为什么 std::vector<char>比 std::string 快? [英] Why is std::vector<char> faster than std::string?

问题描述

我编写了一个小测试，我试图比较调整容器大小然后使用 std::generate_n 填充它的运行速度.我正在比较 std::string 和 std::vector.这是程序:

I have written a small test where I'm trying to compare the run speed of resizing a container and then subsequently using std::generate_n to fill it up. I'm comparing std::string and std::vector<char>. Here is the program:

#include <algorithm>
#include <iostream>
#include <iterator>
#include <random>
#include <vector>

int main()
{
    std::random_device rd;
    std::default_random_engine rde(rd());
    std::uniform_int_distribution<int> uid(0, 25);

    #define N 100000

#ifdef STRING
    std::cout << "String.\n";
    std::string s;
    s.resize(N);
    std::generate_n(s.begin(), N, 
                    [&]() { return (char)(uid(rde) + 65); });
#endif

#ifdef VECTOR
    std::cout << "Vector.\n";
    std::vector<char> v;
    v.resize(N);
    std::generate_n(v.begin(), N, 
                    [&]() { return (char)(uid(rde) + 65); });
#endif

    return 0;
}

还有我的Makefile:

test_string:
    g++ -std=c++11 -O3 -Wall -Wextra -pedantic -pthread -o test test.cpp -DSTRING
    valgrind --tool=callgrind --log-file="test_output" ./test
    cat test_output | grep "refs"

test_vector:
    g++ -std=c++11 -O3 -Wall -Wextra -pedantic -pthread -o test test.cpp -DVECTOR
    valgrind --tool=callgrind --log-file="test_output" ./test
    cat test_output | grep "refs"

以及N的某些值的比较:

N=10000
String: 1,865,367
Vector: 1,860,906

N=100000
String: 5,295,213
Vector: 5,290,757

N=1000000
String: 39,593,564
Vector: 39,589,108

std::vector 每次都领先.既然它似乎性能更高，那么使用 std::string 的意义何在?

std::vector<char> comes out ahead everytime. Since it seems to be more performant, what is even the point of using std::string?

推荐答案

我使用了 #define N 100000000.为每个场景测试了 3 次，并且在所有场景中字符串都更快.不使用 Valgrind，没有意义.

I used #define N 100000000. Tested 3 times for each scenario and in all scenarios string is faster. Not using Valgrind, it does not make sense.

OS: Ubuntu 14.04. Arch:x86_64 CPU: Intel(R) Core(TM) i5-4670 CPU @ 3.40GHz.

$COMPILER -std=c++11 -O3 -Wall -Wextra -pedantic -pthread -o test x.cc -DVECTOR    
$COMPILER -std=c++11 -O3 -Wall -Wextra -pedantic -pthread -o test x.cc -DSTRING

次数:

compiler/variant           | time(1) | time(2) | time(3)
---------------------------+---------+---------+--------
g++ 4.8.2/vector    Times: | 1.724s  | 1.704s  | 1.669s
g++ 4.8.2/string    Times: | 1.675s  | 1.678s  | 1.674s
clang++ 3.5/vector  Times: | 1.929s  | 1.934s  | 1.905s
clang++ 3.5/string  Times: | 1.616s  | 1.612s  | 1.619s

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