vector中push_back和emplace

vector中push_back和emplace_back区别

区别测试代码
vector空间自动增长测试代码

正常情况下push_back是往vector中添加新的元素，只不过添加过程是先利用拷贝构造函数复制目标值，而 emplace_back可以直接在目标位置上生成对象，这也正式emplace的原本放置的意思。
具体在使用上，如果push的对象元素是已经生成好了的（如Nelson），emplace_back和push_back可以认为没有区别，如果原本没有要push的这个元素，则直接原地生成效率较高;

实际在push的时候，由于vector内存需要连续,在vector size达到1, 2,4,8…时其长度需要自动增加，故四二次push时首先生成目标元素，检测到现有存储空间已满，会调用复制构造函数来重头到尾复制前面的vector元素，依次push进去vector之后再析构掉复制的元素，最后再将目标元素push进去;

一般移动构造函数的输入为右值，操作完成后右值内容就为空了（数字除外），具体过程可由std::move(左值) 进行模仿.
如果传入的为右值但没有定义移动构造函数，则可调用拷贝构造函数。即emplace_back和push_back均可接收左值（普通变量）和右值（无名临时对象），左值调用复制构造函数，右值调用移动构造函数，表现一致;

    President joy("joy ", "USA", 2021);elections.emplace_back(std::move(joy));elections.emplace_back(std::move(joy));elections.emplace_back(std::move(joy));
//输出结果
joy  elected president of USA in 2021.elected president of         in 2021.elected president of            in 2021.

emplace_back结果
elections.emplace_back(“Nelson Mandela”, “South Africa”, 1994)
I am being constructed.
elections.emplace_back(Nelson);
I am being constructed.
I am being copy constructed.
moved:
joy USA2021
elections.emplace_back(std::move(Nelson));
I am being constructed.
I am being moved.
moved:
2021

push_back结果
reElections.push_back(President(“Franklin Delano Roosevelt”, “the USA”, 1936))
I am being constructed.
I am being moved.
I am being destructed.
reElections.push_back(obama);
I am being constructed.
I am being copy constructed.
moved:
Franklin USA1966
reElections.push_back(std::move(obama));
I am being constructed.
I am being moved.
moved:
1966

区别测试代码

这里是参考博客 ().

// An highlighted block
#include <vector>
#include <string>
#include <iostream>  struct President
{  std::string name;  std::string country;  int year;  President(std::string p_name, std::string p_country, int p_year) //构造函数:name(p_name), country(p_country), year(p_year)  {  std::cout << "I am being constructed.\n"; }President(const President& other)  //拷贝构造函数:name(other.name), country(other.country), year(other.year){ std::cout << "I am being copy constructed.\n";}President(President&& other)        //移动构造函数:name(std::move(other.name)), country(std::move(other.country)), year(other.year)  { std::cout << "I am being moved.\n"; }  President& operator=(const President& other);  ~President(){ std::cout << "I am being destructed.\n";}
};  int main()
{  std::vector<President> elections;  std::cout << "emplace_back:\n";  elections.emplace_back("Nelson ", "frica", 1994); //没有类的移动elections.emplace_back("Nelson ", "Africa", 1995); elections.emplace_back("Nelson ", "Africa", 1996); President joy("joy ", "USA", 2021);elections.emplace_back(std::move(joy));std::vector<President> reElections;  std::cout << "\npush_back:\n";  reElections.push_back(President("Franklin ", "USA", 1936));  reElections.push_back(President("Franklin ", "USA", 1937));reElections.push_back(President("Franklin ", "USA", 1938));President obama("obama ", "USA", 1966);reElections.push_back(obama);std::cout << "moved:\n" << obama.name<< obama.country << obama.year <<std::endl;std::cout << "\nContents:\n";  for (President const& president: elections) {  std::cout << president.name << " elected president of "  << president.country << " in " << president.year << ".\n";  }  for (President const& president: reElections) {  std::cout << president.name << " re-elected president of "  << president.country << " in " << president.year << ".\n";  }
}

结果输出
emplace_back:
I am being constructed.
I am being constructed.
I am being copy constructed.
I am being destructed.
I am being constructed.
I am being copy constructed.
I am being copy constructed.
I am being destructed.
I am being destructed.

push_back:
I am being constructed.
I am being moved.
I am being destructed.
I am being constructed.
I am being moved.
I am being copy constructed.
I am being destructed.
I am being destructed.
I am being constructed.
I am being moved.
I am being copy constructed.
I am being copy constructed.
I am being destructed.
I am being destructed.
I am being destructed.

Contents:
Nelson elected president of frica in 1994.
Nelson elected president of Africa in 1995.
Nelson elected president of Africa in 1996.
Franklin re-elected president of USA in 1936.
Franklin re-elected president of USA in 1937.
Franklin re-elected president of USA in 1938.
I am being destructed.
I am being destructed.
I am being destructed.
I am being destructed.
I am being destructed.
I am being destructed.

vector空间自动增长测试代码

#include <iostream>
#include <vector>using std::cout;
using std::endl;struct X{X(int a) {x=a;cout << "调用构造函数：X()" << a << endl;}X(const X& Xa) {x=Xa.x; cout << "调用拷贝构造函数：X(const X&)" << x << endl;}~X(){cout << "调用析构函数：~X()" << x << endl;}int x;
};int main(int argc, char **argv)
{cout << "定义局部变量：" << endl;X x(1);X y(2);std::vector<X> vec;cout << "\n存放在容器：" << endl;vec.push_back(y);vec.push_back(x);vec.push_back(y);cout << endl;cout << "程序结束！！！" << endl;return 0;
}

输出结果

定义局部变量：
调用构造函数：X()1
调用构造函数：X()2
存放在容器：
调用拷贝构造函数：X(const X&)2
调用拷贝构造函数：X(const X&)1
调用拷贝构造函数：X(const X&)2
调用析构函数：~X()2
调用拷贝构造函数：X(const X&)2
调用拷贝构造函数：X(const X&)2
调用拷贝构造函数：X(const X&)1
调用析构函数：~X()2
调用析构函数：~X()1
程序结束！！！
调用析构函数：~X()2
调用析构函数：~X()1
调用析构函数：~X()2
调用析构函数：~X()2
调用析构函数：~X()1