STL中的容器作為返回值

分別以函數返回值方式和參數傳引用方式測試了vector、map兩種容器，代碼如下：

// testContainer.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
#include <iostream>
#include <vector>
#include <map>
#include <chrono>
using namespace std;
vector<int> funcVec1(){
    vector<int >vec;
    for (int i = 0; i < 10; ++i){
        vec.push_back(i);
    }
    return vec;
}

void funcVec2(vector<int>&vec){
    for (int i = 0; i < 10; ++i){
        vec.push_back(i);
    }
    return;
}

map<int, int>funcMap1(){
    map<int, int>tmpMap;
    for (int i = 0; i < 10; ++i){
        tmpMap[i] = i;
    }
    return tmpMap;
}

void funcMap2(map<int, int>&tmpMap){
    for (int i = 0; i < 10; ++i){
        tmpMap[i] = i;
    }
}

int _tmain(int argc, _TCHAR* argv[])
{  
    using namespace std::chrono;
    steady_clock::time_point t1 = system_clock::now();
    for (int i = 0; i < 100000; ++i){
        vector<int> vec1 = funcVec1();
    }  
    auto t2 = std::chrono::system_clock::now();
    //count輸出時鍾周期
    cout << "return vec takes " << (t2 - t1).count()<<" tick count" << endl;
    // duration_cast在chrono中,可以將時鍾數轉換為相應的時間間隔
    cout << duration_cast<nanoseconds>(t2 - t1).count() << " nanoseconds" << endl;
    cout << duration_cast<microseconds>(t2 - t1).count() << " microseconds" << endl;
    cout << duration_cast<milliseconds>(t2 - t1).count() << " milliseconds" << endl;
    cout << duration_cast<seconds>(t2 - t1).count() << " seconds" << endl;
    cout << " --------------------------------" << endl;
    vector<int> vec2;
    for (int i = 0; i < 100000; ++i){
        funcVec2(vec2);
    }
    auto t3 = system_clock::now();
    cout << "reference vec takes " << (t3 - t2).count() << " tick count" << endl;
    cout << duration_cast<milliseconds>(t3 - t2).count() << " milliseconds" << endl;
    cout << " --------------------------------" << endl;
    for (int i = 0; i < 100000; ++i){
        map<int,int> tmpMap1 = funcMap1();
    }
    auto t4 = system_clock::now();
    cout << "return map takes " << (t4 - t3).count() << " tick count" << endl;
    cout << duration_cast<milliseconds>(t4 - t3).count() << " milliseconds" << endl;
    cout << " --------------------------------" << endl;
    map<int, int>tmpMap2;
    for (int i = 0; i < 100000; ++i){
        funcMap2(tmpMap2);
    }
    auto t5 = system_clock::now();
    cout << "reference map takes " << (t5 - t4).count() << " tick count" << endl;
    cout << duration_cast<milliseconds>(t5 - t4).count() << " milliseconds" << endl;
    return 0;
}

輸出結果：

在測試代碼中，函數返回值是容器的執行速度比容器作為參數傳遞要慢的多。

可以看到返回容器的函數里，容器頻繁的創建銷毀。

容器作為參數傳遞是項目中常見做法，很少看到函數返回容器。原因就在此。