2021-03-01

C++ new 動態記憶體配置用法與範例

本篇 ShengYu 介紹 C++ new 動態記憶體配置 / delete 釋放記憶體用法，

以下 C++ new 動態記憶體配置內容將分為這幾部份，

new int 的用法
new int 一維陣列
new int 二維陣列
new struct 範例
new struct 一維陣列範例

new int 的用法

以下是動態記憶體配置一個 int 的範例

1	int *p = new int;

也可以加上小括號這樣寫，

1	int *p = new int();

如果要分成兩行寫的話，

1 2	int *p; p = new int();

如果要順便設定這個 int 的初始值的話，可以在 int 的建構子傳入預設值，示範一下如果我要初始值為 5 的用法，

1	int *p = new int(5);

當變數用完後很重要的一件事就是將這個動態配置記憶體的 int 釋放，以下為釋放記憶體的寫法，

delete p;

來看看實際範例吧！

cpp-new-delete.cpp

// g++ cpp-new-delete.cpp -o a.out
#include <iostream>
using namespace std;

int main() {
    int *p = new int();
    *p = 10;
    cout << p << "\n";
    cout << *p << "\n";
    delete p;

    int *p2 = new int(15);
    cout << p2 << "\n";
    cout << *p2 << "\n";
    delete p2;
    
    return 0;
}

輸出如下，

new int 一維陣列

這邊示範動態記憶體配置一個 int 一維陣列的寫法，new 陣列是要用中括號 []，不要跟上述的小括號 () 混淆唷，這兩個可是天差地遠，

1
2
3

int *p = new int[3]; // 不要寫成 int *p = new int(3); 唷！
...
delete[] p;

如果要分成兩行寫的話，

int *p;
p = new int[3];
...
delete[] p;

再來看看怎麼配置一維陣列時同時給初始值，

cpp-new-delete2.cpp

// g++ cpp-new-delete2.cpp -o a.out
#include <iostream>
using namespace std;

int main() {
    int *p1 = new int[3](); // 0,0,0
    int *p2 = new int[3]{1,2,3}; // 1,2,3

    cout << p1[0] << "," << p1[1] << "," << p1[2] << "\n";
    cout << p2[0] << "," << p2[1] << "," << p2[2] << "\n";
    
    delete[] p1;
    delete[] p2;

    return 0;
}

輸出如下，

1 2	0,0,0 1,2,3

new int 二維陣列

動態配置二維陣列這個通常會在影像處理中使用到這個技巧，假設我們要配置 3 * 4 大小的 int 二維陣列，注意在使用完該變數後還是要將其變數 delete 歸還記憶體，二維陣列怎麼 new 的，delete 時就怎麼 delete。

cpp-new-delete3.cpp

// g++ cpp-new-delete3.cpp -o a.out
#include <iostream>
using namespace std;

int main() {
    int **p = new int*[3];
    for (int i = 0; i < 3; i++) {
        p[i] = new int[4]{0};
    }

    p[0][1] = 1;
    p[1][1] = 2;
    p[2][1] = 3;
    
    for (int i = 0; i < 3; i++) {
        for (int j = 0; j < 4; j++) {
            cout << p[i][j] << " ";
        }
        cout << "\n";
    }
    
    for (int i = 0; i < 3; i++) {
        delete[] p[i];
    }
    delete[] p;

    return 0;
}

二維陣列輸出結果如下，

1
2
3

0 1 0 0 
0 2 0 0 
0 3 0 0

new struct 範例

這邊示範怎麼 new struct，以 Point 為例，

cpp-new-delete4.cpp

// g++ cpp-new-delete4.cpp -o a.out
#include <iostream>
using namespace std;

struct Point {
    int x;
    int y;
};

int main() {
    Point *p1 = new Point;
    Point *p2 = new Point();
    //Point *p3 = new Point(3); // error 沒有對應的建構子
    Point *p4 = new Point({10,20});

    cout << p1->x << "," << p1->y << "\n";
    cout << p2->x << "," << p2->y << "\n";
    cout << p4->x << "," << p4->y << "\n";

    delete p1;
    delete p2;
    delete p4;

    return 0;
}

輸出如下，

1
2
3

0,0
0,0
10,20

再舉個 Student 例子，

cpp-new-delete5.cpp

// g++ cpp-new-delete5.cpp -o a.out
#include <iostream>
#include <string>
using namespace std;

struct Student {
    int id;
    string name;
};

int main() {
    Student *p1 = new Student;
    Student *p2 = new Student();
    //Student *p3 = new Student(3); // error 沒有對應的建構子
    Student *p4 = new Student({10, "Amy"});

    cout << p1->id << "," << p1->name << "\n";
    cout << p2->id << "," << p2->name << "\n";
    cout << p4->id << "," << p4->name << "\n";

    delete p1;
    delete p2;
    delete p4;

    return 0;
}

輸出結果如下，

1
2
3

0,
0,
10,Amy

new struct 一維陣列範例

這邊示範怎麼 new struct 一維陣列，以 Point 為例，

cpp-new-delete6.cpp

// g++ cpp-new-delete6.cpp -o a.out
#include <iostream>
using namespace std;

struct Point {
    int x;
    int y;
};

int main() {
    Point *p1 = new Point[3];
    Point *p2 = new Point[3]{{1,2}, {3,4}, {5,6}};

    cout << p1[0].x << "," << p1[0].y << "  " 
        << p1[1].x << "," << p1[1].y << "  " 
        << p1[2].x << "," << p1[2].y << "\n";
    cout << p2[0].x << "," << p2[0].y << "  "
        << p2[1].x << "," << p2[1].y << "  " 
        << p2[2].x << "," << p2[2].y << "\n";

    delete[] p1;
    delete[] p2;

    return 0;
}

輸出如下，

1 2	0,0 0,0 0,0 1,2 3,4 5,6

再舉個 Student 例子，

cpp-new-delete7.cpp

// g++ cpp-new-delete7.cpp -o a.out
#include <iostream>
#include <string>
using namespace std;

struct Student {
    int id;
    string name;
};

int main() {
    Student *p1 = new Student[3];
    Student *p2 = new Student[3]{{10, "Amy"}, {11, "Sam"}, {12, "Tom"}};

    cout << p1[0].id << "," << p1[0].name << "|" 
        << p1[1].id << "," << p1[1].name << "|" 
        << p1[2].id << "," << p1[2].name << "\n";
    cout << p2[0].id << "," << p2[0].name << "|"
        << p2[1].id << "," << p2[1].name << "|" 
        << p2[2].id << "," << p2[2].name << "\n";

    delete[] p1;
    delete[] p2;

    return 0;
}

輸出結果如下，

1 2	0,\|0,\|0, 10,Amy\|11,Sam\|12,Tom

以上就是 C++ new 動態記憶體配置用法與範例的介紹，
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