学步园

C++中的new与delete可以认为是C中的malloc与free的升级版本。

new包含两部分，一部分是与malloc功能相同，是从堆上面申请内存块，第二部是调用类的构造方法来初始化刚申请的内存。

delete是new的逆过程，先调用类的析构方法来反初始化，再把刚申请的内存还给堆。

new [] 与delete []是针对数组操作符，要注意是通过new []分配的对象，不能用delete来释放对象，否则会有内存泄漏。当然通过new分配的对象，不能用delete[]来释放对象。后面我会通过代码来说明为什么。

下面是C++ 中的new与delete函数原型，申请内存失败会抛出异常bad_alloc

 void* operator new(std::size_t) throw (std::bad_alloc);<br /> void* operator new[](std::size_t) throw (std::bad_alloc);<br /> void operator delete(void*) throw();<br /> void operator delete[](void*) throw();

 使用举例

 int* p1 = new int();<br /> delete p2;</p> <p>int* p2 = new int[5];<br /> delete [] p2;

 终于到了用模板来模拟new与delete操作符，代码中有注释说明，其中对于调用类的构造方法，采用一种C++标准中称作in-place construtor的方式。使用原型为T* = new(pbuff) T()，直译的话就是在pbuff这块内存构造T类，而不用再去堆上面申请内存。这种技巧大量应用在对象池的实现中，即pbuff这块内存可以挂在链表中反复地使用（这里先不展开说了）。

 /**<br /> * A simulation of c++ new T() & new T(param) operation<br /> */<br /> struct NewObj<br /> {<br /> template <typename T><br /> inline void operator()(T*& pObj)<br /> {<br /> // allocate memory form heap<br /> void * pBuff = malloc(sizeof(T));<br /> // call constructor<br /> pObj = new (pBuff) T();<br /> }</p> <p> template <typename T, typename P><br /> inline void operator()(T*& pObj, const P& param)<br /> {<br /> // allocate memory form heap<br /> void * pBuff = malloc(sizeof(T));<br /> // call constructor, pass one param<br /> pObj = new(pBuff) T(param);<br /> }<br /> };</p> <p>/**<br /> * A simulation of c++ delete T operation<br /> */<br /> struct DeleteObj<br /> {<br /> template <typename T><br /> inline void operator()(T*& pObj)<br /> {<br /> if ( NULL == pObj ) { return ;}<br /> // call destructor<br /> pObj->~T();<br /> // free memory to heap<br /> free((void*)pObj);<br /> pObj = NULL;<br /> }<br /> };</p> <p>/**<br /> * A simulation of c++ new T[N]() operation<br /> */<br /> struct NewObjArray<br /> {<br /> template <typename T><br /> inline void operator()(T*& pObj, unsigned int size)<br /> {<br /> // save the number of array elements in the beginning of the space.<br /> long * pBuff = (long *) malloc (sizeof(T) * size + sizeof(long));<br /> *((unsigned int *) pBuff) = size;<br /> pBuff++;</p> <p> // change pointer to T type, then can use pT++<br /> T * pT = (T *) pBuff;<br /> // save the pointer to the start of the array.<br /> pObj = pT;<br /> // now iterate and construct every object in place.<br /> for (unsigned int i = 0; i < size; i++)<br /> {<br /> new((void *) pT) T();<br /> pT++;<br /> }<br /> }<br /> };</p> <p>/**<br /> * A simulation of c++ delete [] T operation<br /> */<br /> struct DeleteObjArray<br /> {<br /> template <typename T><br /> inline void operator()(T*& pObj)<br /> {<br /> unsigned int size = *((unsigned int *) ((long *) pObj - 1));</p> <p> T * pT = pObj;<br /> // call destructor on every element in the array.<br /> for (unsigned int i = 0; i < size; i++)<br /> {<br /> pT->~T();<br /> pT++;<br /> }<br /> // free memory to heap.<br /> free ((void *) ((long *) pObj - 1));<br /> pObj = NULL;<br /> }<br /> };

 测试代码

 struct TestClass<br /> {<br /> TestClass() : mem1(0), mem2(0) {}</p> <p> TestClass(int m) : mem1(m), mem2(0) {}</p> <p> int mem1;<br /> long mem2;<br /> };</p> <p>void test_new_delete()<br /> {<br /> TestClass* p1 = NULL;<br /> NewObj()(p1);<br /> printf("%p/n", p1);<br /> DeleteObj()(p1);</p> <p> //<br /> TestClass* p2 = NULL;<br /> NewObj()(p2, 0);<br /> printf("%p/n", p2);<br /> DeleteObj()(p2);</p> <p> //<br /> TestClass* p3 = NULL;<br /> NewObjArray()(p3, 5);<br /> printf("%p/n", p3);<br /> DeleteObjArray()(p3);<br /> }

 Note:测试环境为eclipse+cdt+ubuntu+gcc，注意头文件需要#include <new>，使用#include <stdlib.h>会导致编译不过，因为in-place construtor是C++中的新玩意。