Most efficient way to pass data to a std::valarray from a std::vector

Question

What's the most efficient way to set the data from a std::vector to a std::valarray ? Say we have std::valarray<double> my_valarray; and std::vector<double> my_vector; and we want to copy the data across from my_vector to my_valarray :

Option 1 (using valarray constructor and copy assignment):

my_valarray = std::valarray(my_vector.data(), my_vector.size());

Option 2 (resizing and copying):

my_valarray.resize(my_vector.size());
std::copy(my_vector.begin(), my_vector.end(), std::begin(my_valarray));    

The question arises because in both cases it looks like the complexity is O(2n) . In the first case the data is copied to temporary valarray during construction (one allocation + one pass to copy the data) and then on assignment to the final object (one allocation + one pass to copy the data). In the second case there is one allocation + one pass for initialisation of all elements to zero and another pass to copy the data. Do the move semantic of C++11 applies in the first case making it require only one allocation and one pass to copy the data?

Answer 1

是的，在第一种情况下适用移动语义，因为std::valarray(my_vector.data(), my_vector.size())是一个右值，并且为valarray类定义了移动分配运算符（ http：//en.cppreference .com / w / cpp / numeric / valarray / operator％3D ）。

Answer 2

The first option is more efficient. The reason is that std::valarray::resize zero-initializes all data. But i would assume that any compiler worth it's salt would optimize that redundant zero-initialization away.

You can't prevent copying from the vector to the valarray, there is no way to transfer the ownership of the memory block from my_vector to my_valarray.