这是在C ++中实例化对象的正确方法？ [英] Which is the right way to instantiate objects in C++?

问题描述

在C ++中（我使用QT）我可以创建一个QString类的实例两种方式：

In C++ (I use QT) I can create an instance of QString class two ways:

QString str = "my string";

方法2

method 2

QString *str = new QString("my string");

我知道这是与指针有关。所以我的问题是：

I know this is to do with pointers. So my questions are:

1. 两者之间的区别是什么？


2. 依照？


3. 在方法2中使用方法1时是否正确以及何时正确使用方法2？


4. delete str; 。如何在使用方法1时删除 str 变量？

1. what is the difference between the two?
2. which method should I stick to?
3. when is it correct to use method 1 and when is it correct to use method 2?
4. in method 2 I can destroy the object by calling delete str;. How can I delete the str variable when using method 1?

/ p>

Thanks

推荐答案

1. 主要有不同的生命周期：方法2中创建的对象将生存任意长直到你调用delete;在方法1中，它将在堆栈上创建，并在从函数调用（如果有）返回时被销毁。第二种方法2需要更多的工作，因为非平凡的内存管理。

1. primarily they have different lifetimes: the object created in method 2 will live arbitrarily long until you call delete; in method 1 it will be created on the stack and be destroyed upon return from the function call (if any). secondarily method 2 requires more work because of non-trivial memory management.

使用匹配你想要的生命周期的方法。如果方法1的生命周期足够好，您不使用方法2，因为它需要内存管理的开销。如果你可以重构你的程序，以便你可以做方法1，同时改善设计，这将更有效率和优雅。

use the method that matches the lifetime you want. if the lifetime of method 1 is good enough for you do not use method 2 because it entails the overhead of memory management. if you can restructure your program so that you can do with method 1 while also improving on the design, that will be more efficient and elegant.

请参阅2。以上。特别是使用方法1并存储指向对象并在其生命周期之后访问对象的指针是一个陷阱。这也是可能的方法2，但显式破坏聚焦程序员的注意力（但仍然是因为生命是不直接，它是一个可能的陷阱）方法2的一个陷阱忘记删除它导致内存泄漏（或删除它

see 2. above. in particular, using method 1 and storing a pointer to the object and accessing it after its lifetime is a pitfall. this is also possible with method 2 but the explicit destruction focuses the programmer's attention on it (but still because the lifetime is not straightforward it is a likely pitfall) a pitfall with method 2 is forgetting to delete it causing a memory leak (or deleting it too early and referring to it as described earlier in this paragraph)

在方法1中，对象将在函数返回时自动删除。

in method 1 the object will be automatically deleted when the function returns.

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