C++11中，针对顺序容器(如vector、deque、list)，新标准引入了三个新成员：emplace_front、emplace和emplace_back，这些操作构造而不是拷贝元素。这些操作分别对应push_front、insert和push_back，允许我们将元素放置在容器头部、一个指定位置之前或容器尾部。
当调用push或insert成员函数时，我们将元素类型的对象传递给它们，这些对象被拷贝到容器中。而当我们调用一个emplace成员函数时，则是将参数传递给元素类型的构造函数。emplace成员使用这些参数在容器管理的内存空间中直接构造元素。
emplace函数的参数根据元素类型而变化，参数必须与元素类型的构造函数相匹配。emplace函数在容器中直接构造元素。传递给emplace函数的参数必须与元素类型的构造函数相匹配。
其它容器中，std::forward_list中的emplace_after、emplace_front函数，std::map/std::multimap中的emplace、emplace_hint函数，std::set/std::multiset中的emplace、emplace_hint，std::stack中的emplace函数，等emplace相似函数操作也均是构造而不是拷贝元素。
emplace相关函数可以减少内存拷贝和移动。当插入rvalue，它节约了一次move构造，当插入lvalue，它节约了一次copy构造。
下面是从其他文章中copy的测试代码，详细内容介绍可以参考对应的reference：
#include "emplace.hpp"#include <iostream>#include <vector>#include <string>#include <map>#include <tuple>#include <utility>namespace emplace_ {/// reference: http://www.cplusplus.com/reference/vector/vector/emplace_back/int test_emplace_1(){{/*template <class... Args>void emplace_back (Args&&... args);*/std::vector<int> myvector = { 10, 20, 30 };myvector.emplace_back(100);myvector.emplace_back(200);std::cout << "myvector contains:";for (auto& x : myvector)std::cout << ' ' << x;std::cout << 'n';}{/*template <class... Args>iterator emplace (const_iterator position, Args&&... args);*/std::vector<int> myvector = { 10, 20, 30 };auto it = myvector.emplace(myvector.begin() + 1, 100);myvector.emplace(it, 200);myvector.emplace(myvector.end(), 300);std::cout << "myvector contains:";for (auto& x : myvector)std::cout << ' ' << x;std::cout << 'n';}return 0;}///// reference: http://en.cppreference.com/w/cpp/container/vector/emplace_backnamespace {struct President {std::string name;std::string country;int year;President(std::string p_name, std::string p_country, int p_year): name(std::move(p_name)), country(std::move(p_country)), year(p_year){std::cout << "I am being constructed.n";}President(President&& other): 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) = default;};}int test_emplace_2(){/*The following code uses emplace_back to append an object of type President to a std::vector.It demonstrates how emplace_back forwards parameters to the President constructor and showshow using emplace_back avoids the extra copy or move operation required when using push_back.*/std::vector<President> elections;std::cout << "emplace_back:n";elections.emplace_back("Nelson Mandela", "South Africa", 1994);std::vector<President> reElections;std::cout << "npush_back:n";reElections.push_back(President("Franklin Delano Roosevelt", "the USA", 1936));std::cout << "nContents:n";for (President const& president : elections) {std::cout << president.name << " was elected president of "<< president.country << " in " << president.year << ".n";}for (President const& president : reElections) {std::cout << president.name << " was re-elected president of "<< president.country << " in " << president.year << ".n";}return 0;}// reference: https://stackoverflow.com/questions/4303513/push-back-vs-emplace-backint test_emplace_3(){/*template <class... Args>pair<iterator,bool> emplace (Args&&... args);*/typedef std::tuple<int, double, std::string> Complicated;std::map<int, Complicated> m;int anInt = 4;double aDouble = 5.0;std::string aString = "C++";// cross your finger so that the optimizer is really good//m.insert(/*std::make_pair*/std::pair<int, Complicated>(4, Complicated(anInt, aDouble, aString)));m.insert(std::make_pair(4, Complicated(anInt, aDouble, aString)));// should be easier for the optimizerm.emplace(6, Complicated(anInt, aDouble, aString));/*std::piecewise_construct: This constant value is passed as the first argument to construct a pair objectto select the constructor form that constructs its members in place by forwarding the elements of twotuple objects to their respective constructor.*/m.emplace(std::piecewise_construct, std::make_tuple(8), std::make_tuple(anInt, aDouble, aString));return 0;}//// reference: https://corecplusplustutorial.com/difference-between-emplace_back-and-push_back-function/namespace {class Dat {int i;std::string ss;char c;public:Dat(int ii, std::string s, char cc) :i(ii), ss(s), c(cc) { }~Dat() { }};}int test_emplace_4(){std::vector<Dat> vec;vec.reserve(3);vec.push_back(Dat(89, "New", 'G')); // efficiency lesser//vec.push_back(678, "Newer", 'O'); // error,push_back can’t accept three argumentsvec.emplace_back(890, "Newest", 'D'); // work fine, efficiency is also morereturn 0;}} // namespace emplace_
GitHub： https://github.com/fengbingchun/Messy_Test