概述

為了在容器操作時(shí)盡可能的減少構(gòu)造函數(shù)的調(diào)用和內(nèi)存的拷貝，C++11 引入了emplace_back的方法乓序，該方法可以改善往容器內(nèi)推入元素對(duì)象時(shí)的效率丹鸿。相比push_back付鹿， push_front等成員函數(shù)蜜氨，它可以節(jié)省一次拷貝構(gòu)造函數(shù)的調(diào)用從而提高插入效率。

代碼演示

#include <iostream>
#include <string>
#include <vector>

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(const President& other)
        : name(std::move(other.name)), country(std::move(other.country)), year(other.year)
    {
        std::cout << "I am being copy 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);
};

int main()
{
    std::vector<President> elections;
    std::cout << "emplace_back:\n";
    elections.emplace_back("Nelson Mandela", "South Africa", 1994); //沒有類的創(chuàng)建

    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";
    }
}

執(zhí)行結(jié)果

g++ emplace_back_push_back.cc --std=c++11

guanyunfei@ivan:~/practice/c++$ ./a.out
emplace_back:
I am being constructed.

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

Contents:
Nelson Mandela was elected president of South Africa in 1994.
Franklin Delano Roosevelt was re-elected president of the USA in 1936.

結(jié)果分析

? 從結(jié)果看稠诲，確實(shí)在調(diào)用emplace_back成員函數(shù)的時(shí)候少了一次拷貝構(gòu)造函數(shù)或者移動(dòng)構(gòu)造函數(shù)侦鹏。但是細(xì)心的朋友可以發(fā)現(xiàn)诡曙，emplace_back()成員函數(shù)調(diào)用的時(shí)候我們是直接用類的參數(shù)列表作為形參的(push_back不能這么使用，它必須接收現(xiàn)成的對(duì)象或者對(duì)象在指針)略水，所以確實(shí)可以少一次拷貝構(gòu)造函數(shù)价卤。那么問題來了，如果我們要把一個(gè)已存在的對(duì)象推入容器時(shí)渊涝，會(huì)不會(huì)調(diào)用呢慎璧？

代碼展示

int main()
{
    std::vector<President> elections;
    std::cout << "emplace_back:\n";
    //elections.emplace_back("Nelson Mandela", "South Africa", 1994); //沒有類的創(chuàng)建
    //elections.emplace_back("Nelson Mandela", "South Africa", 1994); //沒有類的創(chuàng)建

    President Nelson("Nelson Mandela", "South Africa", 1994);
    elections.emplace_back(Nelson); //拷貝構(gòu)造函數(shù)

    std::cout << "\nemplace_back2:\n";
    elections.emplace_back(President("Joseph Robinette Biden", "US", 2021)); //構(gòu)造，移動(dòng)構(gòu)造跨释，拷貝構(gòu)造函數(shù)


    std::vector<President> reElections;
    std::cout << "\npush_back:\n";
    President Trump("Donald Trump", "US", 2016);
    reElections.push_back(Trump);

    std::cout << "\npush_back2:\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";
    }
}

結(jié)果展示

emplace_back:
I am being constructed.
I am being copy constructed.

emplace_back2:
I am being constructed.
I am being moved.
I am being copy constructed.

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

push_back2:
I am being constructed.
I am being moved.
I am being copy constructed.

Contents:
Nelson Mandela was elected president of South Africa in 1994.
Joseph Robinette Biden was elected president of US in 2021.
Donald Trump was re-elected president of US in 2016.
Franklin Delano Roosevelt was re-elected president of the USA in 1936.

結(jié)果分析

? 從結(jié)果可以看出當(dāng)我們推入一個(gè)已經(jīng)存在的對(duì)象時(shí)push_back()和emplace_back()的表現(xiàn)完全一樣都要去調(diào)用拷貝構(gòu)造函數(shù)炸卑。當(dāng)讓臨時(shí)對(duì)象作為形參推入容器時(shí)兩者表現(xiàn)也完全一樣，都要調(diào)用拷貝煤傍，移動(dòng)，拷貝構(gòu)造函數(shù)嘱蛋。

總結(jié)

? emplace_back()之所以可能提高效率時(shí)因?yàn)槠涮峁┝丝梢灾苯虞斎腩悈?shù)列表的功能蚯姆，可以直接在容器內(nèi)部新建這個(gè)對(duì)象。而push_back(),push_front()等函數(shù)不具備這個(gè)凡爾賽功能洒敏。

? 當(dāng)然如果我們想提高效率時(shí)龄恋，也可以讓容器變?yōu)橹羔樔萜鳎綍r(shí)候直接推入指針變量即可凶伙。這樣的話郭毕，每次推入的代價(jià)就由真?zhèn)€類的的拷貝構(gòu)造函數(shù)轉(zhuǎn)為了指針的拷貝，效率會(huì)明顯提升函荣。