reserve_vs_resize.cpp

reserve_vs_resize.cpp

#include <time.h>
#include <stdio.h>
#include <vector>

struct A {
    int mem[100];
};

int n = 1000000;

int main(int argc, char** argv) {
    std::vector<A> a;
    clock_t start, end;

    {
        a.reserve(n);
        start = clock();
        for (int i = 0; i < n; i++) {
            a.push_back({});
        }
        end = clock();
        printf("b.size()=%lu\n", a.size());
        printf("reserve only -- %f ms\n", (float)(end - start) / CLOCKS_PER_SEC * 1000);
    }

    std::vector<A> b;
    {
        start = clock();
        for (int i = 0; i < n; i++) {
            b.resize(b.size()+1);
        }
        end = clock();
        printf("b.size()=%lu\n", b.size());
        printf("resize only -- %f ms\n", (float)(end - start) / CLOCKS_PER_SEC * 1000);
    }

    {
        b.clear();
        printf("b.size()=%lu\n", b.size());
        start = clock();
        for (int i = 0; i < n; i++) {
            b.resize(b.size()+1);
        }
        end = clock();
        printf("b.size()=%lu\n", b.size());
        printf("resize after clear -- %f ms\n", (float)(end - start) / CLOCKS_PER_SEC * 1000);
    }

    {
        b.clear();
        b.reserve(n);
        printf("b.size()=%lu\n", b.size());
        start = clock();
        for (int i = 0; i < n; i++) {
            b.emplace_back();
        }
        end = clock();
        printf("b.size()=%lu\n", b.size());
        printf("resize after clear -- %f ms\n", (float)(end - start) / CLOCKS_PER_SEC * 1000);
    }

    return 0;
}

/*
~/Desktop/cpp_tests                                                                                                                                                              
▶ g++ reserve_vs_resize.cpp -std=c++14

~/Desktop/cpp_tests                                                                                                                                                              
▶ ./a.out                             
b.size()=1000000
reserve only -- 216.867004 ms
b.size()=1000000
resize only -- 593.352966 ms
b.size()=0
b.size()=1000000
resize after clear -- 70.511002 ms
b.size()=0
b.size()=1000000
resize after clear -- 45.888000 ms
*/