是 puts 產生.當然，他們也有可能在某一天對此進行優化，但至少現在 (g++ 5.1) 使用 g++ -O3 -S 進行的測試確認它實際上正在調用 printf(前面的代碼編譯為對 puts 的調用).
I've recently edited the code to force a call to printf. Anders Kaseorg was kind enough to point out--that g++ recognizes the specific sequence printf("%s
", foo); is equivalent to puts(foo);, and generates code accordingly (i.e., generates code to call puts instead of printf). Moving the format string to a global array, and passing that as the format string produces identical output, but forces it to be produced via printf instead of puts. Of course, it's possible they might optimize around this some day as well, but at least for now (g++ 5.1) a test with g++ -O3 -S confirms that it's actually calling printf (where the previous code compiled to a call to puts).
這篇關于混合 cout 和 printf 以獲得更快的輸出的文章就介紹到這了，希望我們推薦的答案對大家有所幫助，也希望大家多多支持html5模板網！