5

注意：这不是现有std::ios::sync_with_stdio(false)问题的重复。我已经经历了所有这些，但我无法cout像printf. 示例代码和证据如下所示。

我有三个源代码文件：

// ex1.cpp

#include <cstdio>
#include <chrono>

int main()
{
    auto t1 = std::chrono::high_resolution_clock::now();
    for (int i = 0; i < 10000000; i++) {
        printf("%d\n", i);
    }
    auto t2 = std::chrono::high_resolution_clock::now();
    auto duration = std::chrono::duration_cast<std::chrono::milliseconds>(t2 - t1);
    fprintf(stderr, "%lld\n", duration.count());
}

// ex2.cpp

#include <iostream>
#include <chrono>

int main()
{
    auto t1 = std::chrono::high_resolution_clock::now();
    for (int i = 0; i < 10000000; i++) {
        std::cout << i << '\n';
    }
    auto t2 = std::chrono::high_resolution_clock::now();
    auto duration = std::chrono::duration_cast<std::chrono::milliseconds>(t2 - t1);
    std::cerr << duration.count() << '\n';
}

// ex3.cpp

#include <iostream>
#include <chrono>

int main()
{
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);

    auto t1 = std::chrono::high_resolution_clock::now();
    for (int i = 0; i < 10000000; i++) {
        std::cout << i << '\n';
    }
    auto t2 = std::chrono::high_resolution_clock::now();
    auto duration = std::chrono::duration_cast<std::chrono::milliseconds>(t2 - t1);
    std::cerr << duration.count() << '\n';
}

我不会在我的代码中混用cstdio，iostream所以我使用的 hacksex3.cpp对我来说没问题。

我在带有固态驱动器的 macOS 上使用 clang++ 编译它们。

clang++ -std=c++11 -O2 -Wall -Wextra -pedantic ex1.cpp -o ex1

clang++ -std=c++11 -O2 -Wall -Wextra -pedantic ex2.cpp -o ex2

clang++ -std=c++11 -O2 -Wall -Wextra -pedantic ex3.cpp -o ex3

现在我运行它们并计时。

$ time ./ex1 > out.txt
1282

real    0m1.294s
user    0m1.217s
sys     0m0.071s

$ time ./ex1 > out.txt
1299

real    0m1.333s
user    0m1.221s
sys     0m0.072s

$ time ./ex1 > out.txt
1277

real    0m1.295s
user    0m1.214s
sys     0m0.070s

$ time ./ex2 > out.txt
3102

real    0m3.371s
user    0m3.037s
sys     0m0.075s

$ time ./ex2 > out.txt
3153

real    0m3.164s
user    0m3.073s
sys     0m0.075s

$ time ./ex2 > out.txt
3136

real    0m3.150s
user    0m3.051s
sys     0m0.077s

$ time ./ex3 > out.txt
3118

real    0m3.513s
user    0m3.045s
sys     0m0.080s

$ time ./ex3 > out.txt
3113

real    0m3.124s
user    0m3.042s
sys     0m0.077s

$ time ./ex3 > out.txt
3095

real    0m3.107s
user    0m3.029s
sys     0m0.073s

即使我将输出重定向到/dev/null. 结果也与-O3优化级别非常相似。

两者ex3和ex2都比ex1? 是否有可能以任何方式使用std::cout提供与 相当的速度printf？

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0 回答 0