急!!!哪位大虾知道c++中new的place 语法?

[zhangx105504]

怎样在指定的地址分配内寸.多谢了!!!

[stonespace]

new是一个运算符，可以重载的。

void * CMyClass::operator new(size_t stAllocateBlock)
{
void * ptr=(void *)(0x1003040)
return ptr;
}

[puppet]

摘自more effective c++
Placement new

There are times when you really want to call a constructor directly. Invoking a constructor on an existing object makes no sense, because constructors initialize objects, and an object can only be initialized — given its first value — once. But occasionally you have some raw memory that's already been allocated, and you need to construct an object in the memory you have. A special version of operator new called placement new allows you to do it.

As an example of how placement new might be used, consider this:

class Widget {
public:
Widget(int widgetSize);
...
};

Widget * constructWidgetInBuffer(void *buffer,
int widgetSize)
{
return new (buffer) Widget(widgetSize);
}

This function returns a pointer to a Widget object that's constructed within the buffer passed to the function. Such a function might be useful for applications using shared memory or memory-mapped I/O, because objects in such applications must be placed at specific addresses or in memory allocated by special routines. (For a different example of how placement new can be used, see Item 4.)

Inside constructWidgetInBuffer, the expression being returned is

new (buffer) Widget(widgetSize)

This looks a little strange at first, but it's just a use of the new operator in which an additional argument (buffer) is being specified for the implicit call that the new operator makes to operator new. The operator new thus called must, in addition to the mandatory size_t argument, accept a void* parameter that points to the memory the object being constructed is to occupy. That operator new is placement new, and it looks like this:

void * operator new(size_t, void *location)
{
return location;
}

This is probably simpler than you expected, but this is all placement new needs to do. After all, the purpose of operator new is to find memory for an object and return a pointer to that memory. In the case of placement new, the caller already knows what the pointer to the memory should be, because the caller knows where the object is supposed to be placed. All placement new has to do, then, is return the pointer that's passed into it. (The unused (but mandatory) size_t parameter has no name to keep compilers from complaining about its not being used; see Item 6.) Placement new is part of the standard C++ library (see Item E49). To use placement new, all you have to do is #include <new> (or, if your compilers don't yet support the new-style header names (again, see Item E49), <new.h>).

If we step back from placement new for a moment, we'll see that the relationship between the new operator and operator new, though you want to create an object on the heap, use the new operator. It both allocates memory and calls a constructor for the object. If you only want to allocate memory, call operator new; no constructor will be called. If you want to customize the memory allocation that takes place when heap objects are created, write your own version of operator new and use the new operator; it will automatically invoke your custom version of operator new. If you want to construct an object in memory you've already got a pointer to, use placement new.

[yaotang]

我有<< more effective C++>>的电子中文版看了你对这问题就了解啦
若你要,在csdn上发端信给我,我e-mail发给你

[shu]

学习一下：
你怎样得到“指定的地址”？如果得到，把指针指过去不就完啦。
malloc有相应的函数锁定内存空间，是不是可以用来保护“指定的地址”之后的内存？

[yaotang]

函数operator new 通常这样声明：
void * operator new(size_t size);
返回值类型是void*，因为这个函数返回一个未经处理（raw）的指针，未初始化的内存。（如果你喜欢，你能写一种operator new函数，在返回一个指针之前能够初始化内存以存储一些数值，但是一般不这么做。）参数size_t确定分配多少内存。你能增加额外的参数重载函数operator new，但是第一个参数类型必须是size_t。（有关operator new更多的信息参见Effective C++ 条款8至条款10。）
你一般不会直接调用operator new，但是一旦这么做，你可以象调用其它函数一样调用它：
void *rawMemory = operator new(sizeof(string));
操作符operator new将返回一个指针，指向一块足够容纳一个string类型对象的内存。
就象malloc一样，operator new的职责只是分配内存。它对构造函数一无所知。operator new所了解的是内存分配。把operator new 返回的未经处理的指针传递给一个对象是new操作符的工作。当你的编译器遇见这样的语句：
string *ps = new string("Memory Management");
它生成的代码或多或少与下面的代码相似（更多的细节见Effective C++条款8和条款10，还有我的文章Counting object里的注释。）：
void *memory = // 得到未经处理的内存
operator new(sizeof(string)); // 为String对象
call string::string("Memory Management") //初始化
on *memory; // 内存中
// 的对象
string *ps = // 是ps指针指向
static_cast<string*>(memory); // 新的对象
注意第二步包含了构造函数的调用，你做为一个程序员被禁止这样去做。你的编译器则没有这个约束，它可以做它想做的一切。因此如果你想建立一个堆对象就必须用new操作符，不能直接调用构造函数来初始化对象。
l placement new
有时你确实想直接调用构造函数。在一个已存在的对象上调用构造函数是没有意义的，因为构造函数用来初始化对象，而一个对象仅仅能在给它初值时被初始化一次。但是有时你有一些已经被分配但是尚未处理的(raw)内存，你需要在这些内存中构造一个对象。你可以使用一个特殊的operator new ，它被称为placement new。
下面的例子是placement new如何使用，考虑一下：
class Widget {
public:
Widget(int widgetSize);
...
};
Widget * constructWidgetInBuffer(void *buffer,
int widgetSize)
{
return new (buffer) Widget(widgetSize);
}
这个函数返回一个指针，指向一个Widget对象，对象在转递给函数的buffer里分配。当程序使用共享内存或memory-mapped I/O时这个函数可能有用，因为在这样程序里对象必须被放置在一个确定地址上或一块被例程分配的内存里。（参见条款M4,一个如何使用placement new的一个不同例子。）
在constructWidgetInBuffer里面，返回的表达式是：
new (buffer) Widget(widgetSize)
这初看上去有些陌生，但是它是new操作符的一个用法，需要使用一个额外的变量（buffer），当new操作符隐含调用operator new函数时，把这个变量传递给它。被调用的operator new函数除了待有强制的参数size_t外，还必须接受void*指针参数，指向构造对象占用的内存空间。这个operator new就是placement new，它看上去象这样：
void * operator new(size_t, void *location)
{
return location;
}
这可能比你期望的要简单，但是这就是placement new需要做的事情。毕竟operator new的目的是为对象分配内存然后返回指向该内存的指针。在使用placement new的情况下，调用者已经获得了指向内存的指针，因为调用者知道对象应该放在哪里。placement new必须做的就是返回转递给它的指针。（没有用的（但是强制的）参数size_t没有名字，以防止编译器发出警告说它没有被使用；见条款M6。） placement new是标准C++库的一部分（见Effective C++ 条款49）。为了使用placement new，你必须使用语句#include <new>（或者如果你的编译器还不支持这新风格的头文件名(再参见Effective C++ 条款49)，<new.h>）。
让我们从placement new回来片刻，看看new操作符（new operator）与operator new的关系，你想在堆上建立一个对象，应该用new操作符。它既分配内存又为对象调用构造函数。如果你仅仅想分配内存，就应该调用operator new函数；它不会调用构造函数。如果你想定制自己的在堆对象被建立时的内存分配过程，你应该写你自己的operator new函数，然后使用new操作符，new操作符会调用你定制的operator new。如果你想在一块已经获得指针的内存里建立一个对象，应该用placement new。
（有关更多的不同的new与delete的观点参见Effective C++ 条款7和我的文章Counting objects。）