Using memcpy to copy the contents of a vector<double> into a memory buffer

Question

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; }
....
}

The instance is:

PersistentVector<double> moments;

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];

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

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

Working solution

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

Failed solution

Try data() instead of 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

Still curious as to why my original code is failing!

Answer 1

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));