C++推前迭代

Question

我有一個Tab類，它有一個組件列表。

list<Component*> Tab::getComponents() { 
    return (this->components); 
} 

void Tab::addComponent(Component* comp){ 
    this->components.push_front(comp); 
} 

Tab::Tab() { 
    // x = 25, y = 30 
Button* one = new Button(25,30,300,100, "images/button.png"); 
this->addComponent(one); 

Button* two = new Button(75,100,300,100, "images/button.png"); 
this->addComponent(two); 

    // x = 150, y = 150 
Button* three = new Button(150,150,300,100, "images/button.png"); 
this->addComponent(three); 
} 

現在有問題的代碼：

list<Component*>::iterator it = activeTab->getComponents().begin(); 
for (it ; it != activeTab->getComponents().end(); it++) { 
    offset.x = (*it)->getX(); 
    cout << "offset.x = " << offset.x << endl; 
    offset.y = (*it)->getY(); 
    cout << "offset.y = " << offset.y << endl; 
} 

這是for循環的第一次迭代的輸出：

offset.x = 25 
offset.y = 30 

不過，看到我用push_front()，它應該成爲：

offset.x = 150 
offset.y = 150 

我在做什麼錯？

編輯：for循環打印垃圾的第二次迭代...

offset.x = 16272 
offset.y = 17 

而第三隻打印segmentation fault :(

Answer 1

你Tab::getComponents()回報list<Component*>代替list<Component*>&，這意味着，this->components在返回時被複制。因此，讓我們來看看代碼：

list<Component*>::iterator it = activeTab->getComponents().begin(); 
// ``it'' is already INVALID here since the list returned in the above line 
// is already destructed! 
for (it ; it != activeTab->getComponents().end(); it++) { 
    offset.x = (*it)->getX(); //``it'' is invalid 
    cout << "offset.x = " << offset.x << endl; 
    offset.y = (*it)->getY(); 
    cout << "offset.y = " << offset.y << endl; 
} 

迭代器it是當你取消對它的引用無效。

Answer 2

注意，你的方法getComponents（）返回一個副本，應返回引用。

list<Component*>& Tab::getComponents() { 
    return (this->components); 
}