tl;dr:因为现代c++不需要垃圾收集。

Bjarne Stroustrup对此问题的常见问题解答如下:

我不喜欢垃圾。我不喜欢乱扔垃圾。我的理想是通过不产生任何垃圾来消除对垃圾收集器的需求。现在这是可能的。

现在编写的代码(c++ 17和遵循官方核心指南)的情况如下:

Most memory ownership-related code is in libraries (especially those providing containers). Most use of code involving memory ownership follows the CADRe or RAII pattern, so allocation is made on construction and deallocation on destruction, which happens when exiting the scope in which something was allocated. You do not explicitly allocate or deallocate memory directly. Raw pointers do not own memory (if you've followed the guidelines), so you can't leak by passing them around. If you're wondering how you're going to pass the starting addresses of sequences of values in memory - you can and should prefer span's, obviating the need for raw pointers. You can still use such pointers, they'll just be non-owning. If you really need an owning "pointer", you use C++' standard-library smart pointers - they can't leak, and are decently efficient (although the ABI can get in the way of that). Alternatively, you can pass ownership across scope boundaries with "owner pointers". These are uncommon and must be used explicitly; but when adopted - they allow for nice static checking against leaks.

“哦,是吗?但是……

．.． 如果我只是像以前写c++那样写代码?”

实际上，您可以忽略所有的指导方针，编写有漏洞的应用程序代码——它将像往常一样编译和运行(并泄漏)。

但这并不是一种“不要这么做”的情况，即开发者应该保持良好的自我控制;编写不符合规范的代码并不简单，也没有更快，也没有更好的性能。慢慢地，它也会变得更加难以编写，因为你将面临与符合规范的代码所提供和期望的“阻抗不匹配”的增加。

．.． 如果我retrepret_cast ?或者做复杂的指针运算?还是其他类似的黑客?”

事实上，如果你用心去做，你可以编写一些代码，尽管你很好地遵循了指导原则。但是:

您很少会这样做(就代码中的位置而言，而不一定是就执行时间的比例而言) 你只会故意这么做，而不是意外。 这样做将在符合准则的代码库中脱颖而出。 在这种代码中，您无论如何都可以在另一种语言中绕过GC。

．.． 图书馆发展?”

如果你是c++库开发人员，那么你确实会编写包含原始指针的不安全代码，并且你被要求谨慎而负责地编码——但这些是由专家编写的自包含代码片段(更重要的是，由专家评审)。

所以，就像Bjarne说的:一般来说，没有动机去收集垃圾，因为你只是确保不产生垃圾。GC正在成为c++的一个不成问题的问题。

这并不是说，当您希望使用自定义分配和反分配策略时，GC对于某些特定的应用程序不是一个有趣的问题。对于那些您想要自定义分配和反分配的对象，而不是语言级GC。

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。

强制垃圾收集实际上是一个低级到高级的范式转换。

If you look at the way strings are handled in a language with garbage collection, you will find they ONLY allow high level string manipulation functions and do not allow binary access to the strings. Simply put, all string functions first check the pointers to see where the string is, even if you are only drawing out a byte. So if you are doing a loop that processes each byte in a string in a language with garbage collection, it must compute the base location plus offset for each iteration, because it cannot know when the string has moved. Then you have to think about heaps, stacks, threads, etc etc.

主要有两个原因:

因为它不需要(恕我直言) 因为它与RAII几乎不兼容，RAII是c++的基石

c++已经提供了手动内存管理、堆栈分配、RAII、容器、自动指针、智能指针……这应该足够了。垃圾收集器适合懒惰的程序员，他们不想花5分钟思考谁应该拥有哪些对象或什么时候应该释放资源。这不是我们在c++中做事情的方式。

什么类型?它应该针对嵌入式洗衣机控制器、手机、工作站或超级计算机进行优化吗? 它应该优先考虑gui响应还是服务器加载? 它应该使用大量内存还是大量CPU?

C/c++被用于太多不同的环境。 我怀疑像增强智能指针这样的东西对大多数用户来说就足够了

编辑:自动垃圾收集器并不是一个性能问题(你总是可以购买更多的服务器)，而是一个可预测的性能问题。 不知道GC什么时候会起作用就像雇佣一个嗜睡症的飞行员，大多数时候他们是很棒的-但当你真的需要响应的时候!