类型转换问题

hjack

回到开头的问题,
用base指针放在queue里,想dequeue后按derive来处理,不用dynamic_cast的话,还有什么方法吗/

onttpi

这个不是多态的问题吗？用virtual 成员函数不行？

QQB的代码有问题？

[ 本帖最后由 onttpi 于 2008-5-15 23:33 编辑 ]

hjack

The code in QQB is right.
Shared_ptr is good. Take it. Thanks.
It works now.

hjack

One question,
A is base class, B is derive class.

   typedef boost::shared_ptr<A> eT;
   typedef queue<eT> qT;
   qT q;
   q.push(eT(new B));
Can it work? What if:
B *b = new B;
q.push(eT(b));
will memory leak occur?

iptton

如果它是照我上网查到的资料所说的按引用数来决定是否自动清理（类似于java的回收算法策略）
LS代码应该没问题吧，just have a try..

iptton

#include "boost/shared_ptr.hpp"
#include <cassert>
#include <queue>
#include <iostream>
using boost::shared_ptr;

class A {
public:
  A(){}
  virtual void say(){
          std::cout<<"A saying.."<<std::endl;
  }
};

class B:public A{
public:
  B(){}
  virtual void say(){
          std::cout<<"B saying.."<<std::endl;
  }
};

int main() {
        typedef boost::shared_ptr<A> eT;
        typedef queue<eT> qT;
           qT q;
           q.push(eT(new B));
           eT tmp=q.front();
           (*tmp).say();
}

iptton

#include "boost/shared_ptr.hpp"
#include <cassert>
#include <queue>
#include <iostream>
using boost::shared_ptr;

class A {
public:
  A(){}
  virtual void say(){
          std::cout<<"A saying.."<<std::endl;
  }
  virtual ~A(){
          std::cout<<"A destructed.."<<std::endl;
  }
};

class B:public A{
public:
  B(){}
  virtual void say(){
          std::cout<<"B saying.."<<std::endl;
  }
  ~B(){
          std::cout<<"B destructed.."<<std::endl;
  }
};

int main() {
        typedef boost::shared_ptr<A> eT;
        typedef queue<eT> qT;
           qT q;
           q.push(eT(new B));
           /*
           eT tmp=q.front();//如果用tmp指向，pop不会造成释放，因为tmp还指向它
           tmp->say();
           */
           (q.front())->say();//这样用的话，pop后会被释放
           std::cout<<"Before pop"<<endl;
           q.pop();
           std::cout<<"After pop"<<endl;
}

iptton

#include "boost/shared_ptr.hpp"
#include <cassert>
#include <queue>
#include <iostream>
using boost::shared_ptr;

class A {
public:
  A(){}
  virtual void say(){
          std::cout<<"A saying.."<<std::endl;
  }
  virtual ~A(){
          std::cout<<"A destructed.."<<std::endl;
  }
};

class B:public A{
public:
  B(){}
  virtual void say(){
          std::cout<<"B saying.."<<std::endl;
  }
  ~B(){
          std::cout<<"B destructed.."<<std::endl;
  }
};

typedef boost::shared_ptr<A> eT;
typedef queue<eT> qT;
void test(qT& q){
        eT tmp=q.front();
           tmp->say();
           std::cout<<"Before pop"<<endl;
           q.pop();
           std::cout<<"After pop"<<endl;
}
int main() {
           qT q;
        q.push(eT(new B));
           std::cout<<"Before test.."<<std::endl;
        test(q);
        std::cout<<"After test.."<<std::endl;
           return 0;
}

output:

Befor test...
      B sayiing...
      Before pop...
      After pop..
      B destructed..
      A destructed...
After test

iptton

http://citycowboy.blog.sohu.com/82383240.html

hjack

看起来是这样....

hjack

What happens if using below method:

A *a = static_cast<A*>(q.front().get()));

will the pointer out of control of shared_ptr?

皇家救星

多态必要用指针或者引用来实现
所以应该用Base *pb = q.front();
pb->...