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++ 中也用于相同的目的。 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。
关于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”确实会调用类的构造函数，而“malloc()”则不会。
下面是说明 new 和 malloc() 功能的程序：
CPP
// C++ program to illustrate malloc()
// and new operator in C++
#include "bits/stdc++.h"
using namespace std;
// Class A
class A {
int a;
public:
int* ptr;
// Constructor of class A
A()
{
cout << "Constructor was Called!"
<< endl;
}
};
// Driver Code
int main()
{
// Create an object of class A
// using new operator
A* a = new A;
cout << "Object of class A was "
<< "created using new operator!"
<< endl;
// Create an object of class A
// using malloc operator
A* b = (A*)malloc(sizeof(A));
cout << "Object of class A was "
<< "created using malloc()!"
<< endl;
return 0;
}CPP
// C++ program to illustrate free()
// and delete keyword in C++
#include "bits/stdc++.h"
using namespace std;
// Class A
class A {
int a;
public:
int* ptr;
// Constructor of class A
A()
{
cout << "Constructor was Called!"
<< endl;
}
// Destructor of class A
~A()
{
cout << "Destructor was Called!"
<< endl;
}
};
// Driver Code
int main()
{
// Create an object of class A
// using new operator
A* a = new A;
cout << "Object of class A was "
<< "created using new operator!"
<< endl;
delete (a);
cout << "Object of class A was "
<< "deleted using delete keyword!"
<< endl;
cout << endl;
A* b = (A*)malloc(sizeof(A));
cout << "Object of class A was "
<< "created using malloc()!"
<< endl;
free(b);
cout << "Object of class A was "
<< "deleted using free()!"
<< endl;
return 0;
}CPP
// C++ program to illustrate new, delete
// malloc() and free()
#include "bits/stdc++.h"
using namespace std;
// Class A
class A {
int a;
public:
int* ptr;
// Constructor of class A
A()
{
cout << "Constructor was Called!"
<< endl;
}
// Destructor of class A
~A()
{
cout << "Destructor was Called!"
<< endl;
}
};
// Driver Code
int main()
{
// Object Created of class A
A a;
return 0;
}CPP
// C++ program to illustrate new, delete
// malloc() and free()
#include "bits/stdc++.h"
using namespace std;
// Class A
class A {
int a;
public:
int* ptr;
// Constructor of class A
A()
{
cout << "Constructor was Called!"
<< endl;
}
// Destructor of class A
~A()
{
cout << "Destructor was Called!"
<< endl;
}
};
// Driver Code
int main()
{
// Object Created of class A
A a;
exit(0);
}CPP
// C++ program to illustrate new, delete
// malloc() and free()
#include "bits/stdc++.h"
using namespace std;
// Class A
class A {
int a;
public:
int* ptr;
// Constructor of class A
A()
{
cout << "Constructor was Called!"
<< endl;
}
// Destructor of class A
~A()
{
cout << "Destructor was Called!"
<< endl;
}
};
// Driver Code
int main()
{
// Object Created of class A
A *a = new A;
return 0;
}输出：
Constructor was Called!
Object of class A was created using new operator!
Object of class A was created using malloc()!在上面的程序中我们可以清楚地看到，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函数没有调用默认构造函数。
free() 与删除：
- free()是一个 C 库函数，也可以在 C++ 中使用，而“delete”是一个 C++ 关键字。
- free() 释放内存但不调用类的析构函数，而“删除”释放内存并调用类的析构函数。
下面是说明 new 和 malloc() 功能的程序：
CPP
// C++ program to illustrate free()
// and delete keyword in C++
#include "bits/stdc++.h"
using namespace std;
// Class A
class A {
int a;
public:
int* ptr;
// Constructor of class A
A()
{
cout << "Constructor was Called!"
<< endl;
}
// Destructor of class A
~A()
{
cout << "Destructor was Called!"
<< endl;
}
};
// Driver Code
int main()
{
// Create an object of class A
// using new operator
A* a = new A;
cout << "Object of class A was "
<< "created using new operator!"
<< endl;
delete (a);
cout << "Object of class A was "
<< "deleted using delete keyword!"
<< endl;
cout << endl;
A* b = (A*)malloc(sizeof(A));
cout << "Object of class A was "
<< "created using malloc()!"
<< endl;
free(b);
cout << "Object of class A was "
<< "deleted using free()!"
<< endl;
return 0;
}
输出：
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()!以下是更多插图的程序：
方案一：
CPP
// C++ program to illustrate new, delete
// malloc() and free()
#include "bits/stdc++.h"
using namespace std;
// Class A
class A {
int a;
public:
int* ptr;
// Constructor of class A
A()
{
cout << "Constructor was Called!"
<< endl;
}
// Destructor of class A
~A()
{
cout << "Destructor was Called!"
<< endl;
}
};
// Driver Code
int main()
{
// Object Created of class A
A a;
return 0;
}
输出：
Constructor was Called!
Destructor was Called!在上面的程序中，即使没有使用 delete运算符，仍然会调用析构函数。析构函数调用的原因是语句“return 0” 。该语句在主函数执行时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，我们可以将语句“return 0”替换为“exit(0)”。下面是相同的代码：
方案二：
CPP
// C++ program to illustrate new, delete
// malloc() and free()
#include "bits/stdc++.h"
using namespace std;
// Class A
class A {
int a;
public:
int* ptr;
// Constructor of class A
A()
{
cout << "Constructor was Called!"
<< endl;
}
// Destructor of class A
~A()
{
cout << "Destructor was Called!"
<< endl;
}
};
// Driver Code
int main()
{
// Object Created of class A
A a;
exit(0);
}
输出：
Constructor was Called!方案三：
CPP
// C++ program to illustrate new, delete
// malloc() and free()
#include "bits/stdc++.h"
using namespace std;
// Class A
class A {
int a;
public:
int* ptr;
// Constructor of class A
A()
{
cout << "Constructor was Called!"
<< endl;
}
// Destructor of class A
~A()
{
cout << "Destructor was Called!"
<< endl;
}
};
// Driver Code
int main()
{
// Object Created of class A
A *a = new A;
return 0;
}
输出：
Constructor was Called!即使在使用语句“return 0”之后也没有析构函数调用。原因在于分配一个类的对象不同。当我们在一个块中创建一个带有class_name object_name的对象时，该对象有一个自动存储期，即它会在超出范围时自动销毁。但是当我们使用new class_name 时，该对象具有动态存储期限，这意味着必须使用delete关键字显式删除它。
想要从精选的视频和练习题中学习，请查看C++ 基础课程，从基础到高级 C++ 和C++ STL 课程，了解基础加 STL。要完成从学习语言到 DS Algo 等的准备工作，请参阅完整的面试准备课程。