使用memcpy复制vector< double>的内容.到内存缓冲区 [英] Using memcpy to copy the contents of a vector<double> into a memory buffer

问题描述

我有一个包装矢量的类:

I have a class that wraps a vector:

template <typename T>
class PersistentVector
{
  private:
    std::vector<T> values;
  public:
    std::vector<T> & getValues() throw () { return values; }
....
}

实例为:

PersistentVector<double> moments;

我正在尝试将所有双打的副本复制到其他人分配的缓冲区中.这是我创建缓冲区的地方:

I am trying to make a copy of all the doubles into a buffer allocated by somebody else. Here is where I create the buffer:

// invariant: numMoments = 1
double * data_x = new double[numMoments];

这是我将向量内容复制到缓冲区的尝试:

Here is my attempt at copying the contents of the vector into the buffer:

double theMoment = moments.getValues()[0];
// theMoment = 1.33

std::memcpy(
  data_x, 
  &(moments.getValues().operator[](0)), 
  numMoments * sizeof(double));
// numMoments = 1

double theReadMoment = data_x[0];
// theReadMoment = 6.9533490643693675e-310

如您所见，我从缓冲区中获取了一个垃圾值.为什么会这样?

As you can see, I am getting a garbage value from the buffer. Why is this so?

使用 std :: copy (感谢WhozCraig)

Use std::copy (thanks to WhozCraig)

double theMoment = moments.getValues()[0];
// theMoment = 1.33

std::copy(moments.getValues().begin(), moments.getValues().end(), data_x);

double theReadMoment = data_x[0];
// theReadMoment = 1.33

解决方案失败

尝试使用 data()代替 operator []()

double theMoment = moments.getValues()[0];
// theMoment = 1.33 

std::memcpy(
  data_x, 
  moments.getValues().data(),
  numMoments * sizeof(double));
// numMoments = 1

double theReadMoment = data_x[0];
// theReadMoment = 6.9533490643693675e-310

仍然对为什么我的原始代码失败感到好奇！

推荐答案

我尝试了3种不同的方法，所有这些方法都可以工作.data()方法是正确执行此操作的方法.如果不起作用，请检查数据是否以某种方式损坏.

I tried doing this in 3 different ways and all of them worked. The data() method is the way to do it right. If it does not work - check if the data was corrupted in some way.

 std::vector<double> vec1 = {1.33,2.66,3.99};
 double* vec2 = new double[3];

 int numMoments = 1;

 ::memcpy(
  vec2, 
  vec1.data(),
  numMoments * sizeof(double));

 ::memcpy(
  vec2, 
  &(*vec1.begin()),
  numMoments * sizeof(double));

 ::memcpy(
  vec2, 
  &(vec1[0]),
  numMoments * sizeof(double));

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