C++ 中的 new、malloc() 和 free() 、delete

我们在 C++ 中使用 new 和 delete 运算符来动态分配内存，而 malloc() 和 free() 函数在 C 和 C++ 中也用于相同目的。new 或 malloc() 和 delete 或 free() 的功能似乎相同，但它们在不同方面有所不同。
构造函数和析构函数调用的行为在以下方面有所不同：
malloc() 与 new()：

malloc()： 它是一个 C 库函数，也可以在 C++ 中使用，而new运算符仅适用于 C++。
malloc() 和 new 都用于在堆中动态分配内存。但是new确实调用了类的构造函数，而malloc()没有。

下面是说明 new 和 malloc() 功能的程序：

#include "bits/stdc++.h"/nusing namespace std;

// Class A
class A {
    int a;

public:
    int* ptr;

    // Constructor of class A
    A()
    {
        cout << "Constructor was Called!"/n            << endl;
    }
};

// Driver Code
int main()
{
    A* a = new A;
    cout << "Object of class A was "/n        << "created using new operator!"/n        << endl;
    A* b = (A*)malloc(sizeof(A));
    cout << "Object of class A was "/n        << "created using malloc()!"/n        << endl;

    return 0;
}

运行结果：

Constructor was Called!
Object of class A was created using new operator!
Object of class A was created using malloc()!

在上面的程序中，可以清楚地看到，在使用 new 运算符创建对象时，调用了默认构造函数，而使用 malloc 函数未调用默认构造函数。

free()和delete：

free() 是一个 C 库函数，也可以在 C++ 中使用，而delete是 C++ 关键字。
free() 释放内存但不调用类的析构函数，而delete释放内存并调用类的析构函数。

下面是说明 new 和 malloc() 函数的程序：

#include "bits/stdc++.h"/nusing namespace std;

// Class A
class A {
    int a;

public:
    int* ptr;

    // Constructor of class A
    A()
    {
        cout << "Constructor was Called!"/n            << endl;
    }

    // Destructor of class A
    ~A()
    {
        cout << "Destructor was Called!"/n            << endl;
    }
};

// Driver Code
int main()
{
    // Create an object of class A
    // using new operator
    A* a = new A;
    cout << "Object of class A was "/n        << "created using new operator!"/n        << endl;

    delete (a);
    cout << "Object of class A was "/n        << "deleted using delete keyword!"/n        << endl;

    cout << endl;

    A* b = (A*)malloc(sizeof(A));
    cout << "Object of class A was "/n        << "created using malloc()!"/n        << endl;

    free(b);
    cout << "Object of class A was "/n        << "deleted using free()!"/n        << endl;

    return 0;
}

运行结果如下：

Constructor was Called!
Object of class A was created using new operator!
Destructor was Called!
Object of class A was deleted using delete keyword!

Object of class A was created using malloc()!
Object of class A was deleted using free()!

以下是更多的程序示例：
程序A：

// C++ program to illustrate new, delete
// malloc() and free()
#include "bits/stdc++.h"/nusing namespace std;

// Class A
class A {
    int a;

public:
    int* ptr;

    // Constructor of class A
    A()
    {
        cout << "Constructor was Called!"/n            << endl;
    }

    // Destructor of class A
    ~A()
    {
        cout << "Destructor was Called!"/n            << endl;
    }
};

// Driver Code
int main()
{

    // Object Created of class A
    A a;
    return 0;
}

运行结果：

Constructor was Called!
Destructor was Called!

在上面的程序中，即使没有使用删除操作符，析构函数仍然被调用。析构函数调用的原因是语句return 0。该语句在主函数中执行时会调用为其创建对象的每个类的析构函数。
为了避免析构函数调用，可以将语句return 0替换为exit(0)。下面是相同的代码：
程序代码示例二：

// C++ program to illustrate new, delete
// malloc() and free()
#include "bits/stdc++.h"/nusing namespace std;

// Class A
class A {
    int a;

public:
    int* ptr;

    // Constructor of class A
    A()
    {
        cout << "Constructor was Called!"/n            << endl;
    }

    // Destructor of class A
    ~A()
    {
        cout << "Destructor was Called!"/n            << endl;
    }
};

// Driver Code
int main()
{

    // Object Created of class A
    A a;
    exit(0);
}

运行结果：

Constructor was Called!

示例代码三：

// C++ program to illustrate new, delete
// malloc() and free()
#include "bits/stdc++.h"/nusing namespace std;

// Class A
class A {
    int a;

public:
    int* ptr;

    // Constructor of class A
    A()
    {
        cout << "Constructor was Called!"/n            << endl;
    }

    // Destructor of class A
    ~A()
    {
        cout << "Destructor was Called!"/n            << endl;
    }
};

// Driver Code
int main()
{

    // Object Created of class A
    A *a = new A;
    return 0;
}

运行结果：

Constructor was Called!

即使使用了return 0语句，也没有析构函数调用。原因在于分配类的对象的不同。当我们在一个块中创建一个类名 object_name 的对象时，该对象有一个自动存储持续时间，即它会在超出范围时自动销毁。但是当使用 new class_name 时，该对象具有动态存储持续时间，这意味着必须使用 delete 关键字显式删除它。