# [Boolan] C++第五周重载const，new，delete

2019-11-08 01:31:20

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const 重载

const修饰类成员函数，该成员函数里面不可能修改成员变量其中const也算函数签名的一部分，只用于在类的成员函数const的重载使用条件：如果一个类声明中有const重载的函数，const对象使用const修饰的成员函数，非const对象使用没有const修饰的成员函数#include <iostream>using namespace std;class Foo{public: void PRint() const{ cout << "Foo::Print const" << endl; } void Print(){ cout << "Foo::Print" << endl; }};int main(int argc, char *argv[]){ const Foo fConst; fConst.Print(); Foo f; f.Print(); return 0;}

重载new delete

new ----> Operator new 分配空间调用相应的构造函数delete ---- > 调用析构函数 operator delete#include <iostream>#include <string>using namespace std;class Foo{private: int _id; long _data; string _str;public: Foo():_id(0) { cout << "default ctor. this=" << this << " id="<<_id << endl; } Foo(int i): _id(i) { cout << "ctor.this=" << this << " id=" << _id << endl; } ~Foo() { cout << "dtor.this=" <<this << " id=" << _id <<endl; } static void *operator new(size_t size); static void operator delete(void* pdead, size_t size); static void *operator new[](size_t size); static void operator delete[] (void*pdead, size_t size);};void * Foo::operator new(size_t size){ cout << " Foo::operator new " << endl; Foo *p = (Foo*)malloc(size); return p;}void Foo::operator delete(void* pdead, size_t size){ cout << " Foo::operator delete " << endl; free(pdead);}void * Foo::operator new[](size_t size){ Foo* p = (Foo*)malloc(size); return p;}void Foo::operator delete[](void*pdead, size_t size){ free(pdead);}int main(int argc, char *argv[]){ Foo *pf = new Foo; delete pf; cout << "-----------------" <<endl; //强制调用全局的new/delete Foo *p = ::new Foo; ::delete p; return 0;}

重载new() delete()

我们可以重载类的成员函数operator new(), 写出多个版本，前提是每个版本的声明必须有独特的参数列，其中第一个参数必须是size_t, 其余参数指定的placement arguments为初值 Foo* pf = new (300, 'c')Foo;我们也可以重载类的成员函数operator delete()，写出多个版本，但是他们绝不会被delete调用。只有当new所调用的析构函数抛出exception，才会调用这些重载版的operator delete(). 它只可能这样被调用，主要用来归还未能完全创建成功的object所占的memory.并且重载版的delete是被系统自动调用的#include <iostream>#include <stdio.h>using namespace std;namespace test1{ class Foo { public: void Print() const{ cout << "Foo::Print const" << endl; } void Print(){ cout << "Foo::Print" << endl; } };};namespace test2 { class Bad{}; class Foo{ public: Foo() { cout << "Foo::Foo()" <<endl; } Foo(int) {cout << "Foo::Foo(int)" << endl; throw "抛出异常";} //(1) static void* operator new(size_t size) { return malloc(size); } static void operator delete(void* pdead, size_t size) { cout << "void operator delete(void* pdead, size_t size)" <<endl; free(pdead); } //(2) static void* operator new(size_t size, void *) { return malloc(size); } static void operator delete(void *, void *) { cout << "operator delete(void *, void *)" << endl; } //(3) static void* operator new(size_t size, long extra) { return malloc(size + extra); } static void operator delete(void *pHead, long) { cout << "operator delete(void *, long)" << endl; //free(pHead); } //(4) static void* operator new(size_t size, long extra, char init) { return malloc(size + extra); } static void operator delete(void *pHead, long, char) { cout << "operator delete(void *, long, char)" << endl; free(pHead); } };};int main(int argc, char *argv[]){ test2::Foo start; test2::Foo *p1; test2::Foo *p2; test2::Foo *p3 ; test2::Foo *p4; test2::Foo *p5 ; try { p1 = new test2::Foo; p2 = new (&start) test2::Foo; p3 = new (100)test2::Foo; p4 = new (100, 'a')test2::Foo; p5 = new (100)test2::Foo(1); } catch (const char* msg) { cout << msg<<endl; } return 0;}//////////////////////////////// 环境VC6.0Foo::Foo()Foo::Foo()Foo::Foo()Foo::Foo()Foo::Foo()Foo::Foo(int)operator delete(void *, long)抛出异常Press any key to continue引用场景:string中使用重载operator new()的方式，给一块内存添加引用计数，可节省内存空间

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