使用 shared_ptr 的示例?

Example to use shared_ptr?(使用 shared_ptr 的示例?)

问题描述

我今天问了一个关于 如何插入不同的问题同一向量数组中的对象类型 和我在那个问题中的代码是

Hi I asked a question today about How to insert different types of objects in the same vector array and my code in that question was

 gate* G[1000];
G[0] = new ANDgate() ;
G[1] = new ORgate;
//gate is a class inherited by ANDgate and ORgate classes
class gate
{
 .....
 ......
 virtual void Run()
   {   //A virtual function
   }
};
class ANDgate :public gate 
  {.....
   .......
   void Run()
   {
    //AND version of Run
   }  

};
 class ORgate :public gate 
  {.....
   .......
   void Run()
   {
    //OR version of Run
   }  

};      
//Running the simulator using overloading concept
 for(...;...;..)
 {
  G[i]->Run() ;  //will run perfectly the right Run for the right Gate type
 } 

我想使用向量，所以有人写道我应该这样做:

and I wanted to use vectors so someone wrote that I should do that :

std::vector<gate*> G;
G.push_back(new ANDgate); 
G.push_back(new ORgate);
for(unsigned i=0;i<G.size();++i)
{
  G[i]->Run();
}

但后来他和许多其他人建议我最好使用 Boost 指针容器
或 shared_ptr.在过去的 3 个小时里，我一直在阅读有关此主题的信息，但文档对我来说似乎相当先进.****谁能给我一个关于 shared_ptr 用法的小代码示例，以及为什么他们建议使用 shared_ptr.还有其他类型，如 ptr_vector、ptr_list 和 ptr_deque** **

but then he and many others suggested that I would better use Boost pointer containers
or shared_ptr. I have spent the last 3 hours reading about this topic, but the documentation seems pretty advanced to me . ****Can anyone give me a small code example of shared_ptr usage and why they suggested using shared_ptr. Also are there other types like ptr_vector, ptr_list and ptr_deque** **

Edit1:我也阅读了一个代码示例，其中包括:

I have read a code example too that included:

typedef boost::shared_ptr<Foo> FooPtr;
.......
int main()
{
  std::vector<FooPtr>         foo_vector;
........
FooPtr foo_ptr( new Foo( 2 ) );
  foo_vector.push_back( foo_ptr );
...........
}

而且我不懂语法！

推荐答案

使用 shared_ptr 的 vector 消除了内存泄漏的可能性，因为你忘记遍历 vector 并且对每个元素调用 delete.让我们逐行浏览一个稍微修改过的示例版本.

Using a vector of shared_ptr removes the possibility of leaking memory because you forgot to walk the vector and call delete on each element. Let's walk through a slightly modified version of the example line-by-line.

typedef boost::shared_ptr<gate> gate_ptr;

为共享指针类型创建别名.这避免了 C++ 语言中由于键入 std::vector 而导致的丑陋.> 并忘记结束大于号之间的空格.

Create an alias for the shared pointer type. This avoids the ugliness in the C++ language that results from typing std::vector<boost::shared_ptr<gate> > and forgetting the space between the closing greater-than signs.

    std::vector<gate_ptr> vec;

创建一个 boost::shared_ptr 对象的空向量.

Creates an empty vector of boost::shared_ptr<gate> objects.

    gate_ptr ptr(new ANDgate);

分配一个新的ANDgate 实例并将其存储到shared_ptr 中.单独执行此操作的原因是为了防止在操作抛出时可能出现的问题.在这个例子中这是不可能的.Boost shared_ptr最佳实践" 解释为什么将分配到独立对象而不是临时对象是最佳实践.

Allocate a new ANDgate instance and store it into a shared_ptr. The reason for doing this separately is to prevent a problem that can occur if an operation throws. This isn't possible in this example. The Boost shared_ptr "Best Practices" explain why it is a best practice to allocate into a free-standing object instead of a temporary.

    vec.push_back(ptr);

这会在向量中创建一个新的共享指针并将 ptr 复制到其中.shared_ptr 内部的引用计数确保 ptr 内部分配的对象被安全地传输到向量中.

This creates a new shared pointer in the vector and copies ptr into it. The reference counting in the guts of shared_ptr ensures that the allocated object inside of ptr is safely transferred into the vector.

没有说明的是，shared_ptr的析构函数确保了分配的内存被删除.这是避免内存泄漏的地方.std::vector 的析构函数确保为存储在向量中的每个元素调用 T 的析构函数.但是，指针的析构函数(例如，gate*)不会删除您分配的内存.这就是您试图通过使用 shared_ptr 或 ptr_vector 来避免的.

What is not explained is that the destructor for shared_ptr<gate> ensures that the allocated memory is deleted. This is where the memory leak is avoided. The destructor for std::vector<T> ensures that the destructor for T is called for every element stored in the vector. However, the destructor for a pointer (e.g., gate*) does not delete the memory that you had allocated. That is what you are trying to avoid by using shared_ptr or ptr_vector.

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