std::vector

1. 始めに (push_back)

[概要]

ja.cppreference.com に記載されているサンプルプログラムから作成してみます。
push_back() を使う例です。

https://ja.cppreference.com/w/cpp/container/vector

[環境]

[プログラムソース]

#include <iostream>
#include <vector>
 
int main()
{
    // Create a vector containing integers
    std::vector<int> v = {7, 5, 16, 8};
 
    // Add two more integers to vector
    v.push_back(25);
    v.push_back(13);
 
    // Iterate and print values of vector
    for(int n : v) {
        std::cout << n << '\n';
    }
}

[出力]

7
5
16
8
25
13 

2. insert

[概要]

同じく ja.cppreference.com に記載されているサンプルプログラムから insert() による要素の挿入を行うサンプルを作成してみます。

https://ja.cppreference.com/w/cpp/container/vector/insert

[環境]

[プログラムソース]

#include <iostream>
#include <vector>

void print_vec(const std::vector<int>& vec)
{
    for (auto x : vec) {
        std::cout << ' ' << x;
    }
    std::cout << '\n';
}

int main()
{
    std::vector<int> vec(3, 100);
    print_vec(vec);

    auto it = vec.begin();
    it = vec.insert(it, 200);
    print_vec(vec);

    vec.insert(it, 2, 300);
    print_vec(vec);

    // "it" no longer valid, get a new one:
    it = vec.begin();

    std::vector<int> vec2(2, 400);
    vec.insert(it + 2, vec2.begin(), vec2.end());
    print_vec(vec);

    int arr[] = { 501,502,503 };
    vec.insert(vec.begin(), arr, arr + 3);
    print_vec(vec);
}

[出力]

100 100 100
200 100 100 100
300 300 200 100 100 100
300 300 400 400 200 100 100 100
501 502 503 300 300 400 400 200 100 100 100

3. emplace_back (c++11)

[概要]

emplace_back を使った要素を挿入する例を示します。

emplace_back, emplace_front は c++11 以降で使用可能です。
push_back を使用したときに必要なコピーやムーブを回避するために、どのように emplace_back を使用すれば良いかを示します。

[環境]

[プログラムソース]

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


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) noexcept
        : 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;
};

std::ostream& operator << (std::ostream& os, const President& president)
{
    os << president.name << ", " << president.country << ", " << president.year;
    return os;
}


int main()
{
    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 << "\nElections:" << std::endl;
    for (auto const& president : elections) {
        std::cout << president << std::endl;
    }
    std::cout << "\nRe-Elections:" << std::endl;
    for (auto const& president : reElections) {
        std::cout << president << std::endl;
    }


    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";
    }
}

[出力]

emplace_back:
I am being constructed.

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

Elections:
Nelson Mandela, South Africa, 1994

Re-Elections:
Franklin Delano Roosevelt, the USA, 1936

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

4. emplace (c++11)

[概要]

emplace を使った要素を挿入する例を示します。

[環境]

[プログラムソース]

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

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) noexcept
        : 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;
};

std::ostream& operator << (std::ostream& os, const President& president)
{
    os << president.name << ", " << president.country << ", " << president.year;
    return os;
}


int main()
{
    std::vector<President> elections;
    std::cout << "emplace_back:\n";
    elections.emplace_back("Nelson Mandela", "South Africa", 1994);
    elections.emplace_back("Franklin Delano Roosevelt", "the USA", 1936);
    elections.emplace_back("Foo", "Japan", 2020);

    std::cout << "\nElections:" << std::endl;
    for (auto const& president : elections) {
        std::cout << president << std::endl;
    }

    elections.emplace(elections.begin()+1, "Boo", "Japan", 2021);

    std::cout << "\nElections:" << std::endl;
    for (auto const& president : elections) {
        std::cout << president << std::endl;
    }
}

 [出力]

emplace_back:
I am being constructed.
I am being constructed.
I am being moved.
I am being constructed.
I am being moved.
I am being moved.

Elections:
Nelson Mandela, South Africa, 1994
Franklin Delano Roosevelt, the USA, 1936
Foo, Japan, 2020
I am being constructed.
I am being moved.
I am being moved.
I am being moved.

Elections:
Nelson Mandela, South Africa, 1994
Boo, Japan, 2021
Franklin Delano Roosevelt, the USA, 1936
Foo, Japan, 2020

補足：
新しい size() が現在の capacity() より大きいため、vectorの再構築が発生し、コンテナ内アイテムの移動が発生しているようです。
アイテムの挿入が頻繁に発生する場合、reserve()で必要な領域を事前確保しておく、または std::list を使用する、などを検討することでペナルティを減らせる場合があります。

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