vector初始化对比

对比
vector(sz)
vector(sz,0)
vector(sz,-1u)
vector(sz)然后arr[i]=i
的时间

结果：

performance是跑20次的平均

-O3编译：有initializer的两种差不多，另外两种差不多

no initializer
[ave performance] 1916320 us

init to all 0
[ave performance] 1020553 us

init to all -1
[ave performance] 838372 us

init to pos
[ave performance] 1748269 us

无O3：前三种差不多，最后一种时间大概是前三种的1.5倍

no initializer
[ave performance] 3929123 us

init to all 0
[ave performance] 4024427 us

init to all -1
[ave performance] 3152277 us

init to pos
[ave performance] 6734609 us

代码

#include <iostream>
#include <fstream>
#include <vector>
#include <string.h>
#include <chrono>

using namespace std;
typedef chrono::microseconds UnitType;
chrono::_V2::system_clock::time_point t_start, t_end;
UnitType d;
#define TIMING_START t_start = chrono::system_clock::now();
#define TIMING_END(str)                                   \
    t_end = chrono::system_clock::now();                  \
    d = chrono::duration_cast<UnitType>(t_end - t_start); \
    cout << str << d.count() << " us" << endl;

void moving_average(double &old_ave, double new_val, int &cnt)
{
    ++cnt;
    double tmp1 = old_ave * ((double(cnt - 1)) / cnt);
    double tmp2 = new_val / cnt;
    old_ave = tmp1 + tmp2;
}

#define BENCHMARK_INIT(str, ...)                   \
    cout << str << endl;                           \
    ave = 0;                                       \
    cnt = 0;                                       \
    for (int i = 0; i < repeat_times; ++i)         \
    {                                              \
        std::cout << "rnd " << i;                  \
        TIMING_START                               \
        __VA_ARGS__                                \
        TIMING_END(" ")                            \
        absorb_stdout << arr[rand() % sz] << endl; \
        moving_average(ave, d.count(), cnt);       \
    }                                              \
    cout << "[ave performance] " << (int64_t)ave << " us" << endl;
#define INIT_TO_POS                 \
    vector<unsigned> arr(sz);       \
    for (size_t i = 0; i < sz; ++i) \
        arr[i] = i;
void compare_init_all_zero_and_other(ostream &absorb_stdout)
{
    size_t sz = 1000000000; // 10^9
    int repeat_times = 20;

    double ave = 0;
    int cnt = 0;

    BENCHMARK_INIT("no initializer", vector<unsigned> arr(sz);)
    BENCHMARK_INIT("\n\ninit to all 0", vector<unsigned> arr(sz, 0);)
    BENCHMARK_INIT("\n\ninit to all -1", vector<unsigned> arr(sz, -1u);)
    BENCHMARK_INIT("\n\ninit to pos", INIT_TO_POS)
}

int main()
{
    ofstream absorb_stdout("tmp.txt");
    compare_init_all_zero_and_other(absorb_stdout);
    return 0;
}