本征如何防止生成临时对象？

Question

In eigen documentation under the title why its interesting there is an example mentioning how eigen works. The example is

#include<Eigen/Core>
int main()
{
  int size = 50;
  // VectorXf is a vector of floats, with dynamic size.
  Eigen::VectorXf u(size), v(size), w(size);
  u = v + w;
}

In the doc says, normally due to the design architecture of C++ the above operation would be done using a temporary.

VectorXf tmp = v + w;
VectorXf u = tmp;
for(int i = 0; i < size; i++) tmp[i] = v[i] + w[i];
for(int i = 0; i < size; i++) u[i] = tmp[i];

As we can see, due to the temporary object we would need one more loop. But eigen prevent generating this extra loop by not generating this temporary object. So just one loop is enough for this operation.

for(int i = 0; i < size; i++) u[i] = v[i] + w[i];

Although they explained how this matrix sum works but I could not understand it. It would be nice if someone could explain it in simple words.

Answer 1

The actual implementation of expression templates inside Eigen is quite complicated thus the description can't be "dumbed down" much. If you are just interested in the concept of expression templates, have a look at the corresponding wikipedia article . Also read the referenced links/books if you are interested in more details.