极客教程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。