将read（）直接使用到C ++ std：vector中 [英] Using read() directly into a C++ std:vector

问题描述

我在一个嵌入式系统的某些C ++中包装了用户空间的linux socket功能（是的，这可能是重新发明轮子）。

I'm wrapping up user space linux socket functionality in some C++ for an embedded system (yes, this is probably reinventing the wheel again).

使用向量提供读写实现。

I want to offer a read and write implementation using a vector.

写操作很容易，我可以通过& myvec [0] code>并避免不必要的复制。我想做同样的事情，直接读入一个向量，而不是读入一个字符缓冲区，然后复制到一个新创建的向量。

Doing the write is pretty easy, I can just pass &myvec[0] and avoid unnecessary copying. I'd like to do the same and read directly into a vector, rather than reading into a char buffer then copying all that into a newly created vector.

现在，我知道我想要读多少数据，我可以适当地分配（ vec.reserve（））。我也可以读入& myvec [0] ，虽然这可能是一个非常糟糕的想法。显然这样做不允许myvec.size返回任何合理的。有什么办法这样做：

Now, I know how much data I want to read, and I can allocate appropriately (vec.reserve()). I can also read into &myvec[0], though this is probably a VERY BAD IDEA. Obviously doing this doesn't allow myvec.size to return anything sensible. Is there any way of doing this that:

1. 从安全/ C ++的角度来看，并不完全感觉到幸运


2. 不涉及数据块的两个副本 - 一次从内核到用户空间，一次从C char * 样式缓冲区到C ++向量。 li>
   

1. Doesn't completely feel yucky from a safety/C++ perspective
2. Doesn't involve two copies of the data block - once from kernel to user space and once from a C char * style buffer into a C++ vector.

推荐答案

使用 resize（） reserve（）。这将正确设置向量的大小 - 之后，& myvec [0] 像往常一样，保证指向一个连续的内存块。

Use resize() instead of reserve(). This will set the vector's size correctly -- and after that, &myvec[0] is, as usual, guaranteed to point to a continguous block of memory.

编辑：使用& myvec [0] 作为指向读写基础数组的指针是安全和有保证的以C ++标准工作。这里是Herb Sutter的不得不说： / p>

Using &myvec[0] as a pointer to the underlying array for both reading and writing is safe and guaranteed to work by the C++ standard. Here's what Herb Sutter has to say:

那么，为什么人们不断地询问std :: vector（或std :: array）的元素是否被连续存储？最可能的原因是，他们想知道他们是否可以咳嗽指针到内部共享数据，要么读取或写入，与处理在C数组的其他代码。这是一个有效的用法，一个重要的足以保证标准。

So why do people continually ask whether the elements of a std::vector (or std::array) are stored contiguously? The most likely reason is that they want to know if they can cough up pointers to the internals to share the data, either to read or to write, with other code that deals in C arrays. That’s a valid use, and one important enough to guarantee in the standard.

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