c++11 vector使用emplace_back代替push_back

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_back
namespace {
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 shows
        how 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-back
int 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 optimizer
    m.emplace(6, Complicated(anInt, aDouble, aString));
    /*
        std::piecewise_construct: This constant value is passed as the first argument to construct a pair object
        to select the constructor form that constructs its members in place by forwarding the elements of two
        tuple 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 arguments
    vec.emplace_back(890, "Newest", 'D'); // work fine, efficiency is also more
 
    return 0;
}
 
} // namespace emplace_