There are three ways to use NEW: plain New, nothrow new, placement new.

1.plain New/delete. Ordinary New
The definition is as follows:
void *operator New (std::size_t) throw (Std::bad_alloc);

void operator Delete (void*) throw ();

Note: Standard C + + plain throws a standard exception std::bad_alloc instead of returning null when new fails, so it is futile to check that the return value is null to determine whether the assignment succeeded.


2.nothrow New/delete does not throw an exception in the form of operator new, which returns NULL when new fails.

The definition is as follows:

void *operator New (Std::size_t,const std::nothrow_t&) throw ();

void operator Delete (void*) throw ();

struct nothrow_t{}; Const nothrow_t Nothrow;//nothrow As the symbolic dummy of new

Test program:

#include "stdafx.h"
#include <iostream>
#include <new>
using namespace Std;

Char *getmemory (unsigned long size)
{
Char *p=new (nothrow) char[size];//allocation failed, return null
if (null==p)
cout<< "Alloc failure!" <<endl;
return p;
}

int main ()
{
Try
{
Char *p=getmemory (10E11);
//...........
if (p==null)
cout<< "Failure" <<endl;
delete [] p;

}
catch (const Std::bad_alloc &AMP;EX)
{
Cout<<ex.what () <<endl;
}

return 0;
}


The main purpose of 3.placement New/delete is to construct different types of objects or their arrays repeatedly using a large dynamically allocated memory. For example, you can request an array of characters that is large enough, and then construct different types of objects or arrays on top of it when needed. Placement new does not have to worry about memory allocation failure because it does not allocate memory at all, it simply invokes the object's constructor.

Note: objects or arrays constructed with placement new will be explicitly called to destroy (destructors do not release the object's memory), and do not use Delete. This is because placement The new constructed object or array size does not necessarily equal the size of the original allocated memory, and using delete causes a memory leak or a run-time error when memory is released later.