Difference between "&" and std::reference_wrapper?

Question

I have following code

#include <iostream>
#include <vector>
#include <functional>

int main() {
    std::vector<double> v(5, 10.3);
    std::vector<double>& r = v;
    //std::vector<std::reference_wrapper<double>> r(v.begin(),v.end());
    for (auto& i : v)
        i *= 3;
    for (auto i : r)
       std::cout << i << " ";
    std::cout << std::endl;
}

I am using reference of the vector 'r' using '&' operator and in the second line which I commented out I am using std::reference_wrapper of C++. Both do pretty much the same job? But I think there must be a purpose of making std::reference_wrapper even if we had '&' to do the job. can anyone explain please?

Enlil

Do r.resize(200) and you'll see the difference - one refers to the vector, others refers to the doubles. – Feathers

& here is not an operator. – Incorrect

Answer 1

First, the two lines

std::vector<double>& r = v;
std::vector<std::reference_wrapper<double>> r(v.begin(),v.end());

Don't mean the same thing. One is a reference to a vector of doubles, the other one is a vector of "references" to doubles.

Second, std::reference_wrapper is useful in generic programming (ie: templates) where either a function might take its arguments by copy, but you still want to pass in an argument by reference. Or, if you want a container to have references, but can't use references because they aren't copyable or movable, then std::reference_wrapper can do the job.

Essentially, std::reference_wrapper acts like a "&" reference, except that it's copyable and reassignable

Chesson

Perfect example of the functions taking arguments by copy section is std::async and std::thread – Xylophone