读书笔记《effective stl》item14: 使用reserve来避免不必要的重新分配

在容器刚被构造出来之后就使用reserve，可以减少不必要的重新分配的时间。

#include <iostream>

#include <vector>

#include "time.h"

using namespace std;

typedef vector<int> VI;

template<class T>

T push_int_containers(int large, int small, T c)

{

    for (int i = large; i>small ; i--)

    {

        c.push_back(i);

    }

    return c;

} 

template<class T>

void print_containers(string name,T c)

{

    cout << "\n" << name <<" : ";

    for (auto i = c.begin(); i != c.end(); ++i)

    {

        cout << *i << " ";

    }

    cout << "\n";

}

int main()

{

    VI v1,v2;

    auto b1 = clock();

    v1.reserve(10000);

    for (int i = 10000; i>1 ; i--)

    {

        v1.push_back(i);

    }

    auto e1 = clock();

    cout << "v1 spends " << e1 - b1 << "\n";

    auto b2 = clock();

    for (int i = 10000; i>1 ; i--)

    {

        v2.push_back(i);

    }

    auto e2 = clock();

    cout << "v2 spends " << e2 - b2 << "\n";

    return 0;

}

输出结果为：

v1 spends 278

v2 spends 337