尝试在没有智能指针的情况下使用 STL - 尝试避免临时对象创建
我特别喜欢以直接的方式使用 STL 容器的简单性。
我从来没有真正弄清楚如何让 Boost 库在我的开发平台上工作，事实上我认为我什至没有尝试过。我想你可能会说我只是想推迟不可避免的事情，因为 Boost 显然是一个我应该使用的有用的库。
但我的问题本质上与这个主题相同： 如何在不调用复制构造函数的情况下使用类初始化 STL 向量/列表
mylist; 我只想要一个将新实例推入列表并调用默认构造函数的函数，而不是从它的临时堆栈实例中复制它。在另一个主题中提到了移动构造函数。我查阅了它们，坦率地说，这除了让我心中感到恐惧之外没有任何作用。两个&符号！！
如果我只创建一系列 ExpenseClass 对象，它会起作用吗？ ExpenseClass *mylist = new ExpenseClass[20]; 这会调用构造函数 20 次吗？
在我看来我应该只使用 boost:ptr_list 。
I particularly like the simplicity of using STL containers in the straightforward way.
I have never really figured out how to get the Boost library working on my dev platforms, in fact I don't think I've even tried. I guess you could say I am just trying to delay the inevitable since Boost is clearly a helpful library that I should be using.
But my question is essentially the same as this topic: How to initialise a STL vector/list with a class without invoking the copy constructor
I have std::list<ExpensiveClass> mylist; and I just want a function that pushes a new instance into the list and calls the default constructor, rather than copying it from a temporary stack instance of it. In the other topic there was mention of move constructors. I looked them up and quite frankly it does nothing but strike fear into my heart. Two ampersands!!
Would it work if I just made an array of ExpensiveClass objects? ExpensiveClass *mylist = new ExpensiveClass[20]; Does this call the constructor 20 times?
Seems to me I should just use boost:ptr_list.
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将对象插入到容器中会调用该对象的复制构造函数。目前确实没有办法解决这个问题，因此为什么指针容器用于大型对象：复制指针非常便宜。
如果您选择使用智能指针容器，则可以使用 Boost 指针容器之一，也可以使用
shared_ptrs。回答您的问题：
ExppressiveClass的默认构造函数被调用 20 次：数组的每个元素调用一次。Inserting an object into a container invokes the copy constructor on that object. There's really no way around that (yet), hence why pointer containers are used for large objects: it's dirt cheap to copy a pointer.
If you choose to use a container of smart pointers, you can either use one of the Boost pointer containers or you can use an STL container of
shared_ptrs.To answer your question about:
The default constructor for
ExpensiveClassis called 20 times: once for each element of the array.既然您提到了“两个与号”，我将为您提供 C++0x 解决方案：
如果您想使用不同的构造函数，只需提供参数：
此成员函数就地构造对象，无需复制（或移动）。
Since you mentioned "two ampersands", I am going to give you the C++0x solution:
If you want to use a different constructor, simply provide arguments:
This member function constructs the object in-place, without copying (or moving).
根据对 C++0x 功能的实现支持，请查看
emplace_back和类似的成员函数 - 它们正是您所要求的，即直接在容器内“就地”构造一个对象，没有任何复制。Depending on implementation support for C++0x features, have a look at
emplace_backand similar member functions - they do precisely what you ask for, i.e. construct an object directly inside the container, "in-place", without any copying.