C++ std::vector emplace_back 优于 push_back 的理由

#include <iostream>
#include <vector>
#include <chrono>
#include <windows.h>

using namespace std;
using namespace std::chrono;

// 要考虑 copy ctor 和 copy opt=

class Item
{
public:
	Item() = delete;

	Item(const Item & ths);

	Item(std::string str);

	Item & operator = (const Item & ths);

	~Item();

private:

};

Item::Item(const Item & ths)
{
	//cout << "copy ctor" << endl;
	Sleep(2);
}

Item::Item(std::string str)
{
	//cout << str.c_str() << endl;
	Sleep(2);
}

Item & Item::operator=(const Item & ths)
{
	//cout << "copy opt=" << endl;
	Sleep(2);
}

Item::~Item()
{
}

int main() 
{ 
	{
		auto tbegin = steady_clock::now();
		std::vector<Item> vec8;
		for (size_t i = 0; i < 2000; ++i)
			vec8.push_back(Item("111"));


		auto used = duration_cast<milliseconds>(steady_clock::now() - tbegin);
		std::cout << used.count() << "ms" << std::endl;
	}

	{
		auto tbegin = steady_clock::now();
		std::vector<Item> vec8;
		for (size_t i = 0; i < 2000; ++i)
			vec8.emplace_back("222"); // 少一次 copy ctor 的调用

		auto used = duration_cast<milliseconds>(steady_clock::now() - tbegin);
		std::cout << used.count() << "ms" << std::endl;
	}

}