STL中的容器作为返回值

分别以函数返回值方式和参数传引用方式测试了vector、map两种容器，代码如下：

 // 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 = ; i < ; ++i){
         vec.push_back(i);
     }
     return vec;
 }

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

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

 void funcMap2(map<int, int>&tmpMap){
     for (int i = ; i < ; ++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 = ; i < ; ++i){
         vector<int> vec1 = funcVec1();
     }
     auto t2 = std::chrono::system_clock::now();
     //count输出时钟周期
     cout << "return vec takes " << (t2 - t1).count()<<" tick count" << endl;
     // duration_cast在chrono中,可以将时钟数转换为相应的时间间隔
     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 = ; i < ; ++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 = ; i < ; ++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 = ; i < ; ++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 ;
 }

输出结果：

在测试代码中，函数返回值是容器的执行速度比容器作为参数传递要慢的多。

可以看到返回容器的函数里，容器频繁的创建销毁。

容器作为参数传递是项目中常见做法，很少看到函数返回容器。原因就在此。