所有的技术讨论都使这个概念过于复杂。

如果您将GC自动放入c++以获取所有内存，那么可以考虑使用类似web浏览器的东西。web浏览器必须加载完整的web文档并运行web脚本。web脚本变量可以存储在文档树中。在打开大量选项卡的浏览器中的大文档中，这意味着每次GC必须进行完整收集时，它还必须扫描所有文档元素。

On most computers this means that PAGE FAULTS will occur. So the main reason, to answer the question is that PAGE FAULTS will occur. You will know this as when your PC starts making lots of disk access. This is because the GC must touch lots of memory in order to prove invalid pointers. When you have a bona fide application using lots of memory, having to scan all objects every collection is havoc because of the PAGE FAULTS. A page fault is when virtual memory needs to get read back into RAM from disk.

因此，正确的解决方案是将应用程序分为需要GC的部分和不需要GC的部分。在上面的web浏览器示例中，如果文档树是用malloc分配的，但javascript是用GC运行的，那么每次GC启动时，它只扫描一小部分内存，并且文档树的内存中所有page OUT元素不需要被换回。

为了进一步理解这个问题，请查阅虚拟内存以及它是如何在计算机中实现的。这都是关于当没有那么多RAM时，程序可用2GB的事实。在32BIt系统的2GB RAM的现代计算机上，只要只有一个程序在运行，就不存在这样的问题。

作为另一个示例，考虑一个必须跟踪所有对象的完整集合。首先，您必须扫描所有通过根访问的对象。第二步扫描步骤1中可见的所有对象。然后扫描等待的析构函数。然后再次浏览所有页面，关闭所有不可见对象。这意味着许多页面可能会被多次换出和换回。

因此，我的回答是，由于触及所有内存而发生的PAGE FAULTS的数量导致对程序中所有对象进行完整的GC是不可行的，因此程序员必须将GC视为脚本和数据库工作的辅助，但使用手动内存管理进行正常工作。

另一个很重要的原因当然是全局变量。为了让收集器知道全局变量指针在GC中，它需要特定的关键字，因此现有的c++代码将无法工作。

要回答关于c++的大多数“为什么”问题，请阅读c++的设计与进化

原始C语言背后的一个基本原则是，内存是由一系列字节组成的，代码只需要关心这些字节在被使用的确切时刻意味着什么。现代C语言允许编译器施加额外的限制，但C语言包括——c++保留了——将指针分解为字节序列，将包含相同值的任何字节序列组装为指针，然后使用该指针访问先前的对象。

While that ability can be useful--or even indispensable--in some kinds of applications, a language that includes that ability will be very limited in its ability to support any kind of useful and reliable garbage collection. If a compiler doesn't know everything that has been done with the bits that made up a pointer, it will have no way of knowing whether information sufficient to reconstruct the pointer might exist somewhere in the universe. Since it would be possible for that information to be stored in ways that the computer wouldn't be able to access even if it knew about them (e.g. the bytes making up the pointer might have been shown on the screen long enough for someone to write them down on a piece of paper), it may be literally impossible for a computer to know whether a pointer could possibly be used in the future.

An interesting quirk of many garbage-collected frameworks is that an object reference not defined by the bit patterns contained therein, but by the relationship between the bits held in the object reference and other information held elsewhere. In C and C++, if the bit pattern stored in a pointer identifies an object, that bit pattern will identify that object until the object is explicitly destroyed. In a typical GC system, an object may be represented by a bit pattern 0x1234ABCD at one moment in time, but the next GC cycle might replace all references to 0x1234ABCD with references to 0x4321BABE, whereupon the object would be represented by the latter pattern. Even if one were to display the bit pattern associated with an object reference and then later read it back from the keyboard, there would be no expectation that the same bit pattern would be usable to identify the same object (or any object).

虽然这是一个老问题，但仍然有一个问题我没有看到任何人解决过:垃圾收集几乎不可能指定。

特别是，c++标准非常谨慎地根据外部可观察到的行为来指定语言，而不是实现如何实现该行为。然而，在垃圾收集的情况下，实际上没有外部可观察到的行为。

The general idea of garbage collection is that it should make a reasonable attempt at assuring that a memory allocation will succeed. Unfortunately, it's essentially impossible to guarantee that any memory allocation will succeed, even if you do have a garbage collector in operation. This is true to some extent in any case, but particularly so in the case of C++, because it's (probably) not possible to use a copying collector (or anything similar) that moves objects in memory during a collection cycle.

如果不能移动对象，就不能创建一个单独的、连续的内存空间来进行分配——这意味着您的堆(或自由存储区，或任何您喜欢称呼它的地方)可能会随着时间的推移而变得碎片化。这反过来又会阻止分配成功，即使空闲内存比请求的内存多。

尽管有可能提出某种保证，即(本质上)如果您重复完全相同的分配模式，并且它在第一次成功，那么它将在后续迭代中继续成功，前提是分配的内存在迭代之间变得不可访问。这是一个非常微弱的保证，基本上毫无用处，但我看不到任何加强它的合理希望。

即便如此，它也比为c++所提议的更强大。之前的提议[警告:PDF](被放弃了)根本不能保证任何东西。在28页的提案中，你在外部可观察到的行为中看到的是一个单一的(不规范的)注释:

[注意:对于垃圾收集程序，高质量的托管实现应该尝试最大限度地回收不可访问的内存量。-结束注释]

至少对我来说，这引发了一个关于投资回报的严肃问题。我们将破坏现有的代码(没有人知道具体破坏了多少，但肯定是相当多)，对实现提出新的要求，对代码提出新的限制，而我们得到的回报很可能是什么都没有?

即使在最好的情况下，我们得到的是基于Java测试的程序，以现在相同的速度运行可能需要大约6倍的内存。更糟糕的是，垃圾收集从一开始就是Java的一部分——c++对垃圾收集器施加了足够多的限制，以至于它几乎肯定会有更糟糕的成本/收益比(即使我们超出了提案所保证的范围，并假设会有一些收益)。

我要用数学方法总结一下情况:这是一个复杂的情况。数学家都知道，复数有两部分:实数和虚数。在我看来，我们这里的成本是真实的，但收益(至少大部分)是虚构的。

If you want automatic garbage collection, there are good commercial and public-domain garbage collectors for C++. For applications where garbage collection is suitable, C++ is an excellent garbage collected language with a performance that compares favorably with other garbage collected languages. See The C++ Programming Language (4rd Edition) for a discussion of automatic garbage collection in C++. See also, Hans-J. Boehm's site for C and C++ garbage collection (archive). Also, C++ supports programming techniques that allow memory management to be safe and implicit without a garbage collector. I consider garbage collection a last choice and an imperfect way of handling for resource management. That does not mean that it is never useful, just that there are better approaches in many situations.

来源:http://www.stroustrup.com/bs_faq.html垃圾收集

至于为什么它没有内置它，如果我没记错的话，它是在GC出现之前发明的，我不相信这种语言有GC，有几个原因(我不相信它有GC)。C)向后兼容。

希望这能有所帮助。

c++没有内置垃圾回收的最大原因之一是，让垃圾回收很好地使用析构函数是非常非常困难的。据我所知，还没有人真正知道如何完全解决这个问题。有很多问题需要处理:

deterministic lifetimes of objects (reference counting gives you this, but GC doesn't. Although it may not be that big of a deal). what happens if a destructor throws when the object is being garbage collected? Most languages ignore this exception, since theres really no catch block to be able to transport it to, but this is probably not an acceptable solution for C++. How to enable/disable it? Naturally it'd probably be a compile time decision but code that is written for GC vs code that is written for NOT GC is going to be very different and probably incompatible. How do you reconcile this?

这些只是面临的问题中的一小部分。