operator new

document by C++:

系统的有三个声明：

throwing (1)	void* operator new (std::size_t size);
nothrow (2)	void* operator new (std::size_t size, const std::nothrow_t& nothrow_value) noexcept;
placement (3)	void* operator new (std::size_t size, void* ptr) noexcept;

定义：

　　operator new:全局运算符函数，分配存储空间的接口。只allocate memory，不construction。

new expression:

　　but in C++, new is an operator with a very specific behavior: An expression with the new operator, first calls function operator new (i.e., this function) with the size of its type specifier as first argument, and if this is successful, it then automatically initializes or constructs the object (if needed). 

new xxx：1.call operator new to allocate memory. 2.call constructor on this class.

　　Allocate storage space

　　Default allocation functions (single-object form).

 

特性：

　　1.可替换，可重载

　　2.系统定义的是全局函数

 

如果要写内存的profile工具，就要重载这个allocate的接口函数。

 

例子：

　　

// operator new example
#include <iostream>     // std::cout
#include <new>          // ::operator new

struct MyClass {
  int data[100];
  MyClass() {std::cout << "constructed [" << this << "]\n";}
};

int main () {

  std::cout << "1: ";
  MyClass * p1 = new MyClass;
      // allocates memory by calling: operator new (sizeof(MyClass))
      // and then constructs an object at the newly allocated space

  std::cout << "2: ";
  MyClass * p2 = new (std::nothrow) MyClass;
      // allocates memory by calling: operator new (sizeof(MyClass),std::nothrow)
      // and then constructs an object at the newly allocated space

  std::cout << "3: ";
  new (p2) MyClass;
      // does not allocate memory -- calls: operator new (sizeof(MyClass),p2)
      // but constructs an object at p2

  // Notice though that calling this function directly does not construct an object:
  std::cout << "4: ";
  MyClass * p3 = (MyClass*) ::operator new (sizeof(MyClass));
      // allocates memory by calling: operator new (sizeof(MyClass))
      // but does not call MyClass's constructor

  delete p1;
  delete p2;
  delete p3;

  return 0;
}

 　

Possible output:

1: constructed [0x8f0f70]
2: constructed [0x8f23a8]
3: constructed [0x8f23a8]
4: