C++重载new operator

关于重载new operator，Effective C++ 中给出了new/delete 的几点理由：

• 检测代码中的内存错误
• 优化性能
• 获得内存使用的统计数据

除了基本的重载new operator之外，你可以增加参数，提高函数的灵活性。比如：

void* operator new(size_t size, const char* file, int line);  // 其返回的指针必须能被普通的 ::operator delete(void*) 释放

void operator delete(void* p, const char* file, int line);   // 这个函数只在析构函数抛异常的情况下才会被调用

在使用时，

Foo* p = new (__FILE, __LINE__) Foo;   // 这样能跟踪是哪个文件，哪一行代码分配的内存

我们也可以用宏替换 new 来节省打字。“检测内存错误”和“统计内存使用情况”通常会用这种方式重载。

class MyObj{public:MyObj():m_i(0), m_c('c'),m_ii(1){}MyObj& operator=(const MyObj& obj){if (this != &obj){this->m_i = obj.m_c;this->m_c = obj.m_c;this->m_ii = obj.m_ii;}return *this;}private:int m_i;char m_c;int m_ii;};class MyObjContainer{public:MyObjContainer():m_i(1),m_pObj(NULL){}MyObjContainer(MyObj* pObj):m_i(0),m_pObj(pObj){}MyObjContainer& operator=(const MyObjContainer& obj){if (this != &obj){this->m_i = obj.m_i;this->m_pObj = obj.m_pObj;}return *this;}void* operator new(size_t size, new_handler handle = 0){if (!size)size = 1;while (1){void* pointer = malloc(size);if (pointer != NULL){cout << "Dear, we have new " << size << " memory" << endl;return pointer;}//new_handler global_handler = set_new_handler(0);set_new_handler(handle);if (handle) {(*handle)();return 0;}elsethrow bad_alloc();}}void operator delete(void* pointer){if (NULL == pointer)return;free(pointer);}void destroy() {if (m_pObj != NULL)delete m_pObj;delete this;}private:intm_i;MyObj*m_pObj;intm_arr[1024*1024*510];~MyObjContainer(){}};void no_memory(){cout << "OMG, no memory!!!!" << endl;}void private_decons(){MyObj* pMy = new MyObj();MyObjContainer* pCons = new(no_memory) MyObjContainer(pMy);if (pCons != NULL){pCons->destroy();pCons = NULL;}elsedelete pMy;}

注意：

• 用系统默认的 malloc() 分配的内存要交给系统默认的 free() 去释放；
• 用系统默认的 new 表达式创建的对象要交给系统默认的 delete 表达式去析构并释放；
• 用系统默认的 new[] 表达式创建的对象要交给系统默认的 delete[] 表达式去析构并释放；
• 用系统默认的 ::operator new() 分配的的内存要交给系统默认的 ::operator delete() 去释放；
• 用 placement new 创建的对象要用 placement delete 去析构；

最近看了《C++ In Action》，其中有一节讲述overloading operator new，内容还是蛮精彩的，其中Tracing memory leak印象颇深。遂记录下来。

class Tracer{private:    class Entry    {    public:        Entry (char const * file, int line)            : _file (file), _line (line)        {}        Entry ()            : _file (0), _line (0)        {}        char const * File () const { return _file; }        int Line () const { return _line; }    private:        char const * _file;        int _line;    };    class Lock    {    public:        Lock (Tracer & tracer)            : _tracer (tracer)        {            _tracer.lock ();        }        ~Lock ()        {            _tracer.unlock ();        }    private:        Tracer & _tracer;    };   // friend class Lock;public:    Tracer ();    ~Tracer ();    void Add (void * p, char const * file, int line);    void Remove (void * p);    void Dump ();    static bool Ready;private:    void lock () { _lockCount++; }    void unlock () { _lockCount--; }private:    typedef std::map<void *, Entry>::iterator iterator;    std::map<void *, Entry> _map;    int _lockCount;};bool Tracer::Ready = false;Tracer::Tracer():_lockCount(0){Ready = true;}Tracer::~Tracer(){Ready = false;Dump();}void Tracer::Dump(){if (_map.size() != 0){cout << _map.size() << " memory leaks detected" << endl;for (iterator itor = _map.begin(); itor != _map.end(); ++itor){char const* file = itor->second.File();int line = itor->second.Line();cout << file << ", " << line << endl;}}}void Tracer::Add(void *p, const char *file, int line){if (_lockCount > 0)return;Tracer::Lock lock(*this);_map[p] = Entry(file, line);}void Tracer::Remove(void *p){if (_lockCount > 0)return;Tracer::Lock lock(*this);iterator itor = _map.find(p);if (itor != _map.end())_map.erase(itor);}/*extern*/ Tracer gNewTracer;void* operator new (size_t size, const char* file, int line){void* p = malloc(size);if (Tracer::Ready)gNewTracer.Add(p, file, line);return p;}void operator delete(void* p, const char* file, int line){if (Tracer::Ready)gNewTracer.Remove(p);free(p);}void* operator new (size_t size){void* p = malloc(size);if (Tracer::Ready)gNewTracer.Add(p, "?", 0);return p;}void operator delete(void* p){if (Tracer::Ready)gNewTracer.Remove(p);free(p);}#define new new(__FILE__, __LINE__)

当然这段code中Locker可依据具体实际应用而改变。。。

参考文献：

不要重载全局operator new    http://blog.csdn.net/Solstice/article/details/6198937

Effective C++

C++ In Action