C++拾遗--new delete 重载

                        C++拾遗--new delete 重载

前言

    new和delete是操作动态内存的一对操作。对它们重载可以对内存管理进行有效的定制。

正文

1.局部重载

特别针对某一类型，对new和delete进行重载，可以对该类型对象的动态创建实行监控。如下代码：

代码一

#include <iostream>using namespace std;class MyClass{public:MyClass(){cout << "MyClass()" << endl;}~MyClass(){cout << "~MyClass()" << endl;}void *operator new(std::size_t size){cout << "局部new call" << endl;void *mem = malloc(size);    if (mem)     //内存分配失败，则返回0return mem; //return malloc(size);elsethrow bad_alloc();     //内存分配失败，抛出异常}void operator delete(void *ptr){cout << "局部delete call" << endl;//不为空，则调用free释放内存if (ptr){free(ptr);}}};int main(){cout << "******局部new delete重载演示***by David***" << endl;MyClass *my = new MyClass;delete my;cin.get();return 0;}

运行

运行结果表明

表达式new整合了内存分配和构造函数。先调用malloc分配内存，然后调用指定类型并相匹配的构造函数初始化该段内存。

表达式delete整合了析构函数和内存释放。先调用类的析构函数释放资源，后调用free释放分配的内存。

代码二

下面一个例子提供了对内存分配进行监控的一种方法。

#include <iostream>using namespace std;class MyClass{public://count记录未释放的对象个数static int count;int a;MyClass(){cout << "MyClass()" << endl;count++;}~MyClass(){cout << "~MyClass()" << endl;count--;}//new 局部重载void *operator new(size_t size){cout << "局部new call" << endl;void *mem = malloc(size);    //内存分配失败，则返回0if (mem)return malloc(size);elsethrow bad_alloc();     //内存分配失败，抛出异常}//new[] 局部重载void *operator new[](std::size_t size){cout << "局部new[] call" << endl;void *mem = malloc(size);    //内存分配失败，则返回0if (mem)return malloc(size);elsethrow bad_alloc();     //内存分配失败，抛出异常}//delete 局部重载void operator delete(void *ptr){cout << "局部delete call" << endl;//不为空，则调用free释放内存if (ptr){free(ptr);}}//delete[] 局部重载void operator delete[](void *ptr){cout << "局部delete[] call" << endl;//ptr不为空，则调用free释放内存if (ptr){free(ptr);}}};int MyClass::count = 0;int main(){cout << "******new delete 局部重载演示***by David***" << endl;cout << "起始MyClass::count = " << MyClass::count << endl;MyClass *my = new MyClass;delete my;cout << "-----------------" << endl;MyClass my1;cout << "-----------------" << endl;MyClass *mys = new MyClass[5];cout << "MyClass::count = " << MyClass::count << endl;delete[]mys;cout << "MyClass::count = " << MyClass::count << endl;cin.get();return 0;}

运行

2.全局重载

对全局的new和delete重载可以监控所有类型的内存分配。

#include <iostream>#include <string>using namespace std;class MyClass{public:MyClass(){cout << "MyClass()" << endl;}~MyClass(){cout << "~MyClass()" << endl;}void *operator new(std::size_t size){cout << "MyClass::new重载" << endl;void *mem = malloc(size);if (mem)return mem;elsethrow bad_alloc();}void *operator new[](std::size_t size){cout << "MyClass::new[]重载" << endl;void *mem = malloc(size);if (mem)return mem;elsethrow bad_alloc();}void operator delete(void *ptr){cout << "MyClass::delete重载" << endl;if (ptr){free(ptr);}}void operator delete[](void *ptr){cout << "MyClass::delete[]重载" << endl;if (ptr){free(ptr);}}};//全局new重载void *operator new(std::size_t size){cout << "全局new重载" << endl;void *mem = malloc(size);if (mem)return mem;elsethrow bad_alloc();}//全局new[]重载void *operator new[](std::size_t size){cout << "全局new[]重载" << endl;void *mem = malloc(size);if (mem)return mem;elsethrow bad_alloc();}//全局delete重载void operator delete(void *ptr){cout << "全局delete重载" << endl;if (ptr){free(ptr);}}//全局delete[]重载void operator delete[](void *ptr){cout << "全局delete[]重载" << endl;if (ptr){free(ptr);}}int main(){cout << "******全局/局部new和delete都进行重载***by David***" << endl;int *p = new int;delete p;cout << "-------------------" << endl;double *ds = new double[10];delete[]ds;cout << "-------------------" << endl;MyClass *my = new MyClass;delete my;cout << "-------------------" << endl;MyClass *mys = new MyClass[3];delete[]mys;cin.get();return 0;}

运行

如果类型重新定义了new 和 delete，则调用局部的，否则调用全局的。

细节

• operator new或operator new[]的返回类型必须是void*。
• operator delete或operator delete[]的返回类型必须是void。
• 类中重载的new和delete都是隐式static的。若显式声明，也不会出错。
• size_t就是unsigned int。当编译器调用operator new时，把存储指定类型对象所需的字节数传递给size_t的形参。当调用operator new[]时，就传递数组中所有元素的字节数。

编译器对operator new的扩展，更像如下这般：

void *operator new(std::size_t size){if (size == 0) size = 1;//对于size为0，也要保证返回唯一内存地址void *mem = nullptr;try{if (void *mem = malloc(size)){//调用默认的构造函数对分配的内存进行初始化mem = MyClass::MyClass(mem);}}catch (...){throw;}return mem;}

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