如何获得由vector :: reserve（）分配的缓冲区的地址？ [英] How can I get the address of the buffer allocated by vector::reserve()?

问题描述

我有一个std :: vector的值，我知道最大大小，但实际大小将在使用期间变化：

I have a std::vector of values for which I know the maximum size, but the actual size will vary during usage:

void setupBuffer(const size_t maxSize) {
  myVector.reserve(maxSize);
}

void addToBuffer(const Value& v) {
  myVector.push_back(v);

  if (myVector.size() == maxSize) {
    // process data...
    myVector.clear();
  }
}

但是，在setupBuffer中，到myVector的数据的开始。我使用的第三方库，我必须缓存这个指针前面用于在过程数据...部分中进行的调用。

However, in setupBuffer, I need to obtain a pointer to the start of myVector's data. I'm using a third party library where I must cache this pointer up front for use in a call made during the "process data..." section.

void setupBuffer(const size_t maxSize) {
  myVector.reserve(maxSize);

  cachePtr(&(myVector[0])); // doesn't work, obviously
}

我不想调整大小）向量前面，因为我想使用vector.size（）来表示添加到向量中的元素数量。

I don't want to resize() the vector up front, as I want to use vector.size() to mean the number of elements added to the vector.

所以，有没有办法获得指向分配后的向量缓冲区的指针（reserve（）），但在它有任何元素之前？我想象缓冲区存在（只要我限制push_back'd值的数量就不会移动）....也许这不能保证？

So, is there any way to obtain the pointer to the vector's buffer after allocation (reserve()) but before it has any elements? I would imagine the buffer exists (and won't move as long as I restrict the number of push_back'd values)....maybe this isn't guaranteed?

推荐答案

向量缓冲区在调用保留后不会移动，除非超过保留容量。你的问题是获取指向第一个元素的指针。明显的答案是将一个单一的假条目推入向量，获取指向它的指针，然后删除它。

The vector buffer will not be moved after a call to reserve unless you exceed the reserved capacity. Your problem is getting the pointer to the first element. The obvious answer is to push a single fake entry into the vector, get the pointer to it, and then remove it.

更好的方法是如果库接受函数而不是一个指针，当它需要访问缓冲区时，它会调用 - 你可以使函子推迟获取地址，直到缓冲区有一些真正的内容。但是，我意识到你没有改写图书馆的奢侈。

A nicer approach would be if the library accepted a functor rather than a pointer, which it would call when it needed to access the buffer - you could make the functor put off getting the address until the buffer had some real contents. However, I realise you don't have the luxury of rewriting the library.

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