我们在C++中使用new和delete运算符来动态分配内存，而在C和C++中，malloc()和free()函数也用于相同的目的。 new或malloc()以及delete或free()的功能似乎相同，但是它们在各种方式上有所不同。
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()：这是一个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均用于在堆中动态分配内存。但是“ new”确实调用了一个类的构造函数，而“ malloc()”却没有。
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，在使用新运算符Default Constructor创建对象时，未调用malloc函数Default Constructor。
free()与delete：
- 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Destructor，而“ delete”释放内存，还调用类的Destructor。
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：
程序1：
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，即使不使用delete运算符，析构函数仍会被调用。析构函数调用的原因是语句“ return 0” 。在主函数执行该语句时，将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：
程式2：
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!程序3：
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!即使使用语句“ return 0”，也不会进行Destructor调用。原因在于分配类的对象的不同。当我们创建一个具有class_name object_name的对象时，该对象具有自动存储期限，即，超出范围时将自动销毁该对象。但是，当我们使用新的class_name时，该对象具有动态存储期限，这意味着必须使用delete关键字显式删除该对象。
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