malloc，free是一种库函数，不能被重载，new和delete是运算符，可以被重载
new和delete内部都调用malloc和free函数

new的三种用法：
1.new，不可以被重载直接new一个对象T* ptr = new T();new一个对象时做了两件事情
* 调用了operator new申请空间
* 调用了构造函数，如果括号中为空，会调用默认构造函数2.operator new，可以被重载重载形式：必须为void* operator new(size_t n, . . .); 行为类似于malloc，就是申请一段空间重载后new一个对象会调用这个函数3.placement new原地构造一个对象，相当于显示调用构造函数new (T*) T(args);
使用方式如下：A *pointer = (A*)malloc(sizeof(A));new(pointer)A(10);cout << pointer->i << endl;

delete：类似于new
会首先调用析构函数析构，然后调用free()释放空间
可以被重载，重载形式如下：void operator delete(void* ptr, . . .);

new[]:
首先调用operator new[]申请n段连续的T空间，然后会调用n次默认构造函数，可以被重载
重载形式如下：void* operator new[](size_t n, . . .);
如果重载了此函数，new[]会调用这个函数

delete[]:
与new[]配套使用，会先调用n次析构函数，然后再释放空间，可以被重载
重载形式如下：void operator delete[](void* ptr, . . .);
如果重载了此函数，delete[]会调用这个函数。

测试代码：#include <iostream>
using namespace std;
class A
{
public:A()      : i(1) { cout << "A default constructor" << endl; }A(int j) : i(j) { cout << "A constructor"        << endl; }~A()            { cout << "A destructor"          << endl; }int i;
};class B
{
public:B()      : i(1) { cout << "B default constructor" << endl; }B(int j) : i(j) { cout << "B constructor"        << endl; }~B()            { cout << "B destructor"          << endl; }void* operator new(size_t n){cout << "B my new" << endl;B* p = (B*)malloc(n);return p;}void* operator new[](size_t n){cout << "B my new[]" << endl;B* p = (B*)malloc(n * sizeof(B));return p;}void operator delete(void* ptr){cout << "B my delete" << endl;free(ptr);}void operator delete[](void* ptr){cout << "B my delete[]" << endl;free(ptr);}int i;
};int main()
{//---------------------------A---------------------------//
    A *ptr_A_1 = new A;cout << ptr_A_1->i << endl;delete ptr_A_1;cout << endl;A *ptr_A_2 = new A();cout << ptr_A_2->i << endl;delete ptr_A_2;cout << endl;A *ptr_A_3 = new A(2);cout << ptr_A_3->i << endl;delete ptr_A_3;cout << endl;A *ptr_A_4 = new A[3];auto p = ptr_A_4;for(int i = 0; i < 3; ++i) cout << (p + i)->i << " ";cout << endl;delete[] ptr_A_4;cout << endl;//------------------------B-------------------------//
    B *ptr_B_1 = new B;cout << ptr_B_1->i << endl;delete ptr_B_1;cout << endl;B *ptr_B_2 = new B();cout << ptr_B_2->i << endl;delete ptr_B_2;cout << endl;B *ptr_B_3 = new B(2);cout << ptr_B_3->i << endl;delete ptr_B_3;cout << endl;B *ptr_B_4 = new B[3];auto p1 = ptr_B_4;for(int i = 0; i < 3; ++i) cout << (p1 + i)->i << " ";cout << endl;delete[] ptr_B_4;cout << endl;return 0;
}