当使用std::list<std::string>时,我偶然发现了堆栈溢出问题std::string的内存泄漏,其中一个注释说:

不要用那么多new。我看不出你为什么在任何地方都用新的。你可以在c++中通过值创建对象,这是使用该语言的一个巨大优势。您不必分配堆上的所有内容。不要像Java程序员那样思考。

我不太确定他这么说是什么意思。

为什么在c++中应该尽可能多地根据值创建对象,这在内部有什么不同?我误解了答案吗?


当前回答

我发现有几个重要的原因让我们忽略了尽量少做新事情:

Operator new的执行时间不确定

调用new可能会导致操作系统为进程分配新的物理页,也可能不会。如果你经常这样做,会很慢。或者它可能已经准备好了一个合适的内存位置;我们不知道。如果你的程序需要具有一致且可预测的执行时间(如在实时系统或游戏/物理模拟中),你需要避免在时间关键型循环中添加新的元素。

操作符new是一个隐式线程同步

是的,你听到了。你的操作系统需要确保你的页表是一致的,因此调用new会导致你的线程获得一个隐式互斥锁。如果你一直从许多线程调用new,你实际上是在序列化你的线程(我用32个cpu做过这个,每个cpu都调用new来获得几百个字节,哎呦!那是一个需要调试的皇家p.i.t.a.。)

其余的,比如速度慢、碎片化、容易出错等,其他答案已经提到了。

其他回答

以上所有正确答案还有一点,这取决于你在做什么类型的编程。在Windows中开发内核,例如->堆栈受到严重限制,您可能无法像在用户模式中那样处理页面错误。

在这样的环境中,新的或类似c的API调用是首选的,甚至是必需的。

当然,这只是规则的一个例外。

使用new时,对象被分配到堆中。它通常用于预期扩展时。当你声明一个对象,比如,

Class var;

它被放置在堆栈上。

你总是需要对你用new放在堆上的对象调用destroy。这就有可能导致内存泄漏。放在堆栈上的对象不容易发生内存泄漏!

我倾向于不同意使用new“too much”的观点。尽管最初的海报使用new with system classes有点可笑。(int *我;I = new int[9999];?真的吗?int [9999];这样就清楚多了。)我想这就是让评论者生气的地方。

When you're working with system objects, it's very rare that you'd need more than one reference to the exact same object. As long as the value is the same, that's all that matters. And system objects don't typically take up much space in memory. (one byte per character, in a string). And if they do, the libraries should be designed to take that memory management into account (if they're written well). In these cases, (all but one or two of the news in his code), new is practically pointless and only serves to introduce confusions and potential for bugs.

When you're working with your own classes/objects, however (e.g. the original poster's Line class), then you have to begin thinking about the issues like memory footprint, persistence of data, etc. yourself. At this point, allowing multiple references to the same value is invaluable - it allows for constructs like linked lists, dictionaries, and graphs, where multiple variables need to not only have the same value, but reference the exact same object in memory. However, the Line class doesn't have any of those requirements. So the original poster's code actually has absolutely no needs for new.

C++ doesn't employ any memory manager by its own. Other languages like C# and Java have a garbage collector to handle the memory C++ implementations typically use operating system routines to allocate the memory and too much new/delete could fragment the available memory With any application, if the memory is frequently being used it's advisable to preallocate it and release when not required. Improper memory management could lead memory leaks and it's really hard to track. So using stack objects within the scope of function is a proven technique The downside of using stack objects are, it creates multiple copies of objects on returning, passing to functions, etc. However, smart compilers are well aware of these situations and they've been optimized well for performance It's really tedious in C++ if the memory being allocated and released in two different places. The responsibility for release is always a question and mostly we rely on some commonly accessible pointers, stack objects (maximum possible) and techniques like auto_ptr (RAII objects) The best thing is that, you've control over the memory and the worst thing is that you will not have any control over the memory if we employ an improper memory management for the application. The crashes caused due to memory corruptions are the nastiest and hard to trace.

两个原因:

在这种情况下没有必要。您正在使代码不必要地变得更加复杂。 它在堆上分配空间,这意味着您必须记住稍后删除它,否则将导致内存泄漏。