＆是什么QUOT;单个分配＆QUOT;意味着升压:: make_shared [英] What does "single allocation" mean for boost::make_shared

问题描述

在 make_shared 升压文档，它说：

除了方便和风格，这样的功能也异常安全，大大
  更快，因为它可以使用单一分配为对象及其对应的
  控制块，消除shared_ptr的建设开销显著部分。

Besides convenience and style, such a function is also exception safe and considerably faster because it can use a single allocation for both the object and its corresponding control block, eliminating a significant portion of shared_ptr's construction overhead.

我不明白单分配的含义，这是什么意思？

I don't understand the meaning of "single allocation", what does it mean?

推荐答案

这是分配是指从一个呼叫分配器获得的内存块。

An "allocation" means a block of memory obtained from a call to an allocator.

通常情况下，创建一个的shared_ptr 用指针构造一个控制块，其中包含引用计数和删除器分配内存。该的shared_ptr 所有的副本指的是同一控制块，让他们共享的引用计数。因此，总共有两笔拨款 - 对象本身和的shared_ptr 创建了控制块

Usually, creating a shared_ptr with the pointer constructor allocates memory for a "control block", which holds the reference count and the deleter. Copies of that shared_ptr all refer to the same control block, so that they share the reference count. Hence there are two allocations in total - the object itself and the control block created by shared_ptr.

如果您创建对象和的shared_ptr 连同 make_shared ，那么只有一个分配的。你可以认为这是一个单一的结构有两个成员：

If you create the object and shared_ptr together with make_shared, then only one allocation is made. You can think of this as a single struct with two members:

1. 这是被管理
对象

2. 控制块。

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