smart_ptr

看完這篇的感想

簡單的用 boost 作些實驗

about smart讓我們看摳布拉潔怎麼說

about weak_ptr

Smart_ptr 幫助我們 handle 資源的 delete

所以不必擔心某個東西被new出來後 要到哪裡去 delete

(或是忘記 delete造成 leak)

下面的程式碼可以看出好用之處

class Object {
public:
    Object(){cout << "A Object created" <<endl; };
    ~Object(){cout << "Object deleted" << endl; }
    void print() {std::cout << " Object::print" << std::endl;}
    Object(const Object&);
    Object& operator = (const Object&);
};  
void test_smartpointers() 
{
    int scopeA = 1;
    if(scopeA == 1) {
        boost::shared_ptr<object> management;
        //boost::weak_ptr<object> management;
        //Object* management;
        int ScopeC = 1;
        if (ScopeC == 1) {
            boost::shared_ptr<object> obj_2 (new Object());
            boost::shared_ptr<object> model(obj_2);
            management = obj_2;
            //management = obj_2.get();
            model->print();
        }            
        management->print();
    }   
}

Obj_2離開 ScopeC的時候並沒有真的 delete

因為此時management還有reference到它

一直到離開scopeA的時候 才會被自動的 delete掉

至於甚麼時候用的到小老百姓 weak_ptr?

當我們不希望 obj_2的life time被綁死的時候

所以 management改用了這個宣告

boost::weak_ptr<object> management;

如此一來obj_2就會在離開 ScopeC的時候就destruct

weak_ptr 解決了這個問題   (不知道還有沒有其他的)

另外我比較困惑的是這篇提到的”災難性的選擇”

如果改成這樣

void test_smartpointers() 
{
    int scopeA = 1;
    if(scopeA == 1){
        Object* management;
        int ScopeC = 1;
        if (ScopeC == 1) {
            boost::shared_ptr<object> obj_2 (new Object());
            boost::shared_ptr<object> model(obj_2);
            management = obj_2.get();
            model->print();
        }
        management->print();// should have problem here
    } 
}

真的call到最後一行的 print() 在vc2008的環境也不會有事

Related link

How to Boooost your vc.