How to initialize std::vector from C-style array?

Question

What is the cheapest way to initialize a std::vector from a C-style array?

Example: In the following class, I have a vector, but due to outside restrictions, the data will be passed in as C-style array:

class Foo {
  std::vector<double> w_;
public:
  void set_data(double* w, int len){
   // how to cheaply initialize the std::vector?
}

Obviously, I can call w_.resize() and then loop over the elements, or call std::copy(). Are there any better methods?

Answer 1

Don't forget that you can treat pointers as iterators:

w_.assign(w, w + len);

Answer 2

Not quite a dupe, but see http://stackoverflow.com/questions/231491/how-to-initialize-const-stdvectort-like-a-c-array

Answer 3

You use the word initialize so it's unclear if this is one-time assignment or can happen multiple times.

If you just need a one time initialization, you can put it in the constructor and use the two iterator vector constructor:

Foo::Foo(double* w, int len) : w_(w, w + len) { }

Otherwise use assign as previously suggested:

void set_data(double* w, int len)
{
    w_.assign(w, w + len);
}

Answer 4

std::vector<double>::assign is the way to go, because it's little code. But how does it work, actually? Doesnt't it resize and then copy? In MS implementation of STL I am using it does exactly so.

I'm afraid there's no faster way to implement (re)initializing your std::vector.